CHAPTER 13
SINGLE-PHASE
MOTORS
13.1 Introduction
There are two basic forms of construction for single-phase
motors. One is almost identical to that of the three-phase
induction motor while the other is of a form similar to that
of the d.c. series motor. Both types are popular but for
larger sizes the induction motor is the most highly
regarded because of its simplicity, ruggedness and
reliability. The series or "universal" motor is more
popular in smaller sizes where its high speed and light
weight give it many advantages. The three-phase
induction motor discussed in the previous chapter had an
inherent starting torque because of its rotating field, but
the single-phase induction motor initially has no rotating
field, and so no starting torque. Special techniques have to
be adopted to ensure the starting of the single-phase
induction motor and because of the various starting
methods employed there are several versions of the single-
phase induction motor.
13.2 Producing starting torque
single-phase motor
In the three-phase motor the supply consist<
identical currents being supplied to thre•
windings in the motor. The resultant ma!
rotated at constant speed and strength. Ide•
identical currents at 90° E could be suppli
windings in the two-phase motor. then ~
magnetic field would rotate at synchronorn
motor can be wound with two windings but on
it to a single-phase supply the two cum
probably be in phase with each other and no re
would be produced. The rotating magnel
produced in the single-phase induction
simulating the effects of a two-phase motor.
problem is to ensure the motor has two cur
appropriate phase angle to each other. 11
achieved by having windings of different indrn
sometimes by adding a capacitor in series witt
windings. Once the motor is rotating at a sui
one of the windings can be disconnected and
will continue to rotate. There are two chc
peculiar to single-phase motors. The motor
vibration of twice line frequency when runni
tends to make the motor more noisy in opera ti
three-phase motor. The second is that the mot'
an amount of negative torque which is a func
slip speed. This results in a rather high no-load
low power factor. When a load is applied to th
current changes only marginally, but the pc
improves in a similar fashion to the three-phc
13.3 Single-phase induction m1
Fig. 13.1 A single-phase 180 W motor
244
13.3.1 Split-phase motor
The standard split-phase induction moto
separate windings (start and run) connected
supply during the starting process. For norm
however, only the run winding is used.
The run winding consists of a numb1
connected in series to form a set number <
245
SINGLE-PHASE MOTORS
v
'"
Fig. 13.3 Phase relationships between starting and running
winding currents for a split-phase motor
(a) Start and run windings displaced by 90"E
Run winding
'
r '
Start winding
•
•
y
Centrifugal
switch
-
a.c. supply()-~------'
(b) Internal connections
Fig. 13.2 Electrical details of a split-phase motor
four-pole machine, for example, has four groups of coils
all in series and physically displaced around the stator to
form four separate poles as shown in Figure 13.2(a).
The run winding is wound with a heavier gauge wire to
reduce its resistance. To increase the inductance of the run
winding, the coils are imbedded deep into the slots of the
iron core and usually have more turns than the start
winding. The current lR flowing through the run winding
is highly reactive, and so it lags the applied voltage Vby a
considerable angle </>R (Fig. 13.3).
The start winding also consists of a number of coils
connected in series to form a set number of poles. If the
machine has four poles in the run winding. it will also have
four poles in the start winding. However, the start winding
is physically displaced by 90° E around the stator core, as
shown in Figure 13.2(a).
The start winding of the standard split-phase motor is
wound with finer gauge wire, increasing the resistance of
the winding. When compared with the run winding, the
start winding has fewer turns. and the coils are placed
nearer the surface of the slots in the stator core. reducing
I
I
Run rnding rlux <l>A
Start winding flux <l>s
/
/
/
/
e
a
b
c
d
e
g
h
Fig. 13.4 Flux waveforms
k
/
a
246
ELECTRICAL PRINCIPLES FOR THE ELECTRICAi
Run winding
Start
winding
e --..
(b} Relative field strength
(a) Relative winding positions
Fig. 13.5 Rotating field in a split-phase motor
the inductance of the winding. The net result is that the
The direction of rotation is in the same directi1
current Is flowing in the start winding is more in phase
the resultant magnetic field.
If the direction of current flow through on
reversed, the resultant magnetic field rotates i1
with the voltage V than IR in the run winding. In Figure
13.3, IR lags Vby <l>•and /slags Vby <l>s. This produces a
phase displacement of ¢ between the two currents, so
producing a phase displacement between the respective
fluxes of the two windings.
