TH IRTY-SECOND CONFERENCE
INDUSTRIAL A.C. INDUCTION MOTORS
By E. G. BARNES
The output of any electrical apparatus is governed by the temperature at which it may continuously opexate. British Standard Specification
BS.2613 : 1957 entitled "The Electrical Performance of Rotating Electrical Machinery" assigns a particular temperature rise to various classes
of insulating materials commonly used in the electrical industry. A
further specification B.S.2757 : 1956 entitled "Classification of Insulating
Materials for Electrical Machinery and Apparatus" assigns reference
letters A, E, B etc. to the various classes of insulating materials together
with the maximum hot spot temperature permitted for each class. The
most frequently used are class A, E, B and H and Table I shows an
abridged list of typical materials for each class in addition to the associated hot spot temperature.
(
Clrn of
insulation
1
A
(
E
B
H
l
TABLE I--Insulating
materials.
Typical materials
/
Varnished cotton, paper or nylon; cellulose acetate;
oleo-resinous wire enamel.
Vinyl acetal type and polyurethane type wire enamels
paper and cotton fabric phenolic laminates: cellulose
triacetate film ; 'polyethylene terephthalate (Terylene
and Melinex).
Polyester enamels, mica, glass fibre, or asbestos bonded
with shellac; oil-modified synthetic resins; alkyd resins
and epoxy resins.
Mica, glass fibre o r asbestos bonded with silicone
elastomer.
resins;.silicone
p------
p
p
p
"
p
-
'Hot spot'
temp. OC.
1
105
120
I180
3O
l
The method of temperature measurement is specified in BS.2613 as
either by thermometer, the resistance method or by means of temperature
detectors embedded in the machine windings. The former is defined as
measurement by thermometers applied to the hottest accessible surfaces
of stationary parts during the test period and by other thermometers
applied to the accessible surfaces of rotating parts as soon as the machine
is stopped after the test. In the second alternative the temperature rise
of the machine is computed from the change in winding resistance
measured before and after the load test. Table I on page 20 of BS.2613
indicates which method should be adopted for a particular size of machine.
The thermometer method is most frequently used for open protected
industrial low tension machines but it is becoming increasingly common
to adopt the resistance method for totally enclosed or similar motors with
inaccessible windings. The surface temperatures measured are assumed to
be lower than those occurring at some inaccessible hot spot within the
machine winding and an agreed allowance must be subtracted from the
hot spot temperature shown in Table I. Table I1 shows the build up of
hot spot temperature for class A, E, B and H, as specified in BS.2613 : 1957
and amendments.
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TH IRTY-SECOND CONFERENNCE
1965
TABLE Il-.-Hot spot temperature, 83.2613 : 1957.
insulation class
I * I E I B I H
Ambient temperature "C
Thermometer method: temperature rise ' C
Hot spot allowance "C
Resistance method: temperature rise "C
Hot spot allowance "C
Hot spot temperature "C
The above temperature rise is based on continuous maximum rating
which is the output at which the machine will operate for an unlimited
period without exceeding the given values. The temperature limits of each
class should ensure an operating life of many years under normal operating
conditions. Should the machine become excessively overheated following,
for example, a prolonged stall then, although the windings may not
actually burn out, the useful life of the insulation will be considerably
reduced. For this reason it is essential to enumerate all abnormal operating
conditions when specifying new machinery in order that adequate
machine capacity and protection will be provided.
Whilst discussing insulation it sholxld be explained that it is not
essential to utilize those materials listed in BS.2757 for a given insulation
class if it can be proved that alternative materials are suitable. It is
permissible to use higher thermal grade insulation in lower insulation
classes and the reverse is true if materials are treated to withstand the
higher thermal stresses. Class H insulation consists mainly of silicones and
silicone impregnated materials but although these have a higher thermal
stability they are unlikely to be used much in the range of machines
under consideration. In their present form class I1 silicones apart from
their higher thermal capacity are not superior in other respects to the
best class E materials and are considerably more expensive.
