Post by John on Apr 6, 2016 11:30:05 GMT -5
A paper sent to me by Rob52. Sorry about missing parts, I managed to C&P from a pdf document, first time I've managed it.
I see some characters haven't come out as expected, I'll edit the paper later. I've done a lot of editing, I'll follow up later.
COAL FACE VOLTAGE
-the choice
By J F Wilkie C. Eng. FlEE. MIMinE. Member.
The purpose of this paper is to assess the various factors contributory
to determination of the voltage that should be used at the
coal face. It is attempted to portray fully the technical comparisons
between 565, 1100 and 3300 volts and thereby facilitate decision making.
Some of the technical aspects of system voltage and design
and development are discussed to enable a better appreciation
of the reasons for changing systems or equipment designs. Additionally,
an appreciation of the various technical aspects of
equipment design is: considered necessary.
A.C. equipment i~ most vulnerable to voltage fluctuations.
Desirable limits of voltage drop at starting are 9 % for squirrel
cage motors and 14% for slip ring motors. For a number of years
voltages in use underground have been 2200/6600 volts for H.T.
distribution and 440/550 volts on medium tension. Voltage
drop problems have always been apparent, even with 6600 volts
as the main supply distribution, because the transformers were
generally of 500 KVA capacity and situated outbye.
As the horsepower of the face equipment increased, steps had
to be taken to install a transformer in each roadway to the face,
the minimum distance of the transformer from the face being
limited until the advent of the F.L.P. transformer.
SYSTEM VOLTAGE CRITERIA
For several years consideration has been given to the application
of 6.6 KV for both use and distribution but progress in this
could not he made due to the incompatibility of physical mining
requirements and necessary technical design features. It may he
possible in future to continue this line of action but other distribution
systems, namely 3300 volts and 1130 volts, are being
actively considered.
'
In American mines machines operating on 4160 volts and
1000 volts are being introduced.
Use of two voltages together on or at the face, i.e., 1130 and
550, is fraught with disadvantages of increased space requirements
and complexity of layout (adversely affecting safety) in the
confined space at the roadway end near the face; and it still leaves
the AFC starting problem to be solved. The difficulty arises
where smaller motors cannot be wound suitably for 1100 volts.
For roadway heading machines it may be desirable to fit to the
machine a small capacity 1130/565 volt transformer.
The criterion is to provide the rated voltage at the motor
terminals and it is generally agreed that a voltage higher than
550 volts will give efficiency and economy under the most adverse
conditions. Likewise there will be a proportionate reduction
in current flow with the benefits that result from this reduced
current, namely, smaller conductor size in cables etc. and reduced
heating and maintenance of equipment resulting from less repeated
starting and from the rated voltage being applied to the
motor.
What must be borne in mind, however, is the maximum
permissible voltage drop at the motor terminals, if the design
torque characteristic of the motor is not to be jeopardised.
Therefore, before any decision is made to increase the distance
of the transformer from the face, or reduce the power cable
conductor size, it is essential to ensure that this limit is not over
reached.
Figures I and 2 outline the effects of reduced voltage on a
motor's Torque/Speed, Current/Time and Current/Speed characteristics.
These curves are typical for N.C.B. Spec. 50, 65 and 120
H.P. AFC motors. With a large percentage voltage drop,
particularly in conjunction with an incorrectly filled fluid
coupling, starting problems may arise.
Care must always be taken when designing an electrical system
that the torque and HP are available to overcome the increased
friction that occurs during the anticipated life of the mechanical
equipment, so as to achieve and maintain a low incidence of
stalling and consequent reduction in outage time.
SAFETY
Without any doubt safety is the most important factor to he
considered when assessing the use-level voltage; there must be
adequate protection provided for personnel and equipment.
However with the protective equipment already incorporated
in the 550 volt system and again proved in the 1100 volt systems
now in use, and similar equipment with expected similar results
FIG 1 FIG 2
... IOSC 'Of ................... K ,,~, ._ ............. ... ... • ..... '1'_ ......
.' .' .
: ..•
7
in 3300 volt units, it is apparent that coverage is being provided.
These are, in effect, fail safe protective circuits with attendant
complications of maintenance, short circuit protection (still with
a divergence of setting levels), earth fault current limiting and
lock out, earth fault proving before motor circuit energisation
(which, in addition to earth fault current limiting, reduces equipment
replacement cost and lost production time), limitation of
fault power and pilot core protection.
The major consideration for high voltage systems, 3300 volts
and above, is the limitation of short circuit currents by screened
power cables and components and the limitation of earth fault
currents. Reactors have been utilised in the power circuits but
these limit the flexibility of the load via available through currents;
they are also a major contributory factor to poor voltage
regulation. Neutral Point resistors have been utilised to limit the
earth fault current to the value governed by that required by the
surface or pit bottom distribution, i.e. above 15 Amps. The
question of neutral earthing the main supply is further assessed
under SYSTEM DESIGN later in this paper.
For safety we must have quality components, for operational
reliability, effective interlocking, efficient earthing linked with
good installation practices, effective maintenance and proper
use of plant and equipment.
RELIABILITY AND MAINTENANCE
Reliability infers
1 improved machinery and performance, related to
improved and maintained productivity
2 longer operational life of equipment- preferably
spanning the life of the face or even extending into
use on the next face
3 less maintenance-particularly important where
round the clock working is essential.
In essence, a reliable component or system is or.e which
continues to operate until a scheduled or charge point is reached.
It is not only impractical, but impossible, to design equipment
so that every part will wear out at the same time; but our least life-
expectancy component must have sufficient reliability and
life duration to be equal to a scheduled replacement cycle. It
would be reasonable to expect this minimum to be twelve months.
For GE Boxes the minimum should be 1,000,000 operations (or
some two years), not only for the contactor but also for auxiliary
switches and mechanism, together with all nuts, bolts and
linkages making up the whole. A static control circuit should
provide some means of eliminating mechanically actuated
equipment.
To achieve reliability it is often necessary to increase the
initial cost of the equipment. This may be more economical in
the long run as down time of machinery is very costly. Instances
of such initial cost increases are the steps now being taken to
have motors condensation and water proofed-and the wider
adoption of metal-cased heaters installed in some switchgear
as a means of combating condensation. Designed reliability must
be emphasised - scientific research & development and modification
in design can still be attained at a lower cost level. We
have already achieved improvements in design from two-way
communication between the user and the manufacturer but
there is still much room for progress here.
As machines take over more of the production load, with the
inevitable production improvement expectations, and as more
capacity is concentrated in individual machines, the cost, capacity
and complexity of modern equipment place increased burdens
on maintenance features. Therefore ease of maintenance is an
8
essential aspect of ensured reliability and reduced down time.
Effective maintenance will always be necessary, even with more
reliable equipment, since it has an influence on production costs.
an effect on output (operating time versus downtime) and it
serves to protect capital investment.
Therefore, when choosing equipment, merit should be awarded
to those types which are easily cleaned, inspected, maintained and
overhauled, and which also offer flexibility in load settings. A
typical example is the integral testing of control circuits on coal
production machines, especially those on GE Boxes. The test
facilities must indicate the condition not only of the total circuit
but also of as many sub circuits as possible, e.g., the contactor,
the control relay circuit, the proving relay circuit and the sequence
relay circuits. As electronic equipment control circuits may be
complex, it will be necessary to provide an easily visible indicator
which provides the values which can be related to circuit parameters.
STANDARDISATION AND FLEXIBILITY
The term standardisation is synonymous with variety reduction;
but standardisation does not mean continuous repurchase
of a component of a fixed type; it is also not a cover for
stultification in design. It must be regarded as a definite stepping
stone toward further improvement of existing "standards". The
word standard is the general term; "specification" is a particular
kind of standard which sets down precise requirements in an
objective manner.
Standards should be defined as clearly as possible in qualitative
rather than quantitive terms, i.e., all parts made must measure
up to qualitative finished products. No specification should lend
itself to ambiguous interpretation.
Adequately rated components, with the amount and nature
of the loads and circuit arrangement clearly determined, are
essential to flexibility. With this in mind flexibility for electrical
distribution and for control gear can be achieved by designing
the main items of equipment, such as circuit breakers, contactor
starters, etc., to a rated value, with interchangeable components
to suit the load of the circuit. This is already largely achieved.
Such a provision ensures economical power distribution centres
with flexibility for load growth and change within minimum
time and cost limits.
The units in the system must however be adaptable, without
bulky projections (unless these can be disconnected easily).
With "In Seam" mining- with rectangular roadways, maximum
seam height of 5 ft and maximum effective clearance of 4 ft 6
inches- problems arise, especially if the roadways are not
levelled out in steep cross gradient seams.
The motors, switchgear and transformers must therefore be of
compact design; the external surface must be to a minimum
prescribed area, to avoid the settlement of too much coal dust,
so that the maximum temperature, which can be progressive
with dust layers, does not affect the performance of the equipment
or give rise, at 200 °C, to self ignition of the dust.
Wherever possible the motors should be wound for dual
voltage, with linking arrangement in the terminal box for use
with the voltages of different systems.
SYSTEM DESIGN
System design is a vital part of mining and in many cases
determines the success or failure of an operation. When designing
an electric power distribution system it is essential to provide,
I maximum safety for personnel and equipment
2 stable continuous power supplies
3 low equipment investment cost-at economical
rates
4 minimum maintenance
5 minimum power transmission cost
6 ease of relocation of equipment and the flexibility
to meet varying loads
Stability of voltage is becoming an increasingly important
factor, since there is no evidence that horsepower of face equip
ment will level off in the forseeable future unless a horse
power voltage
impasse is reached.
A plan of the present and anticipated layout is a necessity;
then an assessment of the amount and nature of the load. The
magnitudes and geographical locations of the loads to be encountered
may be well known from experience of other mining
operations but before fixing the working voltage and distribution
and cable rating it must be ascertained as nearly as possible the
future growth of the load, i.e., the expected total connected load,
diversity of use, load factor and method of distribution that will
provide the load at optimum flexibility. There is an economic
limit to the number of KVA miles which can be transmitted at a
given voltage. Very large copper cross sections may have to be
installed to cover anticipated increases in growth of the load and
to provide a desired flexibility.
A generalisation is usually made to standardise given sizes of
cables of the paralleling of two or more cables to a KVA
distribution centre-sub station underground-to provide, in
addition to standby supplies, the KVA transmission capacity at
the required voltage. Where possible, large power rated equipment
should be utilised at the high voltage of the distribution
system.
Paralleled (ring main) supplies are often warranted for load
and/or standby capacity. A typical value above which warranty
would apply is 200 amps per mile of HV power distribution.
However it must be noted that ring main supplies give rise to
their own particular problems. From the load centres to the
seams. and thence to the faces. distance and load are the criteria
of cable cross sectional area (size).
It is clear therefore that considerable judgment must be
exercised. There are variables in mining which cannot always be
forseen; Therefore. in distribution systems, especially underground.
it is not possible to adhere strictly to formulae. Empirical
formulae can however be of valuable assistance.
In addition to load characteristics there are two essential
requirements; that the cables and switchgear will withstand the
short circuit current that can flow in the system; and that the
switchgear is capable of clearing the fault conditions that can
arise. Where contactors are used in the system for motor control
we must
1 limit the current to a value that the contactor is
capable of breaking
or
2 fit back-up protection HBC (high breaking capacity)
fuses to the equipment; When deciding on the rating
of the fuse to be fitted we must however ensure
that sufficient current will flow in the circuit during
a fault to rupture these fuses in the very short time
before the contactor is opening.
Consider a 500 or 750 KV A transformer feeding a complete
face and outbye supplies. With a %age Impedance of 4· 75 %
and assuming unlimited Incoming MV A capacity the secondary
short circuit current is as follows:-
Transformer KVA x lop
1000 x %age Impedance
MVA(S.C)
and amps (S.C) ~
MVA (S.C) x 1000
V (KV) x y3
For 500 K VA Transformer 3·3 K V Secondary
500 x I 00
1000 x 4·75
500 x 0·0210
10·5 MVA
and 1837 amps 3 phase for 3300 Volts system
750 KVA Transformer 3·3 KV Secondary
~ 750 x 0.0210 ~ 15·75 MVA
and" 2755 amps 3 phase RMS.