The currents and fluxes of the start and run windings
can be up to 30° out of phase. Figure 13.4 (p. 245) shows
the waveforn1s of the fluxes of the two windings.
Starting
For a two-pole machine, the windings are also physically
displaced by 90° E. Assuming a 30° phase displacement
bet\veen the two fluxes, at position a in Figure 13.4 the
run flux ii>" is zero and the start flux ii> s is 50% of the
maximum value in a positive direction. The resultant
stator flux at position a is shown in Figure 13.5(b).
At position b in Figure 13.4, ii>" is 50% of its
direction. That is, the direction of rotation of
reversed by changing the direction of cu
through one winding. This is done by exchan1
end connections of any one winding.
As seen in Figure 13.5(b), the rotating s
not of uniform value and an elliptical fie!<
produced. This produces considerable vib:
humming noise during starting.
The rotating stator field cuts the rote
induces a voltage in them and, because they
out, a current flows through the bars and pro(
flux. The stator flux and the rotor flux interac
a force on the rotor bars, causing the rotor tc
direction in which the stator flux is rotating. 1
maximum value and <I> sis 86.6% of its maximum value,
and these combine to form the resultant stator flux b in
Figure 13.5(b). At position cin Figure 13.4, il>.is 86.6%
of its maximum value and il>sis 100%, and the resultant
stator flux c is shown in Figure 13.5(b). By taking each
position from a to I in Figure 13.4 it can be seen that the
stator flux rotates one full revolution for one full cycle.
The stator flux rotates at a speed governed by the
supply frequency and the number of poles in a winding.
In = l~Of I
i.e.
where
f
=
frequency
p = number of poles
n =speed in r/min
For a two-pole machine on a 50 Hz supply,
n = 3000r/min
For a four-pole machine,
n
= 1500 r/min.
Fig. 13.6 Types of switching mechanisms for s,
motors. Also shown is a centrifugal a1
lower left.
SIMPSON AP
SINGLE-PHASE MOTORS
247
A
,,0
/0
10
10
\0
\ 0
'
A
I
I
'
I
4
I
)
I
I
\
t
,
~\
..
I
.-;
/e
r:\ ..
"')
e"'/
\
G> I
;;;
(
I
\
I
I
..
0\
01
t
t
'-
.,
E• \
01
I'
I
I
0;
-'~
c
c
(a) First hall cycle
(b) Second half cycle
Fig. 13.7 Pulsating stator field
force is called the starting torque and largely depends
upon the relative strengths of the start and run fluxes, and
the phase .displacement between the currents flowing
through both windings.
The start and run windings are connected in parallel
across the supply voltage. When the rotor has reached
sufficient speed to provide a strong cross flux, the start
winding can be open-circuited. This is usually done by
connecting a centrifugally operated switch in series with
the start winding (Fig. l 3.2(b )).
The centrifugal switch is usually set to open when the
rotor speed reaches approximately 75% of the rated speed
of the motor. When the motor is switched off, the rotor
slows down and the centrifugal mechanism operates,
closing the switch contacts again in readiness for the next
starting operation.
A
Because the start winding is only connected during the
starting procedure, it is designed for a very short duty
cycle. If the centrifugal switch fails to operate, the start
winding will quickly overheat and burn out.
Running
When the rotor speed of the standard split-phase motor
reaches approximately 75% of the synchronous speed, the
centrifugal switch open-circuits the start winding and only
the run winding is connected to the supply.
For a two-pole motor when the stator current flows in
one direction for one half-cycle, a magnetic field is
produced in the direction C-A in Figure 13.7(a). During
the next half-cycle when the stator current is reversed, the
magnetic field also reverses and is in the direction A-C in
Figure 13.7(b).
A
8
c
'
(a) Stator flux
c
(b) Rotor flux
Fig. 13.8 Magnetic fields in a rotating single-phase motor
248
ELECTRICAL PRINCIPLES FOR THE ELECTRICAL
V rotor
This stator field, produced by the run windings, varies
in strength and direction according to the supply, but it
does not rotate. It is a stationary pulsating field. This is the
reason why some form of starting (i.e. start winding) is
required for split-phase motors.
When the stator winding is connected to the a.c.
supply and the rotor is turning, the rotor bars cut the
stator flux, causing an e.m.f. to be generated in them. In
Figure l 3.8(a) on page 24 7 the rotor is revolving in a
clockwise direction, and the stator field is acting in the
direction C-A. By Fleming's right-hand rule (sect. 6.1.1)
the generated e.m.f. in the rotor acts in the direction
shown (out of the page) in all rotor bars above the axis
D-B (indicated by the dots), and into the page in all rotor
bars below the axis D-B.