Over the past few years it has been the practice to manufacture small
and medium sized induction motors utilizing class E in place of class A
insulating materials. From consideration of heating it is possible to obtain
some seven per cent increase in output from a given motor frame size by
replacing class A with equivalent class E insulated windings. The increased output per frame more than offsets the slightly higher costs of
the class E materials and thus justifies the initial development of standardizing with class E insulated motors. In uprating a standard motor
however, it is not always sufficient to substitute new windings in existing
laminations as an increase in rating would reduce the machine overload
capacity expressed as a percentage of full load torque. BS.2613 specifies
that the momentary overload capacity of continuous maximum rated
induction motors should be not less than 200 per cent of iull load torque
for 15 seconds for motors up to and including 50 hp, and 175 per cent of
full load torque for 15 seconds for motors above 50 hp and up to and
including 500 hp. The maximum torque of an induction motor is governed
largely by the magnetic flux or magnetic strength of the stator winding
and the magnetic flux is, for a given voltage, inversely proportional to the
effective number of stator turns per phase. Substituting class 'E' coils
wound with an identical number of turns to those previously used in the
class 'A'winding will therefore not affect the actual magnitude of the
motor pull out torque. However, the percentage overload capacity, and
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,
with
Pip. &TWO rrcHon d l . 4 ribbon &l#.
possibly two or more straps constituting each coil turn. Figure 2 shows
preformed coils ready for assembly. The coil halves are assembled individually, and on completion it is necwmy to connect each pair of sections.
The coils sa fomed must now be connected in a spcified manner to complete the winding. The wound stator is varnish impregnated as before and
finished by a further spraying of the end-windings.
Rewinding and/or Uprating Exhtlag PIant
Due to limitation in overload capacity discussed previously it is
rarely possible to achieve the output of a machine designed specificdy
1 965
TH IRTY-SECOND CONFERENCE
79
for class E temperature rises by rewinding existing class A motors with
replacement class E coils. A complication arises in motors wound with
rectangular conductors due to the different combination of class E and
class A conductor and slot liner insulation thicknesses. The synthetic
enamel wire covering and high quality class E composite slot liner material
takes less space in the stator slots than the equivalent class A materials.
It is not usually possible to find a standard stock class E copper section
which will permit the coils to fit tightly into laminations designed for a
class A motor.
Improvements in manufacturing techniques have made possible the
use of mush windings in a larger size machine frame, consequently many
motors supplied in the past with two section ribbon windings could now
be designed to incorporate mush type windings. It is possible therefore
to rewind existing ribbon wound class A motors with specially designed
wire windings of equivalent or improved magnetic properties. The size
and number of wires in parallel are selected to fit tightly into the parallelsided slots used in the original .laminations.
It is relatively simple to substitute class E windings in machines
originally supplied with mush type windings.
Class B insulation may also be utilized in rewinding existing plant
although it is not usually possible to achieve the full potential output
afforded by the higher thermal grade insulation without severe reduction
in overIoad capacity. For this reason it may not prove economical to
rewind with class B unless the higher thermal capacity is necessary due
to the machine being moved to a higher ambient temperature site.
Various alternatives are possible in rewinding existing machinery
and it is recommended that specific cases be referred to the manufacturer
in order that the most efficient and economic choice be made.
Rotor Windings
Typical slip-ring motor windings are described very briefly as it is
usually possible to utilize class A, E or R windings in a given rotor core.
The smaller motors may be manufactured with mush windings similar
to those described previously. Larger machines however, have bar wound
wave or lap double layer windings. These consist of pre-cut and insulated
straight copper bars which are assembled in insulated rotor slots before
the end-windings are bent to shape and connected.
Squirrel Cage Motors
It is common practice to manufacture squirrel cage rotors by centrifugally or pressure casting aluminium in assembled core laminations.
The integral bar, end-ring and often the fan blade assembly so formed
provide a virtually indestructable construction.
The performance of motors with aluminium squirrel cages may be
altered considerably by adopting a variety of basic rotor slot shapes and
effectively adjusting the secondary resistance : reactance ratio. A number
of alternatives are illustrated in Table 111.
Design 1 is that of a normal starting torque, current and secondary
loss motor with possible high overload capacity. Design 2 provides a high
starting torque machine with normal starting current and secondary
loss. Design 3 can be considered as either a normal starting torque low
starting current or alternatively high starting torque normal starting
current motor. Design 4 has been included although the characteristics
80
I
TH IRTY-SECOND CONFERENCE
TABLE Ill-Resistance
19.65
: reactance ratio.
Starting
Resistance
low
high
high
high
1
Running
-
.
p
Reactance
-- -
Resistance
-
low
low
-
high
low
low
low
low
high
II
Reactance
low
high
high
low
are not due to rotor slot shape. I t is often advantageous to cast with
aluminium alloys of various electrical conductivity values in order to
increase the secondary resistance and consequent full load slip of the
machine. These high slip motors are most suitable for fly wheel applications. The high running reactance indicated in alternatives 2 and 3
necessitates careful design to ensure that the machine has adequate
overload capacity.