If the fault level at the transformer was not infinite, but
limited to 25 MVA:~
500 KVA Transformer
750 KV A Transformer
5·15 MVA and SC amps~ 901
6·15 MVA and SC amps~1075
Considering the use of 3300 Volts as the "use level voltage",
and before further developing the aspect of HBC fuses, there is
one other feature that must be included in the system. This is
the reduction of earth fault currents.
This can fall into one or two categories, either a limitation of
15 Amps to earth, or sensitive earth fault protection.
If limitation to 15 Amps on a new system is required this is
achieved as follows:-
1 fit neutral point current limiting resistances to the
supply transformers. fit all the main EHV distribution
switchgear with earth protection at 5/7.5
amps.
2 install transformers specially to feed underground
and fit neutral point current limiting resistances to
them and fit all HV switchgear feeding underground
with 5/7.5 amp earth fault protection.
However. if say 3·3 KV was considered for underground use
it would also apply at existing collieries where it would be impossible
or impractical to fit 5/7·5 amp earth fault protection
to the existing 3·3 KV switchgear as generally. since 1939, this
was fitted with protection at 5 amps or 15 %. (whichever was the
greater) of the normal current of the circuit. The main surface
switchgear has time delayed protection, with instantaneous
earth fault protection for underground switchgear.
For established systems, whether 15 Amps restricted earth
fault current or sensitive earth fault (restricted and monitored).
it will be necessary to fit a 3 ·3/3.3 KV transformer underground.
Setting relays at 5 amps or even 15 amps is no guarantee of protection.
With the 1 to 1 ratio double wound transformer the
neutral point is made available underground for necessary
protection and discrimination and connection to the associated
primary circuit breaker for clearance of the system.
Therefore, we have incorporated additionally an inherent
impedance in the system which must be taken into consideration
for voltage regulation in the system and for restriction of short
circuit current.
What must be taken into consideration where HBC fuses are
fitted is this variable, affected by impedance of different lengths
of cable. of short circuit fault capacity to rupture these backup
fuses.
If contactors are of a high current capacity this does not raise
so much of a problem as they can cope generally with a high
fault capacity. The HBC fuse cut-off limit characteristic cannot
be reduced to too low a value as it must be capable also of
carrying a required through current. Several successive starts
must be allowed so that the HBC"back-up fuse does not inadvertently
fail due to the high temperature that would prevail.
As an example, take a 120 HP motor with full load of 20 amps
and stalled current of 120 Amps and 10 seconds run up time per
start (assuming that the current does not fall off, due to stalled
conditions, as the motor tends to run up to speed). Six successive
starts equals 60 seconds and, therefore, the fuse must cover
this current and time.
Suitable fuses (see figs. 3 & 4) are:-
(a) E.M.P. Electric Ltd's Quick Acting 40 amp
(b) English Electric Co Ltd's Type K2 PA 50 amp
The first fuse will cover 120 amps for over 120 seconds, as the
characteristic curve tends to flatten out. In fact it would carry
140 amps for 60 sees., but with the BS.2692 allowable ± 20%
tolerance this could be reduced to 112 amps. It would also
rupture at 750 amps in 0 ·01 sec., or at 500 amps in 0 ·02 sec.
11resecondfuse will fail at 120 amps for 4 mins
140 amps for 2 mins
150 amps for I min
or at-20% rating 120 amps for 1 min
It would also rupture at 1000 amps in 0·01 sec, or at 800 amps
in 0·02 sec.
It must be noted that a fuse may carry up to 1·4 or more
times its rated current without melting, a value which may be
sustained in the protected circuit; Le., Fusing Factor
Minimum Fusing Current
may vary between 1·4 and 2,
Rated Current
dependent upon its design, and therefore up to 3 times the current
may have to flow for fusing to occur.
3300 volt Contactors. The ISO amp size will break 1200 amps or
1600 amps five to ten times as an emergency rating. The 75 amps
size will break 600 amps or 800 amps as an emergency rating.
Therefore, when related to fuse cut-off current, the fuse will
operate first-as in addition to the current there is the inherent
time lag of the mechanical operation of the contactor, which is
normally greater than 0·05 secs. This inherent time lag with
current transformer operated overloads results from the cr
being saturated well below these short circuit currents, with the
oil dashpot consequently slugged in its action. If the contactor
was opening on an earth fault, even via an instantaneous relay,
there is the inherent time lag of 0·02/0·03 secs. This then feeds
the trip relay with an action time lag of 0·02/0·03 sees and the
DC contactor mechanical operation with a further 0 ·02/0 ·03
sees. The quickest time on earth fault is thus 0·06 sees.
If the contactor was opening on earth fault and a phase to
phase fault developed this would present an extreme circumstance
-but the 75 or ISO amp contactors should still clear.
The AEI Vacuum Contactor 300 amps rating is capable of
3000 amps guaranteed
4000 amps 99 % of breaks
4500 amps 50 % of breaks
5000 amps nil
The contacts start to blow off at 8 to 10 KA and will withstand
8000 amps for 10 sees at a non dangerous temperature
rise. It must be assessed whether, with this type of contactor, it is
necessary to have:-
10
1 a 150 and/or 75 amp rating, as a further reduction
in contactor dimensions may result in an appreciable
difference in the size of the enclosure at 3·3 KV.
1 a more efficient means of detecting contact wear
3 a ready means of ascertaining that the vacuum is in
order, or that a contact is not welded.
4 a large gap between contacts; there are definite advantages
to a small gap, e.g., prevention of contact
FIG 3
FIG 4
..0 LOAf! .... TIN' !u .... "
lUaJ«T IQ .T._RO TOlu~ •• cn.
,
'------------~,------------~~~
bounce and operating coil rating; however, linkage
wear may reduce the gap too much within the 2 million
operations life span.
EQUIPMENT DESIGN
A prime problem facing designers, especially with face equipment,
is the virtual impossibility of deciding on the physical
conditions, overload and duty cycle under which equipment
may have to operate. The user must cooperate fully in advising
on these aspects to achieve a good design, acceptable for
standardisation of a nature desirable to collieries.
The primary task in design is to assess the field in which
development must be concentrated. The following comments
are grouped under the heads of 1100 VOLTS, FLP CONTACTOR
CONTROL GEAR, 3·3 KV, OVERLOAD TRIPS &
PROTECTION, MOTORS (OVERLOAD CAPACITY),
MOTORS FOR HIGH VOLTAGE, WATER COOLED
MOTORS AND LIQUID FILLED MOTORS.
1100 volts
Thrustor control and protection must be provided; the
necessity is obvious for practical reasons and also because
thrustors cannot be wound suitably for above 650 volts. Since
the present Gate End Box enclosure can accommodate two contactors
it is possible that one be fitted and the spare space made to
accommodate the thrustor supply step down transformer
(0·5/1'0 KVA) and protection.
The manufacturers GEC/AEI Ltd state that the reason for
limitation to 550 volts as maximum for thrustors arises from
clearances and the mechanical strength of the winding wire.
The following table outlines the transformer & thrustor rating
and bare copper conductor sizes prior to application of THERMEX
'M' covering.
Thrustor Details
RATING THRUSTOR
KVA Watts. Bare Conductor Size ins
Supply Thrustor Size
Transformer l00/110v 500/550 v
0·4 150/180 752/1002 0·0172 0·0136
0·5 170/200 1503/2503 0'018 0'0156
1-0/1'6 420/580 5005/6505 0'024 0'018
1-6 700 8000/12 0·025 0·022
FLP Contactor Control gear
It is generally accepted that higher voltage control gear
requires greater space, due to line to line and earth clearances.
Whatever the actual size, it is essential that gear of different
voltages cannot be butted together.
Space utilisation is critical in switch gear design and is usually
complicated by requirements such as a thrustor transformer and
protection or other control requirements.
AEI Ltd, in an article I referring to volume of vacuum contactors
at HV and MV, state that, because no arc chutes are
utilised, their space requirements are nearly 1/8 and 2/5 respectively
FIG 5
BELOW
FIG 6
~ 1.11"
~"
III
-d
r •. , I II .,1
L''''/J
I ,
•
•
those of air break equivalents-and reduce the clearance required
to adjacent apparatus.
Vacuum contactor sizes and weights compared with air-break
equivalents
j
I CONTACT OR
I
Height Width Depth Weight
(ins) (ins) (ins) (Ibs)
HV Vacuum III 16l 16 60
Air-Break Equivalent 31l 26 291 275
MV Vacuum 8t 91 9t 26
Air Break Equivalent lit 16 10 31l
Difficulties may arise if the 1100 volt contactor gear is re~
designed, due to the utilisation of a vacuum allowing the en~
closure to be smaller; the author considers however that any
space made available can be usefully employed. Baldwin &
Francis in their prototype design and using their block contactor
were able to utilise the 550 volt enclosure.
3·3KV
The main criterion is however with any standard design for
3·3KV.
Belmos Peebles, in their early pioneering of 3·3 KV, have
produced the widest range of equipment. They consider that
they could reduce the volume of their KFG unit, which covers a
single 3·3 KV contactor, thrustor supply and protection and
contactors for 3 other medium or low voltage low HP drives, and
still have room for SEL protection.
Baldwin & Francis consider that by utilising a vacuum con~
tactor they cannot reduce the volume of the enclosure.
Figs. 5 and 6 respectively outline the details of Belmos Peebles
and Baldwin & Francis 3·3 KV equipment enclosures. The large
depth, especially with the door opened, should be noted.
The NCB must now seriously consider the design of 3·3 KV
equipment, so that some standardisation of dimensions and
control duty requirement will be possible before too many
varieties are utilised. The normal:Belmos Peebles' range covers
. ., 8-'
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ocw,; :~, ~ ,' . J .. . @ill
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high enclosures not suited to easy installation and relocation
underground; but the availability of control features meets a
wide need for one motor or up to four-5 HP 550 volt motors
via the designed rating of the 16·5 KVA 3 ·3/0'55 KV transformer
now listed cpable of 27 KV A.
The probable requirements on 3·3 KV are as follows:-
1 Unit Type A 3·3 comprising:-
a reversible "OFF-EARTH" off load Isolator
b 3·3 KV ISO amp motor drive (non reversible)
c 4, KVA of 3 phase 550/11Ov drives for up to 3
motors
d 15 volt IS signal transformer
e individual circuit overload protection
f earth fault protection for main motor drive
g group earth fault protection for auxiliary drives
h HBC fuses for main and auxiliary supplies
auxiliaries switches, remote, local, sequence,
contactor freeze protection, incoming and outgoing
cable arrangements for all circuits, ammeter etc.
and with addition as required of
j KW Hr meter
k 500 VA 240/120 volt lighting transformer and
protection
metal cased anti-condensation heaters, each 60 watt
2 Unit Type B. 3·3
As unit type A 3·3 with addition of second 3·3 KV
ISO amp contactor for two motor or reversible drive.
3 Unit Type C 3·3 comprising
a reversible "OFF-EARTH" off load Isolator
b 25 KV A 3·3 550 volt 3 phase transformer
c 700 watt 550/110 volt transformer for one or two
thrustors
d four 5 HP 550 volt auxiliary drives, one or two
reversible via rotary switches for loop take up
drives
e 4, KV A double drill transformer
f group earth fault protection for d
g individual and short circuit fault protection for e
h ammeters for each circuit
HBC fuses for main and auxiliary supplies
j 15 volt IS signal transformer
k auxiliaries switches, remote local, sequence, incoming
and outgoing cable arrangements for all
circuits
alternative to d for one 20 HP motor
4 General
a a combination of type A, B & C 3·3 to be suitable
for erection in a bank with coupled busbars and
through going 3·3 KV busbar arrangement
b 3·3 KV plug and socket; Belmos Peebles have a design
drawing of a 3·3 KV 50 amp restrained plug
socket. This plug socket should be the restrained
type with screened power cores and interlock and
pilot core protection and possibly compound
filling.
FinalIy, it wi1l be necessary to resolve an approved design of
flexible 3·3 KV trailing cables.