The induced rotor voltages are in phase with the stator
flux and cause a rotor current to flow. Because of the
low resistance and high inductance of the rotor bars, these
currents lag the induced rotor voltage by nearly 90°.
Consequently, the rotor currents produce a rotor flux
lagging almost 90° behind the stator flux, and acting in
the direction D-B, as shown in Figure l 3.8(b).
Because the rotor flux is at right angles to the stator
flux it is often referred to as the "cross field". The two
fields effectively combine to form a rotating field, which
tends to force the rotor bars in the direction in which
the field rotates.
For one full cycle of the a.c. supply the resultant field
rotates 360°E. For the two-pole machine described, this
constitutes one full revolution. For a four-pole machine, it
will rotate a half revolution for each full cycle of the a.c.
supply.
Due to the internal losses within the rotor, however,
the rotor itself will not rotate at synchronous speed, but at
a slightly slower speed.
Figure 13.10 shows a typical torque/ speed characteristic for a split-phase motor. The break in the curve is
caused by the switch operating to disconnect the starting
winding. This is necessary to limit the losses in the motor
and to protect the starting winding. The torque curves
between the running and starting sequences normally do
not coincide, so the speed and torque values have to adjust
when the switch operates. The values shown on the curve
must be considered as representative only. They will vary
from one make to another and even within the one make
because of design changes.
a
c
d
Fig.13.9 Phase relation of <I>s, VR, <l>R
obtained. Any value capacitor will increase th1
values that enable the starting winding tc
resonance must be avoided. For this reason it
to ensure that too large a value of capacitor is
than a small value. The phasors in Figm
indicate the ideal phase displacement of 90° E
of phase displacement between I. and ls prov
uniform strength of stator flux during starting
Figures l 3.5(b) and 13.12(b). Due to this m01
strength, the starting torque is higher than th<
sized split-phase motor. Figure 13.13
speed/torque curve for a capacitor-start n
drawn in the same proportions as that of Figt
give a framework for comparison. It can be see
is a large increase in starting torque due to the
the capacitor, while the torque is the same c:
phase motor after the switch has operated. I
fashion to the split-phase motor the switch
approximately 75% of full-load speed. The act
windings for the two types of motor may
different data. Reversal of rotation is achie
same principles applying to the split-phase mol
the motor can be reversed by changing over th
of any one winding but not both.
Uses
For general-purpose heavy-duty application;
high locked rotor starting torque, such ;
refrigerators and air compressors.
s
Uses
400
Split-phase motors have only moderate starting torque so
they are limited to such typical uses as washing machines,
blowers, buffing machines, grinders and machine tools.
300
13.3.2 Capacitor-start motor
Design limitations restrict the split-phase motor to a
maximum of about 30° E betw~en the starting and
running winding currents. To increase this angle and
produce improved characteristics a capacitor is connected
in series with the starting winding (see Fig. 13.1 l(a)). If
the correct size capacitor is selected then the two currents
are at 90°E to each other and improved starting torque is
b
200
Torque
%
100
I
\
___ ________ _
....._Rated torque
Speed
Flg.13.10 Speed/torque curve fora split-pha~
249
SINGLE-PHASE MOTORS
Run winding
Start winding
90"E
Capacitor
a.c.
supply
(a} Electrical connections
/A
(b) Phasors
Flg.13.11 Capacitor-start, induction-run motor
k
/
h
"
I
\
I
I
\
I
Ia
I
g
I
90°E
\
f
a
b
c
d
e
g
h
k
I
a
d
(a) Waveforms
-
b
/
c
(b) Relative field strength
Fig.13.12 Rotating field in a capacitor-start motor
Switch
speed
400
300
Rated
speed
I
I
200
I
I
Torque
%
100
Rated torque
-----------
I
Speed
Fig. 13.13 Speed/torque curve for a capacitor-start motor
13.3.3 Capacitor-start, capacitor-run motor
This type of motor has both windings permanently
connected across the supply; these are referred to as the
main and auxiliary windings. During starting, additional
capacitance is connected in series with the auxiliary
winding to provide the necessary phase displacement
between the winding currents for maximum torque. The
starting capacitor is therefore connected in para1Iel with
the running capacitor. When the rotor speed reaches
about 75% of the rated speed, the centrifugal switch
disconnects the starting capacitor from the circuit as
shown in Figure 13.14.