It can be seen that a wide variety of performance characteristics can
be achieved by judicious selection of rotor aluminium alloy and slot
shape. The stator winding is designed in conjunction with a suitable rotor
slot to meet specific performance requirements.
Machines above a certain size are generally manufactured with copper
bar squirrel cages. The demand for a particular rating is inversely proportional to the machine size and it is generally considered uneconomical
to produce the necessary tooling equipment for aluminium casting of
larger squirrel cages. The performance of any cast aluminium machine
can be equalled by suitably designed copper bar rotors but the latter
type are more expensive to produce.
Copper conductors may be of rectangular or special section and are
located in appropriately shaped slots. There is a minimum clearance
between bar and slot to ensure a tight fitting assembly which prevents bar
breakage due to excessive movement. On very large machines steel
wedges are often driven into an extended sub-slot beneath the copper
conductor. The bars are extended beyond the core and brazed into heavy
section copper end-rings providing a robust construction requiring very
little maintenance.
Specially shaped bars are employed to accentuate the current displacement effect which thus increases the ratio of starting to running
resistance. The advantages of these bar shapes are numerous and include:
1. Reduction in starting current for a given starting torque.
2. Increase in motor efficiency due to a reduced running resistance
for a given starting resistance.
3. Greater mechanical strength resulting from the increased overall
cross-sectional area.
4. Ability to dissipate more heat generated during starting, or
stalling, again due to the larger section.
The principal disadvantage lies in the increased cost of the special
section. Also, unless the bar is carefully designed, the motor torque-speed
characteristic may show an undesirable dip at approximately 60 to 70
per cent full load speed. In order to achieve a similar torque/speed
characteristic to that of the third cast aluminium rotor alternative
discussed previously, it is necessary to utilize a double squirrel cage
machine in which two independent cage and end-ring assemblies are
1965
THIRTY-SECOND CONFERENCE
81
incorporated. Two common applications of this machine type are for
drives requiring a high starting torque and for normal starting torque
duties with a low starting current.
I t must be stressed that the starting torque and starting current are
interrelated. The minimum ratio of per unit starting current to per unit
starting torque is approximately 2.6-2.0 : 1. As the permissible starting
current is reduced then the above ratio tends to increase.
A machine designed for low starting current would have a high value
of combined inner and outer cage resistance. This introduces additional
rotor losses necessitating in many cases a frame size larger than that of an
identically rated normal starting current machine with starting current
in the order 500 to 600 per cent of full load current.
To obtain the high resistance values it is common to use materials
other than copper which have higher electrical resistivity properties.
Examples of such materials are brass, phosphor-bronze and arsenical
copper.
Machine Enclosures
Various types of machine enclosure are specified in BS.2613 and a
number of alternatives are described below.
Sueelz protected:--- -This type of machine is the most inexpensive and
for a given motor frame and temperature rise can be most highly rated.
All internal live and rotating parts are mechanically protected against
accidental contact by operating personnel. There is virtually no restriction to ventilation which explains the high output possible.
Drip-proof:-A
screen protected motor can usually be simply converted into a drip-proof machine by replacing the mesh screens with
louvred covers. These covers must be designed to prevent vertically
falling water or dirt from entering the machine. Although there is a slight
increase in cost the rating is not usually reduced below that of the screen
protected motor.
Pipe ventilated:---Electrical machines are generally designed with
replaceable stator end-brackets and, by means of suitably shaped endshields, a machine can be arranged for either single or double ended pipe
ventilation. However, for a given temperature rise the increased restriction to ventilation necessitates a reduction in the equivalent size
screen protected machine rating. With a further reduction in output air
filters or cleaners can be mounted on the machine frame to provide a
motor capable of operating in a dusty atmosphere.
Splash-proof :-Splash-proof motors are defined as protected machines
in which the ventilating openings are so constructed that drops of water
or dirt falling through an inclusive angle bounded by the vertical and
100 degrees from the vertical cannot enter the machine. Specially designed
covers are generally mounted on pipe ventilated motor inlet and outlet
openings. The permissible rating for this type of machine would be
somewhat lower than that of a pipe ventilated motor if the specified
temperature rise is not to be exceeded.
Totally cmclosd falz cooled:--T.E.F.C. motors have an externally
mounted fan which blows air over the machine cooling surfaces. Heat
generated by the machine losses must be dissipated through the motor
frame and cooling is often assisted by having a second, internally mounted
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T H I RTY-SECOND CONFERENCE
1965
fan. The rating of T.E.F.C. machines as a percentage of the screen protected frame equivalent depends on the machine size. In the smaller range
up to approximately 50 hp the ratings are identical for screen protected
and T.E.F.C. frames but as the motor size increases the difference between
screen protected and T.E.F.C. machine output also increases.