Overload trips and protection
Unfortunately there has been no overload device yet discovered
which is able to fully protect a motor, largely because of
12
the different thermal capacities of a starter and motor. If a
totally enclosed TEFC or TEFC, FLP motor is heated to its
maximum temperature and then stopped it will take much longer
to cool down than the thermal protection device in the starter.
If the overload device does not work on temperature, e.g.,
adjustable inverse time lag dashpots, then it is automaticalIy
non-responsive to the effects of accummulated peaks of load
and therefore, especially where the design of the motor enclosure
and windings has to be reduced to minimum volume, troubles
can ensue. The motor is not capable of providing several quick
successive starts as is often required and it is necessary to install
a thermal preset temperature trip device in the motor windings
to detect the hot spots.
With the change to motors rated to CMR (continuous maximum
rated) it follows that, if the motor is subject to heavy
starting loads, or has a prolonged start from co1d, which is
within the heating capacity of the motor, the starter would
trip; and if the starter is set to handle this current it will not
protect the motor from a sustained overload when running. The
change of emphasis from the temperature of the outside to the
inside (windings) of the motor is therefore a logical avenue for
further development. If this is done, then hazards such as overloading
on running, stalling, too frequent starting, single
phasing, choked ventilation, etc. should be avoided.
Motors-overload capacity
Motor design has improved over the years and the associated
reductions in dimensions have raised problems which are not
often fully appreciated.
BSS.l68 had a continuous rating permitted overload (CRPO),
or "load-plus-overload" rating; motors over say 10 HP, where
the cooling air does not exceed 35°C, must be capable, after
attaining the temperature rise corresponding to rated load,
voltage and frequency, of 25 % overload in torque for 2 hrs and
100% torque momentarily (i.e. for 15 seconds) without any
appreciable effect on the temperature rise. This 100% overload
is thus permissible and, as the heat produced varies as the square
of the current, successive overloads may produce a temperature
rise exceeding that permitted. If the motor is stalled and therefore
an absence of forced ventilation exists the windings would attain
a very high temperature. There can be a life expectancy of 10/15
years at llO"C and possibly only I year at 135"C for the winding
-not the carcase-of a motor to BSS.168. A relay to BSS.587
can have a tolerance of 10 % on the tripping current and therefore
overload capacity must be used with caution and with full
knowledge of the factors involved, i.e., the ratio of peak loading
time to total running time.
A motor designed to BSS.2613 is a CMR motor and the rerating
was brought about through the use of new synthetic
insulation which altered the concepts L f motor temperature
rise. These temperature rises for windings insulated with Class
A, E, B, F and Hare 55 0, 65°, 75 0, 95 ° and 115°C :espectively.
with a cooling ambient temperature not exceeding 40°C. For
class 'E' the permitted temperature rise is only 10°C, with a
further 25°C allowance for supposed hot spot temperature,
with the final temperature of 120 °C. The hottest spot is the
overhang of the windings, ie. outside the slots. With BSS.l68,
class A and B insulations were not to exceed a temperature rise
of 4O"C and 50"C respectively. CMR motors to BSS.2613 have
no sustained overload capacity, merely a momentary increase
in torque for 15 seconds of 100% up to 50 HP and of 75%
above 50 to 500 HP. Therefore, if the motor is subjected to
overloads in excess of these, it is at the expense of higher insulation
temperature and consequently shorter life.
A further specification change was from BSS.2083 for 'B'
frames to BSS.2096 for D and E frames, which reduced the
dimensions. The motors will thus have lower inertia rotors,
resulting in faster response and shorter accelerating times. If
a CMR motor is not restricted in getting up to full speed this,
with the shorter accelerating time, should help in keeping the
temperature down.
The heat capacity for a prolonged start is less for 'E' frame
motors than for 'B' frame, since the 'E' Frame motors (as 'D'
frame but FLP) have lower inertia, higher permitted temperature
rises and smaller dimensions than motors to 'B' frame size.
Therefore, if the 'E' frame equivalent was required to start up
against a high inertia load, it is possible that it would overheat;
sometimes this has necessitated a change to a larger frame motor.
It will thus be seen that drives must be kept free, connecting
couplings to the motor must be in order and that voltage at the
motor terminals is again critical. An overload capacity and high
pull out torque is necessary in maintaining some drives through
system disturbances.
Motors for high voltage
The lower rating limit for high voltage motors presents design
factors which limit the move from medium voltage of 550 volts.
to a wider adoption of high voltage, namely 3000 volts and
above. Motors designed for 550 volts can generally be utilised
with 1100 volt windings.
There are two important factors which make it impossible to
obtain anything like the normal low-voltage output from a given
frame-size at higher voltages. These are the much thicker slot
insulation necessary and the flux loading reduction necessary
(on account of the almost obligatory use of open slots).
On closer examination it will be found that these factors not
only result in a progressive worsening of the high voltage/low
voltage output ratio as the required output is reduced; they also
dictate-for a given voltage, speed, enclosure, and temperature
rise-an effective minimum output below which no satisfactory
design is possible or economic. In essence, for similar HP and
speeds the motor frame is larger for HT than (MTJ Medium
Tension machines, although there can be exceptions where the
latter is in the lower HP range for a given size frame, or where the
former is in the higher end of the HP range.
There are also mechanical limitations on conductor proportions
which require a minimum output below which no economic
and reliable design is possible for a given voltage, speed, temperature
rise etc. In addition there arises, especially at high
voltage, possible corona effects in the slots. There is also the
factor of the type of slot utilised.
Semi closed slot
This can be slightly offset, i.e., overhanging at the top, and the
type of winding utilised is a multi-turn coil which is placed turn
by turn into an insulated slot adding where necessary inter layer
and inter coil and slot top insulation. The overhang of the slot
in addition is relatively weak.
Open slot
With this slot the effective air gap is considerably reduced in
comparison with the semi-closed. An open slot is generally preferred
for reliability as the coils are completely formed and
insulated before they are placed into the insulated slot. The
magnetising current is a major factor in the design of an induction
motor and in the interests of a reasonably high running
power factor this enforced use of an open slot imposes a severe
limit on the permissible mean flux density in the air gap relative
to that employed with semi-closed slots in a MT winding. In
addition, the increased insulation thickness required and the
need for a separator between the two coil sides increases the
depth of the slot required to such an extent that even at 3·3 KV
it is usually necessary to use a smaller stator bore diameter than
would be necessary for a low voltage machine with the same
outside diameter. Alternatively. dependent upon the pitch requirement
of the slots, the bore may be maintained and the
outside diameter slightly increased.
The reductions in stator bore diameter and in permissible
mean flux density lead inevitably to a reduced output from a
given frame. This does not as yet take into consideration the other
details of the slots and heat dissipation and space for the outhang
from the slots of the stator winding.
As the flux requirements have had to be kept Iowa large number
of slots or a large number of conductors per slot. e.g. 30 or
40, has been required.
Closed slot
This arrangement necessitates the use of mandrills, the
number equal to that of the conductors for the slot, to ensure
easy threading. It also requires that the conductors are equally
and correctly spaced and layered. Special consideration is given
to corona effects by the binding of the coils with tinsel strips.
This arrangement of winding is slow and costly.
In general the slot width/slot pitch arc is a predominant factor
as the maximum conductor depth (depth of slot to be filled with
conductors) also gives a conductor cross section and if this is
too small then the frame size considered is generally too small.
If however the conductor cross section is unnecessarily large
thermally but is just acceptable mechanically this must be used.
The design of the motor would then be cooler than it need be
but would be inherently capable of a higher output. It is however
dependent upon maximum torque or starting performance.
The type of winding (and hence the support of the coil ends
(outhang)) and the method of laying conductors in the slot are
the important factors necessitating a large-longer frame size.
greater envelope volume and weight. This is necessary because
1 the HV stator coils are of more regular section
throughout their length generally through the use
of copper strip rather than a more random disposition
of wire conductors in MT windings.
2 the length of mean t turn of the stator coil is
10/20% greater, e.g., for a 200 HP AFC motor this
can be 16% greater at 3'3 KV than at 550 volts
3 the ends of the winding coils are more regularly
shaped
4 there is larger insulation clearance, coil to frame and
coil to coil
5 there is more coil insulation, therefore more space to
dissipate heat arising from copper losses.
A typical example of the derating of a frame size is that of a
motor on a coal producing machine, say a shearer, which
cannot be increased in any dimension except that of length say
the Anderson Boyes 16 inch machines at 80,100 and 125 HP
and the 200 HP which is approximately 7 inches longer. This
frame size could be capable of 270 HP but if utlised on 3·3 KV
the HP would be reduced by 10/15 % because of the above design
factors (namely capable of 243/230 HP. Figs. 7 and 8 overleaf
compare motor dimensions at different voltages.
Water cooled motors
Water cooled motors can be made smaller in volume than
air cooled. However, with water cooled rotors special glandS
are necessary.
If the smallest design is r~Quired there are aspects of electrical
13
efficiency to be considered. As such a motor would be working
at conditions of maximum permissible flux and current densities
the losses for a given HP would be higher and the power factor
lower than for a standard air cooled motor. This penalty is
slightly offset by a reduction in windage losses, as there is no
fan. If a less extreme reduction in size is envisaged and mainly
less temperature rise or increased HP in the frame size, the design
can be based on normal flux and current density levels.
GeneraUy the improvement in dimensions of an FLP motor
with TEFC versus water cooled stator is 50 % in favour of the
latter. Ramsden2 (page 34) gives the amount of water required
for a difference of inlet and outlet cooling water of 10°C as
(100 % - % Effy) . .
HP x x 0·38 ~ gallons per mm whIch for a
100
100 HP at 1500 RPM (Syn) could be 2/2t galls. per min. If no
water flow, but still in the jacket, the output would have to be
restricted to 15 % of its continuous load, i.e. 15 HP or 62 HP for
I Hr or 100 HP for t hour. The main disadvantage of this type
of motor is the difficulty of arranging an efficient and adequate
supply of clean cooling water with the inherent flow and return
losses and temperature protection devices.
Uquld lIlled motors
Such motors, similar to submersible pump units, may be a
design possibility for the future. What however are the important
factors?
1 insulation to be proof against liquid filling
2 the increase in losses i.e. hydraulic in lieu of windage,
with the increase in drag on the rotor caused by
rotation induced turbulence. The rotor ends must
also be smooth and the stator end windings encapsulated
to a smooth evenly curved surface and
the stator slots made smooth with the laminations.
3 Internal components must becapableofwithstanding
corrosion
4 a high ratio of core length to stator bore diameter, to
keep down -the peripheral speed of the rotor
5 a larger air gap, because of longer core length, with
the resultant reduction in power factor and efficiency
6 a longer motor is generally a cooler running motor
7 heat exchangers may have to be added, as it would be
necessary to have a closed circuit for the cooling
medium, unlike water filled submersible pump
motors which are immersed in the pumped water.
S The need for effective cooling via heat exchangers
to cope with the temperature rise that comes from
frequent starting
9 a high degree of reliability for continued use for long
periods (the construction will not conveniently
allow shut down at frequent intervals)
10 dependent upon the type of cooling fluid utilised, it
may also lubricate the bearings; it may be necessary
to seal only at the outer bearing caps and other
joints; internal pressure would prevent the ingress
of extraneous material via bearings; but an effective
topping up system would be necessary.
COST AND ECONOMICS
To quantify the overall economics of a piece of equipment
it is necessary to consider many factors such as:first
cost ....
efficiency during normal working
maintenance costs
14
FIG 7
.. '
• - • 10 ,.
I
.~
·~f ...n. In:
i "\- I ,-ru
I --$--- 2';~:
I ;;1:). Jl
FIG 8
"A\C[ PfEIL.u • co. U'
',O"p UOO'i sOU ..... 0"0 '4'0_~" 1'-~)/4.
+--+- .--~- -_.
.>O .. ~ "O/,"OOY ., ....... 0 14'0 ~""
_ e.... 100 1300 '4&0 II /. 11'/4
,Go, ... 00 0100 ... 0 H II
J' U
.. '12 H
_ , ...... no 1100 , .. 0 .,'/. 21'/ •• 0 ,"'1>
_ ........ 110
.... .... taO .. 00 ,.10 •• 11
_ ., ,./. __ n/ •• ,.AI' I~ JIll __ nl ... uc .......
cost of lost production arising from breakdowns ....
true depreciation costs ... .
cost of stand-by equipment ... .