During operating conditions the running capacitor
ensures the correct phase displacement between the two
currents in the windings, so providing a constant strength
rotating magnetic field.
It should be noted that the starting capacitor can be
rated for intermittent duty, but the running capacitor
must be of a construction suitable for continuous rating
250
ELECTRICAL PRINCIPLES FOR THE ELECTRICAi
Running
capacitor
Auxiliary winding
Starting
capacitor
Main
winding
Fig. 13.14 Capacitor-start, capacitor-run motor
such as the paper-spaced oil-filled type. The twocapacitor motor provides substantially the same running
torque as the capacitor-starting, induction-run type, but
there are beneficial effects. Adding the second capacitor:
I. increases the breakdown torque;
2. improves full-load efficiency and power factor;
3. reduces operational noise and vibration;
4. increases locked rotor torque.
The direction of rotation can be reversed by changing
over the two leads of any one winding but not both. This
changes the direction of rotation of the magnetic field in
the stator.
Uses
Heavy-duty loads where quietness is a consideration and
substantial starting torque is necessary: wall mounted
air-conditioning units where high head pressures are
encountered in hot weather, for example.
13.3.4 Permanently split capacitor motor
The permanently split motor also has both windings
permanently connected across the supply, with a
capacitor in series with one of them as shown in Figure
13. 15.
For this type of niotor, both windings are identical
in wire size and the nun1ber of turns, and are also referred
to as the main and auxiliary windings. Because the
capacitor is in series with one winding, th~
that winding leads the current in the other, p·
necessary phase displacement to produce a ro
field. However, the phase displacement betw
fluxes is relatively small, and so the startir.
low.
By interchanging the line connection fro1
Figure 13.15, the capacitor is then in series w
instead of the auxiliary winding. The current
winding leads that in the auxiliary winding a1
runs in the reverse direction.
These motors are suitable for unit heate
because their speed can be varied fairly easil~
inductances.
Uses
Light applications with low starting torque, e
blowers which may need to be reversed fre1
remote control of induction regulators and c
regulating air flow in air-conditioning system
13.3.5 Shaded-pole motor
The shaded-pole motor has a cage rotor with'
in the stator. On one side of each pole, a slot
shading ring is embedded into it, as show1
13.16. The shading rings are made of copper
into a closed loop, providing a low resis
through the ring.
The supply current produces an alterna·
each pole. This alternating flux cuts the sh
inducing an e.m.f. in it. Because of the low resi~
the current flowing through the ring is rela
Also, according to Lenz's Jaw, the induced cu
shading ring will produce a flux that will tern
the change of the main flux.
When the supply current rises rapidly fro1
in Figure I 3. I 7(d) an induced voltage is establ
shading ring. The current in the ring prod
which opposes the build-up of the main flux.
the main flux is concentrated in the unshadet
the pole, as in Figure I 3. I 7(a).
A
Main
winding
L,
c
\
)t---0
L,
\
\
Shading ring
\
\
\
\
\
', B
'
Fig. 13.15 Permanently split motor circuit
Flg.13.16 Salient poles and shading
rin~
251
SINGLE-PHASE MOTORS
B
D
A
Time
(a)
(d)
(c)
(b)
Flg.13.17 Centre line of flux moves towards the shading ring, giving the effect of a moving field
When the current changes from B to C, there is little
change in value ofcurrent and very little voltage is induced
in the shading ring. Consequently, practically no current
nor flux is produced in the shading ring. The main flux is
at this time nearly always at maximum value, and is
uniformly distributed over the whole pole face, as seen in
Figure 13.17(b).
When the supply current drops rapidly from C to D,
an induced voltage is established in the shading ring. The
current in the shading ring produces a flux which opposes
the collapse of the main flux. The concentration of flux
therefore occurs in the shaded action of the pole, as shown
in Figure 13.17(c).
The magnetic axis shifts across the pole face, from the
unshaded part to the shaded part of the pole. This shifting
flux is similar to a rotating field, and produces a small
torque, causing the rotor to rotate in the direction of the
flux, towards the shaded section of the pole.