Closed air circuit naotms -A totally enclosed air circuit machine may
have either an air or water cooled air cooler mounted on the machine
frame. To date there has been little demand for such machines in the small
and medium size range under consideration although the present tendency
is to specify either type for use on power station auxiliary machines.
For a given frame size the output of a CACW machine is usually higher
than that of a CACA alternative, and both types can generally be more
highly rated than an equivalent T.E.F.C. motor.
Ventilation
Two basic types of ventilation are encountered in rotating electrical
machinery.
Radial ventilation:-This
is most commonly used in very large
machines and consists of radial air ducts constructed through the core
laminations. The core length is divided into a number of sections by small
spacing strips and air entering the ducts radially, passes through the core
and is expelled through an opening in the machine frame. Machines can
have either single or double ended ventilation.
Axial ventilation :-Most machines in the industrial size range are
single ended axially ventilated. Cooling ducts are manufactured by
punching holes in individual core laminations which on assembly provide
an air circuit running parallel to the machine shaft. Figure 3 shows a
skeleton cross section through various machine enclosures and illustrates
the associated air circuits. The diagrams illustrate:(a) Axially ventilated screen protected or drip-proof enclosure with
a single radial fan.
(b) Single ended radially ventilated screen protected or drip-proof
enclosure with a single propellor fan.
(c) Double ended radially ventilated screen protected or drip-proof
enclosure incorporating a propellor type fan at each end.
(d) Axi.ally ventilated double pipe ventilated enclosure with a single
radlal fan.
(e) CACA enclosure axially ventilated with air cooler mounted on
top of the machine frame. Two radial fans are incorporated.
(f) CACW axially ventilated with air cooler mounted on top of the
machine frame. A single radial fan is utilized.
(g) Axially ventilated T.E.F.C. enclosure with single radial fan.
(h) Axially ventilated T.E.F.C. enclosure with cooling assisted by
adding an internally mounted radial fan.
Belt Drives
Due to the increased rating of modem electrical machines it is worth
commenting on the subject of selecting motors suitable for drives through
pulleys and belts. Special consideration must be paid to the mechanical
loading imposed by radial forces acting on the machine shaft. To determine the suitability of a standard machine with ball and/or roller bearings
it is necessary to calculate the anticipated operating life of the bearings.
1965
THIRTY-SECOND CONFERENCE
Fig. 3-Ventilation
83
schemes.
This is achieved by estimating the magnitude of the radial load and the
bearing life expressed in revolutions is then computed from tabulated
data provided by bearing manufacturers. Vertically mounted machines
are usually located by a ball bearing and the combined thrust and radial
load must be estimated before the bearing life can be predicted. The life
in hours for a given load application is dependent on the operating speed
of the machine. It is also necessary that the magnitude of shaft deflection
expressed as a percentage of the machine air gap does not exceed a generally accepted limit of 10 per cent. The combined bending and twisting
stresses set up in the machine shaft must be calculated and a sufficient
safety margin allowed to ensure satisfactory operation.
The use of V section belts is generally recommended as flat belts must
be tightened to a greater extent than V belt alternatives in order to transmit the required horsepower. The output of a standard machine must in
many cases be reduced for satisfactory V belt operation and must be
84
TH IRTY-SECOND CONFERENCE
1965
reduced still more if the use of flat belts is contemplated. I t is often
necessary to utilize an outrigger pedestal mounted bearing with machine
and pedestal accurately aligned on a common bedplate during manufacture. However, in some cases where bearing life is the limiting feature
it is possible to achieve the required output by substituting heavier grade
or larger bearings. The maximum output of a standard machine when
driving through V or flat belts can be calculated based on average operating conditions and the use of recommended motor pulleys. However, it
is recommended that if abnormal conditions are suspected as, for example,
with machines operating in damp or salty atmospheres, specific applications be referred to the motor manufacturer.
Conclusion
The subject of industrial a.c. induction motors cannot be adequately
covered in a short paper but an attempt has been made to answer a few
of the many questions frequently asked of motor manufacturers. The
paper is by no means a comprehensive treatise on induction machines
but it is hoped that the subjects covered will prove of interest to sugar
industry personnel.
Acknowledgements
The author wishes to thank the management of A.E.I. Engineering
Pty. Limited for permission to publish this paper.
A.E.I. Engineering Pty. Ltd.,
Sydney, N.S.W.