For the distribution system any changes would also have to
be included; if 3300 volts were used at the face the main cable
cross section could be reduced by half with a 40 % reduction
in cost. The costs outlined exclude those for the distribution
transformers. although the author appreciates that there is an
increase of £75 for dual voltage secondary and £200 for dual
voltage 3 '3/6·6 KV primary. If changes are anticipated a dual
voltage transformer is economical as an initial purchase.
Instances still arise where power is required in the tail gate
and therefore a comparison of costs for switchgear is based on
this requirement (no allowance is made for spares)
Main Gate (a) (b) (c)
SSOv ll00v 3300V
1 section switch 320 430 600
5 GE Boxes 1700 2250 1 double drill panel 280 388 } 71501
1 signal transformer 31 104
3 300 amp cable adaptors 105 105 105
£2436 £3277 £7855
Tail Gale
1 section switch 320 430 600
3 GE boxes 1020 1350 1 double drill panel 280 388 } 4400)
1 signal transformer 31 104
3 300 amp cable adaptors 105 105 105
1756 2377 5105
2436 3277 7855
Grand Totals £4192 £5654 £12960
Differences 550 to 1100 volts ~ £1462
550 to 3300 volts ~ £8768
1130 to 3300 volts = £7306
~ Technical estimated costs of composite units as typical
specification already outlined, e.g.
Main Gate
I-A 3 '3, I-B 3·3 and 2-C 3·3
Tail Gate
2-A 3·3 and l-C 3'3
(Personal Technical estimates A 3·3 unit £1300
B 3'3 unit £2250
C 3·3 unit £1800)
A comparative cost of motors is Dual Voltage 550/1100 volts
plus £20
100 HP 1470 RPM Similar Frame Size
550 volts £867 with plug socket
3300 volts £1120 with cable coupler
120 HP 1480 RPM AFC motor
550 volts £650 with plug socket
3300 volts £1075 with cable coupler
It must however be noted that as equipment becomes more
widespread in usc the costs are less.
COAL FACE VOLTAGE-THE CHOICE
Before any factual reason can be givcn for selection of one
system against another the characteristics of results must be
ascertained.
The Table overleaf gives tl:c comparison of results for the
system outlined in tl~.c adjacent Fig. 9, for open circuit volts at
565, I I 30 and 3300 for various conditions. In line with normal
practice the size of the secondary distribution cable is taken as
0·1 sq ins on 3·3 KV as against 0·2 sq ir.s on 565 volts and the
shearer cable 0·04 sq ins as against 0·1 sq in and these values
are used in the calculation.
WI~en it was expEcted that 1130 volts would become the use
level voltage, dual voltage transFormers were obtained because
of the small percentage if-crease in cost. However, as the percentage
impedance value is regulated by the lower voltage
windings, the results are not as attractive as a straight 1130 volts
winding. In the Table, taking Case 3(iii) as a comparison and
substituting the impedance per phase for 565 volts and converting
this to 1130volts the results are:-
Total Secondary Amps
Volts drop due to:supply
HT cable
transformer
secondary Cable
voltage at GE box
shearer motor amps
V D due to shearer cable
volts at shearer motor VI
383
32'6
67·3
105·8
89·1
835·2
321·0
26·0
805·0
voltage available as %age of motor rating 76·8 %
Yoi1Ce starting torque available 200 x (if)2 117·0 %
As a comparison of Case l(iii), Case 3(iii), Case 5 and the
above detailed Case 7 it will be noted that there is a decided
advantage for 1130 volts either with straight 1130 volts or dual
565/1130 volts secondary transformer and little to choose between
1130 volts and 3300 volts-although with a larger cable,
possibly 0 ·15 sq in, at 3300 volts, further improvements would
result.
With the 1130 volts system it is possible to utilise a 500 KVA
transformer and for the section switches still to be capable of
clearing any short circuit fault current.
FIG 9 Power System for detailed calculation for
comparison of 565 V, 1130 V and 3300 Volts
Outbye transformershort
circuit level 25 mVA 0.3 P.F.
+----- 3.3 kV cable 4000 yds 0.1 sq. in.
Inbye dry type transformer 300 kVA 3300/
565[1130V and Outbye Transformer 750
KVA 3.3/3.3 K. V.
+----~ L.V. fixed cable (a) 100 yds 0.2 sq. in.
For 565 & 1130 (b) 500 yds 0.2 sq. in.
Volts (c) 1000 yds 0.2 sq. in.
3300 Volts (d) 1000 yds 0.1 sq. in.
four 50 hp motors each fed 1<-+-+0-1<--- through 50 yds. 0.03 sq. in.
trailer.
125 hp cutter motor fed
+-___________ through 300 yds 0.1 sq. in.
trailer. and 0.04 sq. in. for
3.3 K.V.
CONCLUSION
A change to 1100 volts is relatively easy, as equipment is
available, and the design aspects for the motors are similar to
550 volts. If it is contemplated that 3300 volts will be utilised it
will be essential to formulate some development work. The design,
development and delivery of 3·3 KV switchgear of a more
15
COMPARISOptOF SYSTEM CONDITION WITH NOMINAL VOLTAGES AT 565 V,1130V, & 3300V
565 Volts Case 1 565 Volts Case 2 1 130 Volts Case 3 1130 Volts Case 4 3300 Volts
-_('_'_I~I~ --(-,,-]Oi>I(iii) -1-1---1----'---
(i) (il) (iii) (I) ] (II) (iii) Case 5 CaSeS
----------------------------------
125 HP Starting &. 4 x 50HP All Motors Running 125 HP Starting &. 4 x 50 HP All Motors Running 125 HP All
Running Running Starting Motors
&4 Running
Running
---,---
With Secondary Cable of With Secondary Cable of With Secondary Cable of With Secondary Cable of With Secondary
Cable of
----------------------------
100 yds 500 yds 1 000 yds 100 yds 500 yds 1 000 yds 100 yds SOO yds 1 000 yds 100 yds 500 yds 1 000 yds 1000 yds 1 000 yds ----------------------------------------------------
a + jb e t· jb a + jb a + jb a t- jb a + jb a + jb a + jb a + jb a + jb a + jb a + jb a .j jb a + jb ---------------------------------------------
(e' Impedance 0.00385+ 0.00385+ 0.00385+ 0.00385+ 0.00385+ 0.00385+ 0.01543+ 0.01543+ 0.01543+ 0.01543+ 0.01543+ 0.01543+ 0.1317 + 0.1317 +
Incoming 0.0122 0.0122 0.0122 0.0122 0.0122 0.0122 0.04888 0.04888 0.04888 0.04888 0.04888 0.04888 0.41705 0.41705
(b, Impedance HT 0.03504+ 0,03504+ 0,03504+ 0.03504+ 0.0350t+ 0,03504+ 0,14064+ 0.14064+ 0.14064+ 0.14064+ 0,14064+ 0.14064+ 1.2+ 1.2+
Cable 0.00853 0.00853 0.00853 0.00853 0.00853 0.00853 0.03422 0.03422 0.03422 0.03422 0.03422 0.03422 0.292 0.292
(e, Impedance 0.00707 + 0.00707+ 0,00707+ 0.00707+ 0.00707+ 0.00707+ 0.0101 + 0.0101 + 0.0101 + 0.0101 + 0.0101+ 0.0101 + 0.1532+ 0.1532+
Transformer 0,0417 0.0417 0.0417 0.0417 0.0417 0.0417 0.059 0.059 0,059 0.059 0.059 0.059 0.41 0.41
(d, Imp. Secondary 0.Q16 t- 0.080+ 0.16+ 0.016+ 0.080+ 0.16 + 0.016+ 0.080+ 0.16+ 0.016+ O.08O·t 0.16+ 0.3+ 0.3+
Cable 0.0068 0.0340 0.068 0.0068 0.034 0.068 0.0068 0.034 0.068 0.0068 0.034 0.068 0.073 0.073
(e' Impedance 0.1973+ 0.1973+ 0.1973+ 0.8583+ 0.8583+ 0.8583+ 0.590+ 0.596+ 0.596+ 3.378+ 3.373+ 3.378+ 4.665+ 28.756+
Motors &. Cables 0.2739 0.2739 0.2739 0.4590 0,4590 0.4590 1.102 1.102 1.102 1.827 1.827 1.827 9.31 15.586
-------------------------------------------
Total Secondary
amps 158 663 511 301 288 268 443 426 401 160 158 154 143 55
Line Volt drop due
to supply 16 13 11 5 5 4 38 3. 34 10 10 10 60 24
HT cable 35 34 30 18 11 16 14 14 14 39 38 31 240 100
Transformer 50 42 34 14 13 12 45 43 39 10 10 10 108 42
Secondary cable 21 85 148 9 43 81 10 48 93 5 24 46 60 25
Voltage at GE boxes 444 391 342 519 481 452 9 .. 929 890 1066 1048 1027 2830 3110
Shearer motor amps 612 "9 411 111 104 91 310 351 342 59 58 58 121 19.85
Voltage drop due to
shearer cable 50 44 39 18 16 15 30 29 28 9 9 9 40 1
Volts at shearer
motor 384 339 296 500 468 431 930 891 859 1053 1045 1009 2790 3103
Volts available as
%age of motor
ratln;;! 73.1% 64.6% 56.4% 95.2% 97% 83.2% 88,6% 85.4% 81.8% 101.2% 99.5% 96.1% 85% 94%
%age starting torque
available =
(VI)2 107% 83.4% 63.6% - - - 156.9% 146% 133.9% - - - 149% - 200 x
(V,
Voltage Drops are referred to the Secondary Side of the Transformer i.e. 565v, 1130v or 3.3 KV
Impedance in OHMS Per Phase
Vl = Actual Volts on Motor Terminals
V = Mean of Rated Voltage Range of Motor
compact, flexible and comprehensive arrangement could take
five years.
If a quicker change to 1130 volts had been undertaken the
economics of changeover would have been easier, as during
the past 3 years a large number of GE Boxes have beenpurchdsed
or modified when in fact 550 volt equipment released by obtaining
1130 volt equipment would have provided the necessary
requirements.
The next step is to 1130 volts applied to both advancing and
retreating systems of mining. Also the transformer should be
situated not more than 500 yds from the face; it is not comparison
with a 565 volts transformer situated 100/200 yds from the face,
but an improvement in the operational characteristics that
should be considered. It has been stated that because 3300 volts
equipment is bulky GE Boxes should be situated 1000 yds from
the face but the query of local isolation arises-a.nd the maintenance
electrician travelling this distance in say 4 ft. 6 in high
"in seam" mining roadway. Further, there is the question of
type and method of connection of trailing cables to the G E Boxes.
The high torque motors should continue to be utili:.ed. As this
motor has a high starting torque voltage effect is not so marked
as in the standard motor, where the starting torque could be less
than full load torque.
Close co-ordination is required in the design of equipmentbetween
users, manufacturers and the Ministry of Power.
16
Irrespective of the level of use voltage at the face the capability
of the high voltage main systems to transmit the loads efficiently
must be considered. This is to further enhance economics,
reliability and a greater degree of assurance of the rated voltage
at the motor terminals.
The level of prosperity of the mining industry stiH depends to a
large extent on the vigour with which new ideas are met.
References
1 "Vacuum Contactors" article by AEI Ltd
February 1968 "Works Engineering and Faciory Services"
2 "The Design and Application of Water-Cooled Motors" by D
Ramsden "LSE Engineering Bulletin" Vol 6 No 3,4 March 1962 Pages
24/35
The author wishes to acknowledge assistance received from
Baldwin & Francis Lid., Belmos Peebles Ltd, Bruce Peebles
Lid, GEC/AEI Lid, EMP Electric Lid and English Electric Co Ltd,
in the form of information from which diagrams of plant details
and the first table in this article have been produced.
This paper was commissioned by the technical committee but
the views expressed are entirely those of the author and do not
necessarily represent those of the committee.