The starting torque is very low as indicated in Figure
I 3. 18 and the motor runs with a slip speed slightly higher
than the single-phase motors described above. It is simple
in construction, low in cost and reliable. There are no
switches, slip-rings, brushes or capacitors that may
require maintenance. The motor efficiency is down and
this tends to restrict its use to low power ratings.
Direction of rotation has to be reversed by altering the
direction of the rotating magnetic field across the pole
face. This is done by shifting the shading ring to the other
side of the pole face. Some poles are fitted with slots on
both sides for this purpose but with others the only
method is to remove the stator from its housing and
replace it the other way around in the frame.
Uses
Because its speed can be varied within a limited range by a
series resistor or inductor it is suitable for fans and
blowers, advertising signs, damper controllers, hair dryers
and other uses where the starting torque requirements are
minimal.
1
13.4 Commutator induction motors
This type of single-phase motor is becoming scarce but is
included here for interest. Called repulsion or repulsioninduction motors, all have a generally common build with
minor variations only. The motor has a smooth bore
stator with concentric windings forming the field poles.
The rotor is somewhat similar to the armature of a d.c.
machine, with either a radial or axial commutator (see
Fig. I 3. I 9). The stator winding is connected to the supply
while the rotor winding is connected to the bars of the
300
Rated
200
speed
I
Torque
%
I
100
Speed
n,,,.
Fig. 13.18 Speed/torque curve for a shaded/pole motor
(a) Radial
(b) Axial
Fig. 13.19 Repulsion motor armatures
252
ELECTRICAL PRINCIPLES FOR THE ELECTRICA
(±)@ ®®i:B
e
A
N
®
s_@
®0~
®r:i
0
0
8
s
A
0
0
0
80000
No rotation
Fig. 13.20 Neutral brush position in a repulsion motor
commutator with permanently short-circuited brushes to
provide current paths. Construction costs are high when
compared to the normal cage-rotor induction motor but
for amperes of starting current the starting torque is very
high. There are three major forms of repulsion motor, all
variations of the one type.
13.4.1 Repulsion motor
This motor behaves in a similar fashion to the three-phase
wound-rotor motor in that the speed is dependent to some
extent on the applied load. Note that the windings on
the repulsion-type rotor are not shorted out by a switch
but by the brushes. In Figure 13.20 where the field poles
are shown as salient poles for the sake of clarity, the
rotor winding acts as the secondary of a transformer, and
an e.m.f. is induced in each conductor. By Lenz's law
the direction of these induced voltages is such that they
tend to oppose the stator flux. If the brushes are along
the axis A-A in Figure 13.20, the direction of current
flow is as shown. The resultant rotor flux is also along
the same axis A-A, but opposing the stator flux so no
tangential force is produced, no torque is developed, and
the motor cannot rotate.
If the brush axis is rotated in a clockwise direction to
position B-B, the direction of current flow in the rotor
conductors is as indicated in Figure l3_2l(a). The rotor
flux is along the axis B-B while the stator flux is still along
axis A-A. The two fluxes interact, causing repulsion
between them and a torque is developed causing the rotor
to move in a clockwise direction and bringing successive
armature conductors into an appropriate position to
continue creating torque.
If the brush axis is rotated anticlockwise to position
C-C, as shown in Figure 13.21(b), the repulsion between
the two fluxes causes the rotor to move in an
anticlockwise direction. The normal method for changing
the direction of rotation is by changing the brush position
to either side of the marked centre position.
Because of the high induced voltages in the rotor, the
rotor current and flux is large. Consequently the repulsion
motor develops a high starting torque. As the motor
accelerates so the induced voltage, rotor currj
decrease. The motor torque then reduces dow
running torque. Starting torque typically is i
that of rated running torque.
13.4.2 Repulsion-start, induction-run m1
This motor has the same basic construction 1
addition of a centrifugally operated switch.
commences rotation as a repulsion moto1
attaining about 75% of its rated speed the swil
a mechanism that short-circuits all the segn
commutator, effectively converting it into a
motor. Reversal of rotation is effected by ~
brush position as for the previous type. Starti
typically about 450% that of rated runn'
although high-torque versions will give up to a
A characteristic speed/torque curve for suet
given in Figure 13.22.
13.4.3 Repulsion-induction motor
This is the true RI motor, although theterm is
applied to all forms of repulsion motor.
construction to the repulsion motor, it has
mechanism-instead it has a modified form
cage under the rotor windings. Its operation
the windings being more effective for startin
high inductance cage becomes more effecti·
motor has accelerated up to its rated speed. Th
to give the more constant running speed of
cage-induction motor. Reversal of rotation i
the other two types in that direction is altered I
the brush position. Starting torque is typicall~
of rated running torque.