I see some characters haven't come out as expected, I'll edit the paper later. I've done a lot of editing, I'll follow up later.
COAL FACE VOLTAGE
-the choice
By J F Wilkie C. Eng. FlEE. MIMinE. Member.
The purpose of this paper is to assess the various factors contributory
to determination of the voltage that should be used at the
coal face. It is attempted to portray fully the technical comparisons
between 565, 1100 and 3300 volts and thereby facilitate decision making.
Some of the technical aspects of system voltage and design
and development are discussed to enable a better appreciation
of the reasons for changing systems or equipment designs. Additionally,
an appreciation of the various technical aspects of
equipment design is: considered necessary.
A.C. equipment i~ most vulnerable to voltage fluctuations.
Desirable limits of voltage drop at starting are 9 % for squirrel
cage motors and 14% for slip ring motors. For a number of years
voltages in use underground have been 2200/6600 volts for H.T.
distribution and 440/550 volts on medium tension. Voltage
drop problems have always been apparent, even with 6600 volts
as the main supply distribution, because the transformers were
generally of 500 KVA capacity and situated outbye.
As the horsepower of the face equipment increased, steps had
to be taken to install a transformer in each roadway to the face,
the minimum distance of the transformer from the face being
limited until the advent of the F.L.P. transformer.
SYSTEM VOLTAGE CRITERIA
For several years consideration has been given to the application
of 6.6 KV for both use and distribution but progress in this
could not he made due to the incompatibility of physical mining
requirements and necessary technical design features. It may he
possible in future to continue this line of action but other distribution
systems, namely 3300 volts and 1130 volts, are being
actively considered.
'
In American mines machines operating on 4160 volts and
1000 volts are being introduced.
Use of two voltages together on or at the face, i.e., 1130 and
550, is fraught with disadvantages of increased space requirements
and complexity of layout (adversely affecting safety) in the
confined space at the roadway end near the face; and it still leaves
the AFC starting problem to be solved. The difficulty arises
where smaller motors cannot be wound suitably for 1100 volts.
For roadway heading machines it may be desirable to fit to the
machine a small capacity 1130/565 volt transformer.
The criterion is to provide the rated voltage at the motor
terminals and it is generally agreed that a voltage higher than
550 volts will give efficiency and economy under the most adverse
conditions. Likewise there will be a proportionate reduction
in current flow with the benefits that result from this reduced
current, namely, smaller conductor size in cables etc. and reduced
heating and maintenance of equipment resulting from less repeated
starting and from the rated voltage being applied to the
motor.
What must be borne in mind, however, is the maximum
permissible voltage drop at the motor terminals, if the design
torque characteristic of the motor is not to be jeopardised.
Therefore, before any decision is made to increase the distance
of the transformer from the face, or reduce the power cable
conductor size, it is essential to ensure that this limit is not over
reached.
Figures I and 2 outline the effects of reduced voltage on a
motor's Torque/Speed, Current/Time and Current/Speed characteristics.
These curves are typical for N.C.B. Spec. 50, 65 and 120
H.P. AFC motors. With a large percentage voltage drop,
particularly in conjunction with an incorrectly filled fluid
coupling, starting problems may arise.
Care must always be taken when designing an electrical system
that the torque and HP are available to overcome the increased
friction that occurs during the anticipated life of the mechanical
equipment, so as to achieve and maintain a low incidence of
stalling and consequent reduction in outage time.
SAFETY
Without any doubt safety is the most important factor to he
considered when assessing the use-level voltage; there must be
adequate protection provided for personnel and equipment.
However with the protective equipment already incorporated
in the 550 volt system and again proved in the 1100 volt systems
now in use, and similar equipment with expected similar results
FIG 1 FIG 2
... IOSC 'Of ................... K ,,~, ._ ............. ... ... • ..... '1'_ ......
.' .' .
: ..•
7
in 3300 volt units, it is apparent that coverage is being provided.
These are, in effect, fail safe protective circuits with attendant
complications of maintenance, short circuit protection (still with
a divergence of setting levels), earth fault current limiting and
lock out, earth fault proving before motor circuit energisation
(which, in addition to earth fault current limiting, reduces equipment
replacement cost and lost production time), limitation of
fault power and pilot core protection.
The major consideration for high voltage systems, 3300 volts
and above, is the limitation of short circuit currents by screened
power cables and components and the limitation of earth fault
currents. Reactors have been utilised in the power circuits but
these limit the flexibility of the load via available through currents;
they are also a major contributory factor to poor voltage
regulation. Neutral Point resistors have been utilised to limit the
earth fault current to the value governed by that required by the
surface or pit bottom distribution, i.e. above 15 Amps. The
question of neutral earthing the main supply is further assessed
under SYSTEM DESIGN later in this paper.
For safety we must have quality components, for operational
reliability, effective interlocking, efficient earthing linked with
good installation practices, effective maintenance and proper
use of plant and equipment.
RELIABILITY AND MAINTENANCE
Reliability infers
1 improved machinery and performance, related to
improved and maintained productivity
2 longer operational life of equipment- preferably
spanning the life of the face or even extending into
use on the next face
3 less maintenance-particularly important where
round the clock working is essential.
In essence, a reliable component or system is or.e which
continues to operate until a scheduled or charge point is reached.
It is not only impractical, but impossible, to design equipment
so that every part will wear out at the same time; but our least life-
expectancy component must have sufficient reliability and
life duration to be equal to a scheduled replacement cycle. It
would be reasonable to expect this minimum to be twelve months.
For GE Boxes the minimum should be 1,000,000 operations (or
some two years), not only for the contactor but also for auxiliary
switches and mechanism, together with all nuts, bolts and
linkages making up the whole. A static control circuit should
provide some means of eliminating mechanically actuated
equipment.
To achieve reliability it is often necessary to increase the
initial cost of the equipment. This may be more economical in
the long run as down time of machinery is very costly. Instances
of such initial cost increases are the steps now being taken to
have motors condensation and water proofed-and the wider
adoption of metal-cased heaters installed in some switchgear
as a means of combating condensation. Designed reliability must
be emphasised - scientific research & development and modification
in design can still be attained at a lower cost level. We
have already achieved improvements in design from two-way
communication between the user and the manufacturer but
there is still much room for progress here.
As machines take over more of the production load, with the
inevitable production improvement expectations, and as more
capacity is concentrated in individual machines, the cost, capacity
and complexity of modern equipment place increased burdens
on maintenance features. Therefore ease of maintenance is an
8
essential aspect of ensured reliability and reduced down time.
Effective maintenance will always be necessary, even with more
reliable equipment, since it has an influence on production costs.
an effect on output (operating time versus downtime) and it
serves to protect capital investment.
Therefore, when choosing equipment, merit should be awarded
to those types which are easily cleaned, inspected, maintained and
overhauled, and which also offer flexibility in load settings. A
typical example is the integral testing of control circuits on coal
production machines, especially those on GE Boxes. The test
facilities must indicate the condition not only of the total circuit
but also of as many sub circuits as possible, e.g., the contactor,
the control relay circuit, the proving relay circuit and the sequence
relay circuits. As electronic equipment control circuits may be
complex, it will be necessary to provide an easily visible indicator
which provides the values which can be related to circuit parameters.
STANDARDISATION AND FLEXIBILITY
The term standardisation is synonymous with variety reduction;
but standardisation does not mean continuous repurchase
of a component of a fixed type; it is also not a cover for
stultification in design. It must be regarded as a definite stepping
stone toward further improvement of existing "standards". The
word standard is the general term; "specification" is a particular
kind of standard which sets down precise requirements in an
objective manner.
Standards should be defined as clearly as possible in qualitative
rather than quantitive terms, i.e., all parts made must measure
up to qualitative finished products. No specification should lend
itself to ambiguous interpretation.
Adequately rated components, with the amount and nature
of the loads and circuit arrangement clearly determined, are
essential to flexibility. With this in mind flexibility for electrical
distribution and for control gear can be achieved by designing
the main items of equipment, such as circuit breakers, contactor
starters, etc., to a rated value, with interchangeable components
to suit the load of the circuit. This is already largely achieved.
Such a provision ensures economical power distribution centres
with flexibility for load growth and change within minimum
time and cost limits.
The units in the system must however be adaptable, without
bulky projections (unless these can be disconnected easily).
With "In Seam" mining- with rectangular roadways, maximum
seam height of 5 ft and maximum effective clearance of 4 ft 6
inches- problems arise, especially if the roadways are not
levelled out in steep cross gradient seams.
The motors, switchgear and transformers must therefore be of
compact design; the external surface must be to a minimum
prescribed area, to avoid the settlement of too much coal dust,
so that the maximum temperature, which can be progressive
with dust layers, does not affect the performance of the equipment
or give rise, at 200 °C, to self ignition of the dust.
Wherever possible the motors should be wound for dual
voltage, with linking arrangement in the terminal box for use
with the voltages of different systems.
SYSTEM DESIGN
System design is a vital part of mining and in many cases
determines the success or failure of an operation. When designing
an electric power distribution system it is essential to provide,
I maximum safety for personnel and equipment
2 stable continuous power supplies
3 low equipment investment cost-at economical
rates
4 minimum maintenance
5 minimum power transmission cost
6 ease of relocation of equipment and the flexibility
to meet varying loads
Stability of voltage is becoming an increasingly important
factor, since there is no evidence that horsepower of face equip
ment will level off in the forseeable future unless a horse
power voltage
impasse is reached.
A plan of the present and anticipated layout is a necessity;
then an assessment of the amount and nature of the load. The
magnitudes and geographical locations of the loads to be encountered
may be well known from experience of other mining
operations but before fixing the working voltage and distribution
and cable rating it must be ascertained as nearly as possible the
future growth of the load, i.e., the expected total connected load,
diversity of use, load factor and method of distribution that will
provide the load at optimum flexibility. There is an economic
limit to the number of KVA miles which can be transmitted at a
given voltage. Very large copper cross sections may have to be
installed to cover anticipated increases in growth of the load and
to provide a desired flexibility.
A generalisation is usually made to standardise given sizes of
cables of the paralleling of two or more cables to a KVA
distribution centre-sub station underground-to provide, in
addition to standby supplies, the KVA transmission capacity at
the required voltage. Where possible, large power rated equipment
should be utilised at the high voltage of the distribution
system.
Paralleled (ring main) supplies are often warranted for load
and/or standby capacity. A typical value above which warranty
would apply is 200 amps per mile of HV power distribution.
However it must be noted that ring main supplies give rise to
their own particular problems. From the load centres to the
seams. and thence to the faces. distance and load are the criteria
of cable cross sectional area (size).
It is clear therefore that considerable judgment must be
exercised. There are variables in mining which cannot always be
forseen; Therefore. in distribution systems, especially underground.
it is not possible to adhere strictly to formulae. Empirical
formulae can however be of valuable assistance.
In addition to load characteristics there are two essential
requirements; that the cables and switchgear will withstand the
short circuit current that can flow in the system; and that the
switchgear is capable of clearing the fault conditions that can
arise. Where contactors are used in the system for motor control
we must
1 limit the current to a value that the contactor is
capable of breaking
or
2 fit back-up protection HBC (high breaking capacity)
fuses to the equipment; When deciding on the rating
of the fuse to be fitted we must however ensure
that sufficient current will flow in the circuit during
a fault to rupture these fuses in the very short time
before the contactor is opening.
Consider a 500 or 750 KV A transformer feeding a complete
face and outbye supplies. With a %age Impedance of 4· 75 %
and assuming unlimited Incoming MV A capacity the secondary
short circuit current is as follows:-
Transformer KVA x lop
1000 x %age Impedance
MVA(S.C)
and amps (S.C) ~
MVA (S.C) x 1000
V (KV) x y3
For 500 K VA Transformer 3·3 K V Secondary
500 x I 00
1000 x 4·75
500 x 0·0210
10·5 MVA
and 1837 amps 3 phase for 3300 Volts system
750 KVA Transformer 3·3 KV Secondary
~ 750 x 0.0210 ~ 15·75 MVA
and" 2755 amps 3 phase RMS.