Uses
A very popular motor until three-phase s1
three-phase motors became readily availabl
now restricted. Its attractiveness lay in its h
torque with comparatively low starting cu1
power factor about 10% higher than the
induction motor. Any high inertia load that ti
253
SINGLE-PHASE MOTORS
B
A
N
A
B
(a) Clockwise rotation
~
@G:l (±)(fl EB s
A
N
c
©@a
(±) -
-_ 0
G:l
00
-
8
s
A
0
80000
c
(b) Anticlockwise rotation
Fig. 13.21 Reversal of rotation by shifting the brush position in a repulsion motor
be accelerated up to speed may require such a motor.
Many commercial grade refrigerators used them, as did
older style industrial plants that used a central motor with
many belts and pulleys.
600
500
13.5 Series motor
The series motor is often called a universal motor because
it can operate effectively on d.c. and a.c. up to power line
frequencies. Like the normal d.c. series motor, it has a
highly variable speed characteristic, with speeds up to
15 000 r/min in domestic appliances. Under some
circumstances governors have to be used to restrict speeds
to safe values. Field pole construction consists of a
number of lamination stampings riveted together to form
salient poles. The field coils are concentrated-type
windings fitting closely around the salient poles.
The armature construction is similar to that of a d.c.
armature, with laminations, commutator and windings.
The armature windings are connected in series with the
Switch
speed
400
300
Torque
%
Rated
speed
200
I
100
Rated torque
I
Speed
Fig. 13.22 Speed/torque curve for a high-torque repulsion
start induction-run motor
254
ELECTRICAL PRINCIPLES FOR THE ELECTRICAi
A
s
N
N
s
B
B
(a) First half cycle
(b) Second half cycle
Fig. 13.23 Torque production in a universal motor
two field coils by means of carbon brushes running on the
commutator (see Fig. 13.23).
There is a common current flowing through both
windings, so the two magnetic fluxes produced are in
phase with each other. Interaction of the fluxes produces
the speed is low; at light loads the speed is very
the very small series motor in domestic use, t
losses (such as friction and windage) are large
limit the speed to a safe value. The motor
torque to turn the armature. As the a.c. supply alternates
the fluxes change in unison so remaining in phase.
When the line current flows from A to B in Figure
l 3.23(a), north and south poles are produced as shown.
Assuming the armature current is in the direction
indicated by the dots and crosses, the flux produced
around the armature conductors interacts with the field
flux producing an anticlockwise rotation.
When the line current flows from B to A on the
alternate half-cycle, the polarities of the main fields are
reversed as shown in Figure 13.23(b). The current
through the armature also reverses, so reversing the
Speed
armature flux. The resultant torque is still in the
anticlockwise direction so a steady rotation in one
direction is maintained. Reversal of rotation is obtained
by changing the direction of current flow through the
armature with respect to the field. That is, changing over
the leads to the armature or the fields but not both (refer
to Fig. 13.25). The speed/load characteristic for the series
motor is shown in Figure 13.24. When the load is heavy,
Load
Fig. 13.24 Universal motor speed/load chara1
255~
SINGLE-PHASE MOTORS
'}...
Lt'.)
;,_
""
loc
,_,
-1-
,_
/ ::;,
0-..
lo
(a)
(b)
Fig. 13.25 Reversing direction of rotation in a universal motor
relatively high speed and has good starting and running
torque characteristics considering its small size.
\.
~
reduced starting, running and breakdown torques,
increased running noise and vibration, full load speed
reduction and a higher operating temperature.
Uses
Popular in portable appliances such as saws and drills,
sewing machines, business machines, food mixers, small
washing machines and vacuum cleaners.
13.6 Abnormal operating conditions
In section 12. l l, abnormal operating conditions
applicable only to three-phase motors were discussed. In
this section more conditions are discussed, and apply to
both single- and three-phase motors.
13.6.1 Voltage fluctuation
This can be of two types: voltage rise and fall where the
voltages remain symmetrical; and variation in individual
phase voltages. This latter is especially detrimental to the
performance of three-phase motors.