If the fault level at the transformer was not infinite, but
limited to 25 MVA:~
500 KVA Transformer
750 KV A Transformer
5·15 MVA and SC amps~ 901
6·15 MVA and SC amps~1075
Considering the use of 3300 Volts as the "use level voltage",
and before further developing the aspect of HBC fuses, there is
one other feature that must be included in the system. This is
the reduction of earth fault currents.
This can fall into one or two categories, either a limitation of
15 Amps to earth, or sensitive earth fault protection.
If limitation to 15 Amps on a new system is required this is
achieved as follows:-
1 fit neutral point current limiting resistances to the
supply transformers. fit all the main EHV distribution
switchgear with earth protection at 5/7.5
amps.
2 install transformers specially to feed underground
and fit neutral point current limiting resistances to
them and fit all HV switchgear feeding underground
with 5/7.5 amp earth fault protection.
However. if say 3·3 KV was considered for underground use
it would also apply at existing collieries where it would be impossible
or impractical to fit 5/7·5 amp earth fault protection
to the existing 3·3 KV switchgear as generally. since 1939, this
was fitted with protection at 5 amps or 15 %. (whichever was the
greater) of the normal current of the circuit. The main surface
switchgear has time delayed protection, with instantaneous
earth fault protection for underground switchgear.
For established systems, whether 15 Amps restricted earth
fault current or sensitive earth fault (restricted and monitored).
it will be necessary to fit a 3 ·3/3.3 KV transformer underground.
Setting relays at 5 amps or even 15 amps is no guarantee of protection.
With the 1 to 1 ratio double wound transformer the
neutral point is made available underground for necessary
protection and discrimination and connection to the associated
primary circuit breaker for clearance of the system.
Therefore, we have incorporated additionally an inherent
impedance in the system which must be taken into consideration
for voltage regulation in the system and for restriction of short
circuit current.
What must be taken into consideration where HBC fuses are
fitted is this variable, affected by impedance of different lengths
of cable. of short circuit fault capacity to rupture these backup
fuses.
If contactors are of a high current capacity this does not raise
so much of a problem as they can cope generally with a high
fault capacity. The HBC fuse cut-off limit characteristic cannot
be reduced to too low a value as it must be capable also of
carrying a required through current. Several successive starts
must be allowed so that the HBC"back-up fuse does not inadvertently
fail due to the high temperature that would prevail.
As an example, take a 120 HP motor with full load of 20 amps
and stalled current of 120 Amps and 10 seconds run up time per
start (assuming that the current does not fall off, due to stalled
conditions, as the motor tends to run up to speed). Six successive
starts equals 60 seconds and, therefore, the fuse must cover
this current and time.
Suitable fuses (see figs. 3 & 4) are:-
(a) E.M.P. Electric Ltd's Quick Acting 40 amp
(b) English Electric Co Ltd's Type K2 PA 50 amp
The first fuse will cover 120 amps for over 120 seconds, as the
characteristic curve tends to flatten out. In fact it would carry
140 amps for 60 sees., but with the BS.2692 allowable ± 20%
tolerance this could be reduced to 112 amps. It would also
rupture at 750 amps in 0 ·01 sec., or at 500 amps in 0 ·02 sec.
11resecondfuse will fail at 120 amps for 4 mins
140 amps for 2 mins
150 amps for I min
or at-20% rating 120 amps for 1 min
It would also rupture at 1000 amps in 0·01 sec, or at 800 amps
in 0·02 sec.
It must be noted that a fuse may carry up to 1·4 or more
times its rated current without melting, a value which may be
sustained in the protected circuit; Le., Fusing Factor
Minimum Fusing Current
may vary between 1·4 and 2,
Rated Current
dependent upon its design, and therefore up to 3 times the current
may have to flow for fusing to occur.
3300 volt Contactors. The ISO amp size will break 1200 amps or
1600 amps five to ten times as an emergency rating. The 75 amps
size will break 600 amps or 800 amps as an emergency rating.
Therefore, when related to fuse cut-off current, the fuse will
operate first-as in addition to the current there is the inherent
time lag of the mechanical operation of the contactor, which is
normally greater than 0·05 secs. This inherent time lag with
current transformer operated overloads results from the cr
being saturated well below these short circuit currents, with the
oil dashpot consequently slugged in its action. If the contactor
was opening on an earth fault, even via an instantaneous relay,
there is the inherent time lag of 0·02/0·03 secs. This then feeds
the trip relay with an action time lag of 0·02/0·03 sees and the
DC contactor mechanical operation with a further 0 ·02/0 ·03
sees. The quickest time on earth fault is thus 0·06 sees.
If the contactor was opening on earth fault and a phase to
phase fault developed this would present an extreme circumstance
-but the 75 or ISO amp contactors should still clear.
The AEI Vacuum Contactor 300 amps rating is capable of
3000 amps guaranteed
4000 amps 99 % of breaks
4500 amps 50 % of breaks
5000 amps nil
The contacts start to blow off at 8 to 10 KA and will withstand
8000 amps for 10 sees at a non dangerous temperature
rise. It must be assessed whether, with this type of contactor, it is
necessary to have:-
10
1 a 150 and/or 75 amp rating, as a further reduction
in contactor dimensions may result in an appreciable
difference in the size of the enclosure at 3·3 KV.
1 a more efficient means of detecting contact wear
3 a ready means of ascertaining that the vacuum is in
order, or that a contact is not welded.
4 a large gap between contacts; there are definite advantages
to a small gap, e.g., prevention of contact
FIG 3
FIG 4
..0 LOAf! .... TIN' !u .... "
lUaJ«T IQ .T._RO TOlu~ •• cn.
,
'------------~,------------~~~
bounce and operating coil rating; however, linkage
wear may reduce the gap too much within the 2 million
operations life span.
EQUIPMENT DESIGN
A prime problem facing designers, especially with face equipment,
is the virtual impossibility of deciding on the physical
conditions, overload and duty cycle under which equipment
may have to operate. The user must cooperate fully in advising
on these aspects to achieve a good design, acceptable for
standardisation of a nature desirable to collieries.
The primary task in design is to assess the field in which
development must be concentrated. The following comments
are grouped under the heads of 1100 VOLTS, FLP CONTACTOR
CONTROL GEAR, 3·3 KV, OVERLOAD TRIPS &
PROTECTION, MOTORS (OVERLOAD CAPACITY),
MOTORS FOR HIGH VOLTAGE, WATER COOLED
MOTORS AND LIQUID FILLED MOTORS.
1100 volts
Thrustor control and protection must be provided; the
necessity is obvious for practical reasons and also because
thrustors cannot be wound suitably for above 650 volts. Since
the present Gate End Box enclosure can accommodate two contactors
it is possible that one be fitted and the spare space made to
accommodate the thrustor supply step down transformer
(0·5/1'0 KVA) and protection.
The manufacturers GEC/AEI Ltd state that the reason for
limitation to 550 volts as maximum for thrustors arises from
clearances and the mechanical strength of the winding wire.
The following table outlines the transformer & thrustor rating
and bare copper conductor sizes prior to application of THERMEX
'M' covering.
Thrustor Details
RATING THRUSTOR
KVA Watts. Bare Conductor Size ins
Supply Thrustor Size
Transformer l00/110v 500/550 v
0·4 150/180 752/1002 0·0172 0·0136
0·5 170/200 1503/2503 0'018 0'0156
1-0/1'6 420/580 5005/6505 0'024 0'018
1-6 700 8000/12 0·025 0·022
FLP Contactor Control gear
It is generally accepted that higher voltage control gear
requires greater space, due to line to line and earth clearances.
Whatever the actual size, it is essential that gear of different
voltages cannot be butted together.
Space utilisation is critical in switch gear design and is usually
complicated by requirements such as a thrustor transformer and
protection or other control requirements.
AEI Ltd, in an article I referring to volume of vacuum contactors
at HV and MV, state that, because no arc chutes are
utilised, their space requirements are nearly 1/8 and 2/5 respectively
FIG 5
BELOW
FIG 6
~ 1.11"
~"
III
-d
r •. , I II .,1
L''''/J
I ,
•
•
those of air break equivalents-and reduce the clearance required
to adjacent apparatus.
Vacuum contactor sizes and weights compared with air-break
equivalents
j
I CONTACT OR
I
Height Width Depth Weight
(ins) (ins) (ins) (Ibs)
HV Vacuum III 16l 16 60
Air-Break Equivalent 31l 26 291 275
MV Vacuum 8t 91 9t 26
Air Break Equivalent lit 16 10 31l
Difficulties may arise if the 1100 volt contactor gear is re~
designed, due to the utilisation of a vacuum allowing the en~
closure to be smaller; the author considers however that any
space made available can be usefully employed. Baldwin &
Francis in their prototype design and using their block contactor
were able to utilise the 550 volt enclosure.
3·3KV
The main criterion is however with any standard design for
3·3KV.
Belmos Peebles, in their early pioneering of 3·3 KV, have
produced the widest range of equipment. They consider that
they could reduce the volume of their KFG unit, which covers a
single 3·3 KV contactor, thrustor supply and protection and
contactors for 3 other medium or low voltage low HP drives, and
still have room for SEL protection.
Baldwin & Francis consider that by utilising a vacuum con~
tactor they cannot reduce the volume of the enclosure.
Figs. 5 and 6 respectively outline the details of Belmos Peebles
and Baldwin & Francis 3·3 KV equipment enclosures. The large
depth, especially with the door opened, should be noted.
The NCB must now seriously consider the design of 3·3 KV
equipment, so that some standardisation of dimensions and
control duty requirement will be possible before too many
varieties are utilised. The normal:Belmos Peebles' range covers
. ., 8-'
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o Q & '" •
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ocw,; :~, ~ ,' . J .. . @ill
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,.,~ J!.'-, Z'· 414-"
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~~ ,,' ..... 90.00, ...... .... "' .. "'II"
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j ,.. j ~ j .~ - ::: '011","_"0""" j ~ " ... ~
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iJ .. t;l '[ _"l;
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high enclosures not suited to easy installation and relocation
underground; but the availability of control features meets a
wide need for one motor or up to four-5 HP 550 volt motors
via the designed rating of the 16·5 KVA 3 ·3/0'55 KV transformer
now listed cpable of 27 KV A.
The probable requirements on 3·3 KV are as follows:-
1 Unit Type A 3·3 comprising:-
a reversible "OFF-EARTH" off load Isolator
b 3·3 KV ISO amp motor drive (non reversible)
c 4, KVA of 3 phase 550/11Ov drives for up to 3
motors
d 15 volt IS signal transformer
e individual circuit overload protection
f earth fault protection for main motor drive
g group earth fault protection for auxiliary drives
h HBC fuses for main and auxiliary supplies
auxiliaries switches, remote, local, sequence,
contactor freeze protection, incoming and outgoing
cable arrangements for all circuits, ammeter etc.
and with addition as required of
j KW Hr meter
k 500 VA 240/120 volt lighting transformer and
protection
metal cased anti-condensation heaters, each 60 watt
2 Unit Type B. 3·3
As unit type A 3·3 with addition of second 3·3 KV
ISO amp contactor for two motor or reversible drive.
3 Unit Type C 3·3 comprising
a reversible "OFF-EARTH" off load Isolator
b 25 KV A 3·3 550 volt 3 phase transformer
c 700 watt 550/110 volt transformer for one or two
thrustors
d four 5 HP 550 volt auxiliary drives, one or two
reversible via rotary switches for loop take up
drives
e 4, KV A double drill transformer
f group earth fault protection for d
g individual and short circuit fault protection for e
h ammeters for each circuit
HBC fuses for main and auxiliary supplies
j 15 volt IS signal transformer
k auxiliaries switches, remote local, sequence, incoming
and outgoing cable arrangements for all
circuits
alternative to d for one 20 HP motor
4 General
a a combination of type A, B & C 3·3 to be suitable
for erection in a bank with coupled busbars and
through going 3·3 KV busbar arrangement
b 3·3 KV plug and socket; Belmos Peebles have a design
drawing of a 3·3 KV 50 amp restrained plug
socket. This plug socket should be the restrained
type with screened power cores and interlock and
pilot core protection and possibly compound
filling.
FinalIy, it wi1l be necessary to resolve an approved design of
flexible 3·3 KV trailing cables.
Overload trips and protection
Unfortunately there has been no overload device yet discovered
which is able to fully protect a motor, largely because of
12
the different thermal capacities of a starter and motor. If a
totally enclosed TEFC or TEFC, FLP motor is heated to its
maximum temperature and then stopped it will take much longer
to cool down than the thermal protection device in the starter.
If the overload device does not work on temperature, e.g.,
adjustable inverse time lag dashpots, then it is automaticalIy
non-responsive to the effects of accummulated peaks of load
and therefore, especially where the design of the motor enclosure
and windings has to be reduced to minimum volume, troubles
can ensue. The motor is not capable of providing several quick
successive starts as is often required and it is necessary to install
a thermal preset temperature trip device in the motor windings
to detect the hot spots.
With the change to motors rated to CMR (continuous maximum
rated) it follows that, if the motor is subject to heavy
starting loads, or has a prolonged start from co1d, which is
within the heating capacity of the motor, the starter would
trip; and if the starter is set to handle this current it will not
protect the motor from a sustained overload when running. The
change of emphasis from the temperature of the outside to the
inside (windings) of the motor is therefore a logical avenue for
further development. If this is done, then hazards such as overloading
on running, stalling, too frequent starting, single
phasing, choked ventilation, etc. should be avoided.
Motors-overload capacity
Motor design has improved over the years and the associated
reductions in dimensions have raised problems which are not
often fully appreciated.
BSS.l68 had a continuous rating permitted overload (CRPO),
or "load-plus-overload" rating; motors over say 10 HP, where
the cooling air does not exceed 35°C, must be capable, after
attaining the temperature rise corresponding to rated load,
voltage and frequency, of 25 % overload in torque for 2 hrs and
100% torque momentarily (i.e. for 15 seconds) without any
appreciable effect on the temperature rise. This 100% overload
is thus permissible and, as the heat produced varies as the square
of the current, successive overloads may produce a temperature
rise exceeding that permitted. If the motor is stalled and therefore
an absence of forced ventilation exists the windings would attain
a very high temperature. There can be a life expectancy of 10/15
years at llO"C and possibly only I year at 135"C for the winding
-not the carcase-of a motor to BSS.168. A relay to BSS.587
can have a tolerance of 10 % on the tripping current and therefore
overload capacity must be used with caution and with full
knowledge of the factors involved, i.e., the ratio of peak loading
time to total running time.
A motor designed to BSS.2613 is a CMR motor and the rerating
was brought about through the use of new synthetic
insulation which altered the concepts L f motor temperature
rise. These temperature rises for windings insulated with Class
A, E, B, F and Hare 55 0, 65°, 75 0, 95 ° and 115°C :espectively.
with a cooling ambient temperature not exceeding 40°C. For
class 'E' the permitted temperature rise is only 10°C, with a
further 25°C allowance for supposed hot spot temperature,
with the final temperature of 120 °C. The hottest spot is the
overhang of the windings, ie. outside the slots. With BSS.l68,
class A and B insulations were not to exceed a temperature rise
of 4O"C and 50"C respectively. CMR motors to BSS.2613 have
no sustained overload capacity, merely a momentary increase
in torque for 15 seconds of 100% up to 50 HP and of 75%
above 50 to 500 HP. Therefore, if the motor is subjected to
overloads in excess of these, it is at the expense of higher insulation
temperature and consequently shorter life.
A further specification change was from BSS.2083 for 'B'
frames to BSS.2096 for D and E frames, which reduced the
dimensions. The motors will thus have lower inertia rotors,
resulting in faster response and shorter accelerating times. If
a CMR motor is not restricted in getting up to full speed this,
with the shorter accelerating time, should help in keeping the
temperature down.
The heat capacity for a prolonged start is less for 'E' frame
motors than for 'B' frame, since the 'E' Frame motors (as 'D'
frame but FLP) have lower inertia, higher permitted temperature
rises and smaller dimensions than motors to 'B' frame size.
Therefore, if the 'E' frame equivalent was required to start up
against a high inertia load, it is possible that it would overheat;
sometimes this has necessitated a change to a larger frame motor.
It will thus be seen that drives must be kept free, connecting
couplings to the motor must be in order and that voltage at the
motor terminals is again critical. An overload capacity and high
pull out torque is necessary in maintaining some drives through
system disturbances.
Motors for high voltage
The lower rating limit for high voltage motors presents design
factors which limit the move from medium voltage of 550 volts.
to a wider adoption of high voltage, namely 3000 volts and
above. Motors designed for 550 volts can generally be utilised
with 1100 volt windings.
There are two important factors which make it impossible to
obtain anything like the normal low-voltage output from a given
frame-size at higher voltages. These are the much thicker slot
insulation necessary and the flux loading reduction necessary
(on account of the almost obligatory use of open slots).
On closer examination it will be found that these factors not
only result in a progressive worsening of the high voltage/low
voltage output ratio as the required output is reduced; they also
dictate-for a given voltage, speed, enclosure, and temperature
rise-an effective minimum output below which no satisfactory
design is possible or economic. In essence, for similar HP and
speeds the motor frame is larger for HT than (MTJ Medium
Tension machines, although there can be exceptions where the
latter is in the lower HP range for a given size frame, or where the
former is in the higher end of the HP range.
There are also mechanical limitations on conductor proportions
which require a minimum output below which no economic
and reliable design is possible for a given voltage, speed, temperature
rise etc. In addition there arises, especially at high
voltage, possible corona effects in the slots. There is also the
factor of the type of slot utilised.
Semi closed slot
This can be slightly offset, i.e., overhanging at the top, and the
type of winding utilised is a multi-turn coil which is placed turn
by turn into an insulated slot adding where necessary inter layer
and inter coil and slot top insulation. The overhang of the slot
in addition is relatively weak.
Open slot
With this slot the effective air gap is considerably reduced in
comparison with the semi-closed. An open slot is generally preferred
for reliability as the coils are completely formed and
insulated before they are placed into the insulated slot. The
magnetising current is a major factor in the design of an induction
motor and in the interests of a reasonably high running
power factor this enforced use of an open slot imposes a severe
limit on the permissible mean flux density in the air gap relative
to that employed with semi-closed slots in a MT winding. In
addition, the increased insulation thickness required and the
need for a separator between the two coil sides increases the
depth of the slot required to such an extent that even at 3·3 KV
it is usually necessary to use a smaller stator bore diameter than
would be necessary for a low voltage machine with the same
outside diameter. Alternatively. dependent upon the pitch requirement
of the slots, the bore may be maintained and the
outside diameter slightly increased.
The reductions in stator bore diameter and in permissible
mean flux density lead inevitably to a reduced output from a
given frame. This does not as yet take into consideration the other
details of the slots and heat dissipation and space for the outhang
from the slots of the stator winding.
As the flux requirements have had to be kept Iowa large number
of slots or a large number of conductors per slot. e.g. 30 or
40, has been required.
Closed slot
This arrangement necessitates the use of mandrills, the
number equal to that of the conductors for the slot, to ensure
easy threading. It also requires that the conductors are equally
and correctly spaced and layered. Special consideration is given
to corona effects by the binding of the coils with tinsel strips.
This arrangement of winding is slow and costly.
In general the slot width/slot pitch arc is a predominant factor
as the maximum conductor depth (depth of slot to be filled with
conductors) also gives a conductor cross section and if this is
too small then the frame size considered is generally too small.
If however the conductor cross section is unnecessarily large
thermally but is just acceptable mechanically this must be used.
The design of the motor would then be cooler than it need be
but would be inherently capable of a higher output. It is however
dependent upon maximum torque or starting performance.
The type of winding (and hence the support of the coil ends
(outhang)) and the method of laying conductors in the slot are
the important factors necessitating a large-longer frame size.
greater envelope volume and weight. This is necessary because
1 the HV stator coils are of more regular section
throughout their length generally through the use
of copper strip rather than a more random disposition
of wire conductors in MT windings.
2 the length of mean t turn of the stator coil is
10/20% greater, e.g., for a 200 HP AFC motor this
can be 16% greater at 3'3 KV than at 550 volts
3 the ends of the winding coils are more regularly
shaped
4 there is larger insulation clearance, coil to frame and
coil to coil
5 there is more coil insulation, therefore more space to
dissipate heat arising from copper losses.
A typical example of the derating of a frame size is that of a
motor on a coal producing machine, say a shearer, which
cannot be increased in any dimension except that of length say
the Anderson Boyes 16 inch machines at 80,100 and 125 HP
and the 200 HP which is approximately 7 inches longer. This
frame size could be capable of 270 HP but if utlised on 3·3 KV
the HP would be reduced by 10/15 % because of the above design
factors (namely capable of 243/230 HP. Figs. 7 and 8 overleaf
compare motor dimensions at different voltages.
Water cooled motors
Water cooled motors can be made smaller in volume than
air cooled. However, with water cooled rotors special glandS
are necessary.
If the smallest design is r~Quired there are aspects of electrical
13
efficiency to be considered. As such a motor would be working
at conditions of maximum permissible flux and current densities
the losses for a given HP would be higher and the power factor
lower than for a standard air cooled motor. This penalty is
slightly offset by a reduction in windage losses, as there is no
fan. If a less extreme reduction in size is envisaged and mainly
less temperature rise or increased HP in the frame size, the design
can be based on normal flux and current density levels.
GeneraUy the improvement in dimensions of an FLP motor
with TEFC versus water cooled stator is 50 % in favour of the
latter. Ramsden2 (page 34) gives the amount of water required
for a difference of inlet and outlet cooling water of 10°C as
(100 % - % Effy) . .
HP x x 0·38 ~ gallons per mm whIch for a
100
100 HP at 1500 RPM (Syn) could be 2/2t galls. per min. If no
water flow, but still in the jacket, the output would have to be
restricted to 15 % of its continuous load, i.e. 15 HP or 62 HP for
I Hr or 100 HP for t hour. The main disadvantage of this type
of motor is the difficulty of arranging an efficient and adequate
supply of clean cooling water with the inherent flow and return
losses and temperature protection devices.
Uquld lIlled motors
Such motors, similar to submersible pump units, may be a
design possibility for the future. What however are the important
factors?
1 insulation to be proof against liquid filling
2 the increase in losses i.e. hydraulic in lieu of windage,
with the increase in drag on the rotor caused by
rotation induced turbulence. The rotor ends must
also be smooth and the stator end windings encapsulated
to a smooth evenly curved surface and
the stator slots made smooth with the laminations.
3 Internal components must becapableofwithstanding
corrosion
4 a high ratio of core length to stator bore diameter, to
keep down -the peripheral speed of the rotor
5 a larger air gap, because of longer core length, with
the resultant reduction in power factor and efficiency
6 a longer motor is generally a cooler running motor
7 heat exchangers may have to be added, as it would be
necessary to have a closed circuit for the cooling
medium, unlike water filled submersible pump
motors which are immersed in the pumped water.
S The need for effective cooling via heat exchangers
to cope with the temperature rise that comes from
frequent starting
9 a high degree of reliability for continued use for long
periods (the construction will not conveniently
allow shut down at frequent intervals)
10 dependent upon the type of cooling fluid utilised, it
may also lubricate the bearings; it may be necessary
to seal only at the outer bearing caps and other
joints; internal pressure would prevent the ingress
of extraneous material via bearings; but an effective
topping up system would be necessary.
COST AND ECONOMICS
To quantify the overall economics of a piece of equipment
it is necessary to consider many factors such as:first
cost ....
efficiency during normal working
maintenance costs
14
FIG 7
.. '
• - • 10 ,.
I
.~
·~f ...n. In:
i "\- I ,-ru
I --$--- 2';~:
I ;;1:). Jl
FIG 8
"A\C[ PfEIL.u • co. U'
',O"p UOO'i sOU ..... 0"0 '4'0_~" 1'-~)/4.
+--+- .--~- -_.
.>O .. ~ "O/,"OOY ., ....... 0 14'0 ~""
_ e.... 100 1300 '4&0 II /. 11'/4
,Go, ... 00 0100 ... 0 H II
J' U
.. '12 H
_ , ...... no 1100 , .. 0 .,'/. 21'/ •• 0 ,"'1>
_ ........ 110
.... .... taO .. 00 ,.10 •• 11
_ ., ,./. __ n/ •• ,.AI' I~ JIll __ nl ... uc .......
cost of lost production arising from breakdowns ....
true depreciation costs ... .
cost of stand-by equipment ... .