Previously it has been shown that the torque produced
is proportional to the square of the voltage. That is, ifthe
voltage drops to 90% of its nominal value the torque
reduces to 81 % of its rated value, Similarly, if the voltage
rises to I 10%, then the torque increases to 121 % of its
rated value. For example, a IO kW motor with a 10%
voltage variation should now be rated at 8.1 kW or
12.1 kW. Under normal operating conditions a voltage
variation of this magnitude should make only minor
differences to the motor's characteristics. With a voltage
increase for example, the increased torque reduces the slip
only slightly so a motor rotatingat 1450 r /min on 50 Hz
would increase its speed to approximately 1455 r /min. If
advantage is taken of the increased torque, then the
operating temperature could be expected to increase.
With voltage variations greater than I 0% the motor must
be derated to prevent excessive temperature rise. Motors
are normally given a full-time rating for a specified
temperature rise, and under these conditions may have to
be switched off after a duty period and be allowed to cool
down.
Starting and breakdown torque values are also
affected. A voltage rise increases torque while a voltage
reduction decreases torque. In the latter case care must be
taken to see that the motor does not stall under load.
For three-phase motors a more serious problem
occurs when only one phase shifts in value. The phase
current is affected to a greater proportion, which affects
the rotating magnetic field in the motor. This results in
~
.\-
13.6.2 Higher operating temperatures
Common causes of overheating in motors are inadequate
or restricted ventilation and overloading, Apart from
accelerated deterioration of lubricants, possibly the most
serious effect is on the insulation. At increased
temperatures there is a marked reduction in the life of
insu1ation. For example, insulation designed to work at
90°C may have an expected life of 25 years. If the
operating temperature is doubled to 180°C, the life
expectancy is reduced to about 1.25 years. The cure is
increased efficiency of the cooling system and a decrease
of the load applied to the motor.
13.6.3 Frequency variation
Motor speed can be regulated by the frequency of the
supply and allowances are made when selecting a motor
for this purpose, but a variation in supply frequency under
other circumstances can affect motor operation. The
obvious effect is a change in speed, but there are also
changes in power factor, efficiency and torque. A higher
frequency causes an increase in power factor, a slight
increase in efficiency and a decrease in torque. The
opposite occurs, with a decreased frequency. A variation
in the frequency of supply usually occurs where there
are comparatively few large loads connected to a smaller
supply.
13.6.4 Overloading
Manufacturers build into their motors the capability to
handle short duration overloads as specified in AS
1359.41, In broad general terms the standard requires the
motor to be able to withstand 1.5 times full load for a
period of 15 seconds without appreciable change in speed
or excessive heating. For a motor to operate under these
conditions means that it must also have a breakdown
torque in excess of the overload test figure,
The heating effect in a machine winding is related to
the square of the current and the time it is flowing, so any
excess current must result in a temperature rise. With
short time overloads the amount of heat generated is small
and can expect to be dissipated by the normal cooling
process. Long periods of overload, however, can lead to
excessive increases in temperature, in turn leading to a
shortened motor life. Other effects are a slight decrease in
speed, decreased efficiency, decreased power factor, and
an increased possibility of stalling because the working
\
'\__L
256
ELECTRICAL PRINCIPLES FOR THE ELECTRIC,!
torque is closer to the breakdown torque. When the motor
stalls it draws starting current at full line voltage until its
protection system operates. Large amounts of heat can be
generated in short periods of time under these
circumstances. It should be noted that special types of
motors are given restricted duty cycles. In effect they are
overloaded for short periods of time, after which they
must be switched off and allowed to cool down to room
temperature. If this type of motor is required to run on a
continuous duty cycle, then it must be derated to a lower
power value.
13.6.5 Frequent starting
Motors in general are mechanically strong enough to
handle normal loads with a fair safety factor. The number
of times a motor is started, however, is not within the
scope of the manufacturer unless specifically requested
at time of purchase. When a motor is started there is
a high current flow that decreases as the motor
accelerates up to its operational speed. While this current
is flowing, heat is being generated within the windings
at a rate in excess of the usual heat dissipation rate. Under
normal conditions this excess heat is removed by the
cooling system while the motor is in operation. With
repetitive starting, however, the heat generated does not
have sufficient time to be removed and the temperature
of the motor rises. The circumstances are similar for
repeated reversing and plug braking. Starting current
values, whether being used for starting, reversing or
braking, repeatedly stress the windings. Unless the coils
are firmly braced they rub against one another, eventually
rubbing through the insulation and so causing short
circuits within the windings of the motor.