For the distribution system any changes would also have to
be included; if 3300 volts were used at the face the main cable
cross section could be reduced by half with a 40 % reduction
in cost. The costs outlined exclude those for the distribution
transformers. although the author appreciates that there is an
increase of £75 for dual voltage secondary and £200 for dual
voltage 3 '3/6·6 KV primary. If changes are anticipated a dual
voltage transformer is economical as an initial purchase.
Instances still arise where power is required in the tail gate
and therefore a comparison of costs for switchgear is based on
this requirement (no allowance is made for spares)
Main Gate (a) (b) (c)
SSOv ll00v 3300V
1 section switch 320 430 600
5 GE Boxes 1700 2250 1 double drill panel 280 388 } 71501
1 signal transformer 31 104
3 300 amp cable adaptors 105 105 105
£2436 £3277 £7855
Tail Gale
1 section switch 320 430 600
3 GE boxes 1020 1350 1 double drill panel 280 388 } 4400)
1 signal transformer 31 104
3 300 amp cable adaptors 105 105 105
1756 2377 5105
2436 3277 7855
Grand Totals £4192 £5654 £12960
Differences 550 to 1100 volts ~ £1462
550 to 3300 volts ~ £8768
1130 to 3300 volts = £7306
~ Technical estimated costs of composite units as typical
specification already outlined, e.g.
Main Gate
I-A 3 '3, I-B 3·3 and 2-C 3·3
Tail Gate
2-A 3·3 and l-C 3'3
(Personal Technical estimates A 3·3 unit £1300
B 3'3 unit £2250
C 3·3 unit £1800)
A comparative cost of motors is Dual Voltage 550/1100 volts
plus £20
100 HP 1470 RPM Similar Frame Size
550 volts £867 with plug socket
3300 volts £1120 with cable coupler
120 HP 1480 RPM AFC motor
550 volts £650 with plug socket
3300 volts £1075 with cable coupler
It must however be noted that as equipment becomes more
widespread in usc the costs are less.
COAL FACE VOLTAGE-THE CHOICE
Before any factual reason can be givcn for selection of one
system against another the characteristics of results must be
ascertained.
The Table overleaf gives tl:c comparison of results for the
system outlined in tl~.c adjacent Fig. 9, for open circuit volts at
565, I I 30 and 3300 for various conditions. In line with normal
practice the size of the secondary distribution cable is taken as
0·1 sq ins on 3·3 KV as against 0·2 sq ir.s on 565 volts and the
shearer cable 0·04 sq ins as against 0·1 sq in and these values
are used in the calculation.
WI~en it was expEcted that 1130 volts would become the use
level voltage, dual voltage transFormers were obtained because
of the small percentage if-crease in cost. However, as the percentage
impedance value is regulated by the lower voltage
windings, the results are not as attractive as a straight 1130 volts
winding. In the Table, taking Case 3(iii) as a comparison and
substituting the impedance per phase for 565 volts and converting
this to 1130volts the results are:-
Total Secondary Amps
Volts drop due to:supply
HT cable
transformer
secondary Cable
voltage at GE box
shearer motor amps
V D due to shearer cable
volts at shearer motor VI
383
32'6
67·3
105·8
89·1
835·2
321·0
26·0
805·0
voltage available as %age of motor rating 76·8 %
Yoi1Ce starting torque available 200 x (if)2 117·0 %
As a comparison of Case l(iii), Case 3(iii), Case 5 and the
above detailed Case 7 it will be noted that there is a decided
advantage for 1130 volts either with straight 1130 volts or dual
565/1130 volts secondary transformer and little to choose between
1130 volts and 3300 volts-although with a larger cable,
possibly 0 ·15 sq in, at 3300 volts, further improvements would
result.
With the 1130 volts system it is possible to utilise a 500 KVA
transformer and for the section switches still to be capable of
clearing any short circuit fault current.
FIG 9 Power System for detailed calculation for
comparison of 565 V, 1130 V and 3300 Volts
Outbye transformershort
circuit level 25 mVA 0.3 P.F.
+----- 3.3 kV cable 4000 yds 0.1 sq. in.
Inbye dry type transformer 300 kVA 3300/
565[1130V and Outbye Transformer 750
KVA 3.3/3.3 K. V.
+----~ L.V. fixed cable (a) 100 yds 0.2 sq. in.
For 565 & 1130 (b) 500 yds 0.2 sq. in.
Volts (c) 1000 yds 0.2 sq. in.
3300 Volts (d) 1000 yds 0.1 sq. in.
four 50 hp motors each fed 1<-+-+0-1<--- through 50 yds. 0.03 sq. in.
trailer.
125 hp cutter motor fed
+-___________ through 300 yds 0.1 sq. in.
trailer. and 0.04 sq. in. for
3.3 K.V.
CONCLUSION
A change to 1100 volts is relatively easy, as equipment is
available, and the design aspects for the motors are similar to
550 volts. If it is contemplated that 3300 volts will be utilised it
will be essential to formulate some development work. The design,
development and delivery of 3·3 KV switchgear of a more
15
COMPARISOptOF SYSTEM CONDITION WITH NOMINAL VOLTAGES AT 565 V,1130V, & 3300V
565 Volts Case 1 565 Volts Case 2 1 130 Volts Case 3 1130 Volts Case 4 3300 Volts
-_('_'_I~I~ --(-,,-]Oi>I(iii) -1-1---1----'---
(i) (il) (iii) (I) ] (II) (iii) Case 5 CaSeS
----------------------------------
125 HP Starting &. 4 x 50HP All Motors Running 125 HP Starting &. 4 x 50 HP All Motors Running 125 HP All
Running Running Starting Motors
&4 Running
Running
---,---
With Secondary Cable of With Secondary Cable of With Secondary Cable of With Secondary Cable of With Secondary
Cable of
----------------------------
100 yds 500 yds 1 000 yds 100 yds 500 yds 1 000 yds 100 yds SOO yds 1 000 yds 100 yds 500 yds 1 000 yds 1000 yds 1 000 yds ----------------------------------------------------
a + jb e t· jb a + jb a + jb a t- jb a + jb a + jb a + jb a + jb a + jb a + jb a + jb a .j jb a + jb ---------------------------------------------
(e' Impedance 0.00385+ 0.00385+ 0.00385+ 0.00385+ 0.00385+ 0.00385+ 0.01543+ 0.01543+ 0.01543+ 0.01543+ 0.01543+ 0.01543+ 0.1317 + 0.1317 +
Incoming 0.0122 0.0122 0.0122 0.0122 0.0122 0.0122 0.04888 0.04888 0.04888 0.04888 0.04888 0.04888 0.41705 0.41705
(b, Impedance HT 0.03504+ 0,03504+ 0,03504+ 0.03504+ 0.0350t+ 0,03504+ 0,14064+ 0.14064+ 0.14064+ 0.14064+ 0,14064+ 0.14064+ 1.2+ 1.2+
Cable 0.00853 0.00853 0.00853 0.00853 0.00853 0.00853 0.03422 0.03422 0.03422 0.03422 0.03422 0.03422 0.292 0.292
(e, Impedance 0.00707 + 0.00707+ 0,00707+ 0.00707+ 0.00707+ 0.00707+ 0.0101 + 0.0101 + 0.0101 + 0.0101 + 0.0101+ 0.0101 + 0.1532+ 0.1532+
Transformer 0,0417 0.0417 0.0417 0.0417 0.0417 0.0417 0.059 0.059 0,059 0.059 0.059 0.059 0.41 0.41
(d, Imp. Secondary 0.Q16 t- 0.080+ 0.16+ 0.016+ 0.080+ 0.16 + 0.016+ 0.080+ 0.16+ 0.016+ O.08O·t 0.16+ 0.3+ 0.3+
Cable 0.0068 0.0340 0.068 0.0068 0.034 0.068 0.0068 0.034 0.068 0.0068 0.034 0.068 0.073 0.073
(e' Impedance 0.1973+ 0.1973+ 0.1973+ 0.8583+ 0.8583+ 0.8583+ 0.590+ 0.596+ 0.596+ 3.378+ 3.373+ 3.378+ 4.665+ 28.756+
Motors &. Cables 0.2739 0.2739 0.2739 0.4590 0,4590 0.4590 1.102 1.102 1.102 1.827 1.827 1.827 9.31 15.586
-------------------------------------------
Total Secondary
amps 158 663 511 301 288 268 443 426 401 160 158 154 143 55
Line Volt drop due
to supply 16 13 11 5 5 4 38 3. 34 10 10 10 60 24
HT cable 35 34 30 18 11 16 14 14 14 39 38 31 240 100
Transformer 50 42 34 14 13 12 45 43 39 10 10 10 108 42
Secondary cable 21 85 148 9 43 81 10 48 93 5 24 46 60 25
Voltage at GE boxes 444 391 342 519 481 452 9 .. 929 890 1066 1048 1027 2830 3110
Shearer motor amps 612 "9 411 111 104 91 310 351 342 59 58 58 121 19.85
Voltage drop due to
shearer cable 50 44 39 18 16 15 30 29 28 9 9 9 40 1
Volts at shearer
motor 384 339 296 500 468 431 930 891 859 1053 1045 1009 2790 3103
Volts available as
%age of motor
ratln;;! 73.1% 64.6% 56.4% 95.2% 97% 83.2% 88,6% 85.4% 81.8% 101.2% 99.5% 96.1% 85% 94%
%age starting torque
available =
(VI)2 107% 83.4% 63.6% - - - 156.9% 146% 133.9% - - - 149% - 200 x
(V,
Voltage Drops are referred to the Secondary Side of the Transformer i.e. 565v, 1130v or 3.3 KV
Impedance in OHMS Per Phase
Vl = Actual Volts on Motor Terminals
V = Mean of Rated Voltage Range of Motor
compact, flexible and comprehensive arrangement could take
five years.
If a quicker change to 1130 volts had been undertaken the
economics of changeover would have been easier, as during
the past 3 years a large number of GE Boxes have beenpurchdsed
or modified when in fact 550 volt equipment released by obtaining
1130 volt equipment would have provided the necessary
requirements.
The next step is to 1130 volts applied to both advancing and
retreating systems of mining. Also the transformer should be
situated not more than 500 yds from the face; it is not comparison
with a 565 volts transformer situated 100/200 yds from the face,
but an improvement in the operational characteristics that
should be considered. It has been stated that because 3300 volts
equipment is bulky GE Boxes should be situated 1000 yds from
the face but the query of local isolation arises-a.nd the maintenance
electrician travelling this distance in say 4 ft. 6 in high
"in seam" mining roadway. Further, there is the question of
type and method of connection of trailing cables to the G E Boxes.
The high torque motors should continue to be utili:.ed. As this
motor has a high starting torque voltage effect is not so marked
as in the standard motor, where the starting torque could be less
than full load torque.
Close co-ordination is required in the design of equipmentbetween
users, manufacturers and the Ministry of Power.
16
Irrespective of the level of use voltage at the face the capability
of the high voltage main systems to transmit the loads efficiently
must be considered. This is to further enhance economics,
reliability and a greater degree of assurance of the rated voltage
at the motor terminals.
The level of prosperity of the mining industry stiH depends to a
large extent on the vigour with which new ideas are met.
References
1 "Vacuum Contactors" article by AEI Ltd
February 1968 "Works Engineering and Faciory Services"
2 "The Design and Application of Water-Cooled Motors" by D
Ramsden "LSE Engineering Bulletin" Vol 6 No 3,4 March 1962 Pages
24/35
The author wishes to acknowledge assistance received from
Baldwin & Francis Lid., Belmos Peebles Ltd, Bruce Peebles
Lid, GEC/AEI Lid, EMP Electric Lid and English Electric Co Ltd,
in the form of information from which diagrams of plant details
and the first table in this article have been produced.
This paper was commissioned by the technical committee but
the views expressed are entirely those of the author and do not
necessarily represent those of the committee.