13.6.6 Other factors
Motors are designed to withstand normal operating
conditions. Conditions other than these must be
considered abnormal and need special consideration.
Some of these are exposure to corrosive fumes, explosive
vapours, dust, steam, salt air, high humidity, operation in
ambient temperature of below approximately 10° C or
above 40°C, or operation at altitudes in excess of !000
metres. Generally, all these factors can subject a motor to
damage of some kind, but initially they are due to
selection of the wrong type of stator housing at the time of
purchase. Even motors selected for operation at elevated
altitudes are subject to the same restriction and must be
rated by the manufacturer at the time of purchase for the
conditions under which they will work.
13.7 Single-phase synchronous
motors
Unlike other types of motors, the speed of synchronous
motors is constant and is determined by the number of
poles and the frequency of the supply, whicr
within very fine limits to the standard freqw
Hz). This constant speed is the feature tha
small single-phase synchronous motor suita
applications as clocks, timers or recording d
13.7.1 Reluctance motors
The stator winding of the reluctance motor
the split-phase or capacitor-start motor.
however, is assembled from laminations fr
number of teeth are cut to form definite salie1
the windings are of the usual squirrel-cage t;
The motor starts as an induction mol
starting winding is open-circuited by the
switch at approximately 75% synchror
Because the load applied to this type <
comparatively light, there is small slip. The
poles tend to become permanently magne
stator poles and become "locked" together
poles are changing at the rate of twice
frequency. The rotor is attracted by the stator
the periods of the cycle when they are fully
During the period when the stator flux is 10\
of the rotor carries it past the position of one
and it is then attracted by the next stator pol
build-up of the stator flux. Each rotor pc
travels through the space of two stator poles
supply frequency.
The reluctance motor starts as an induc
locks into synchronism and continues to run;
synchronous speed. If the number of salient
rotor is some multiple of the stator poles, th
operate at a constant speed which is a subm1
synchronous speed. This is called a sub'
reluctance motor.
13.7.2 Hysteresis motors
With this type of motor, the rotor is canst
specially hardened steel rings instead of the
laminations. The effect of hysteresis is theret
and opposes any change in magnetic polaritie.
once they are established. The rotor poles "le
stator poles of the opposite polarities.
Normally, the synchronous motor is not;
One method that is used to provide mover
rotor is the shaded-pole principle. The mov<
stator flux across the pole face pulls the rota
it. Because the stator and rotor fluxes are 1
"locked" together, the rotor runs at a synchr
determined by the number of stator poles an
frequency.
There are many variations to the singlt
chronous motors mentioned, most of which
basic principle of either the reluctance or hyst<
257
SINGLE-PHASE MOTORS
Exercises
13.1
13.2
13.3
13.4
13.5
13.6
13.7
What is meant by the term split phase?
Briefly describe the split-phase method of
starting single-phase induction motors.
Why is a centrifugal switch used in most
single-phase induction motors?
Name one type of single-phase induction
motor which does not use a centrifugal
switch.
Why is the starting winding of a split-phase
motor disconnected under running conditions?
Briefly describe the principle of operation of
the capacitor-start, induction-run motor.
What is the main advantage of the capacitorstart, induction-run motor as compared with
the standard split-phase motor?
13.13 With the aid of diagrams, explain how a
shading ring provides starting torque for a
shaded-pole motor.
13.14 Discuss, with the aid of circuit diagrams, how
the following motors can be reversed: split
phase, series type, shaded pole.
13.15 Why are the armature and field fluxes in
phase with each other in an a.c. series motor?
Explain also why it is necessary that they are
in phase with each other.
13.16 Explain why a capacitor-start motor has more
starting torque than a split-phase motor.
13.17 Draw a simple circuit diagram for a permanently split capacitor motor. Give the
typical operating characteristics and list an
application for this type of motor.
13.8
How is the direction of rotation reversed in a
split-phase motor?
13.9 Explain the principle of operation of the
shaded-pole motor.
13.10 Explain why it is possible for a d.c. series
motor to operate on an a.c. supply.
13.11 How can the direction of rotation be reversed
in an a.c. series motor?
13.12 What is the basic principle of operation of one
type of single-phase synchronous motor?
13.18 What is the major characteristic of a
synchronous motor?
13.19 How can the repulsion-start, induction-run
motor be distinguished from the normal
repulsion motor? Can the difference be
recognised by a visual inspection?
13.20 Name one type of single-phase synchronous
motor and describe its construction and
operation.