This document discusses different types of single-phase induction motors, including split-phase motors, capacitor-start motors, capacitor-start capacitor-run motors, permanent split-capacitor motors, and shaded-pole motors. It explains the operating principles of each type of motor and how they achieve self-starting. Key details are provided on the windings, capacitors, and speed-torque characteristics of each motor type. The document also covers universal motors and their ability to operate on AC or DC power supplies.

1. Types of Single-Phase Induction
Motors
By: Tirffneh Y.(M.Tech)
May 2014
Mekelle University- MIT
1

2. Introduction
• Each single-phase induction motor derives its
name from the method used to make it self-
starting
• Some are
split-phase motor
 capacitor-start motor
 capacitor-start capacitor-run motor
 permanent split-capacitor motor
 the shaded-pole motor
2

3. • For an IM to be self-starting, it must have at
least two phase windings in space quadrature
and must be excited by a 2-Ph or 3-ph source
• The currents in the 2-ph windings are 900
electrical out of phase with each other
• The placement of the two phase windings in
space quadrature in a 1-ph motor is no problem
• However, the artificial creation of a second
phase requires some basic understanding of
resistive, inductive, and capacitive networks
3

4. Split-Phase Motor
• employs 2 separate windings that are placed in
space quadrature and are connected in parallel to
a 1-ph source
• One winding, known as the main winding, has a
low R and high L. This winding carries current and
establishes the needed flux at the rated speed
• The second winding, called the auxiliary winding,
has a high R and low L. This winding is
disconnected from the supply when the motor
attains a N of nearly 75% of its Ns
4

5. • The disconnection is necessary to avoid the
excessive power loss in the auxiliary winding at
full load
• A centrifugal switch is commonly used to
disconnect the auxiliary winding from the source
at a predetermined speed
5

6. • At the time of starting, the two windings draw
currents from the supply
• The main-winding current lags the applied
voltage by almost 900
• The auxiliary-winding current is approximately in
phase with the applied voltage
• In practice a well-designed split-phase motor, the
phase difference between the two currents may
be as high as 600
• It is from this phase-splitting action that the
split-phase motor derives its name
6

7. • Since the two phase-windings are wound in space
quadrature and carry out of-phase currents, they
set up an unbalanced revolving field
• It is this revolving field, albeit unbalanced, that
enables the motor to start
• The starting torque developed by a split-phase
motor is typically 150% to 200% of the full-load
torque
• The starting current is about 6 to 8 times the full-
load current
7

8. • speed-torque characteristic of split-phase motor:
Note the drop in torque at the time the auxiliary
winding is disconnected from the supply
8

9. Capacitor-start Motor
9

10. • In a capacitor-start motor a capacitor is included
in series with the auxiliary winding
• If the capacitor value is properly chosen, it is
possible to design a capacitor-start motor such
that the main-winding current lags the auxiliary-
winding current by exactly 900
• Therefore, the starting torque developed by a
capacitor motor can be as good as that of any
poly-phase motor
• The need for an external capacitor makes the
capacitor-start motor somewhat more expensive
than a split-phase motor 10

11. • However, a capacitor-start motor is used when
the starting torque requirements are 4 to 5 times
the rated torque. Such a high starting torque is
not within the realm of a split-phase motor
• Since the capacitor is used only during starting,
its duty cycle is very intermittent. Thus, an
inexpensive and relatively small ac electrolytic-
type capacitor can be used for all capacitor-start
motors
11

12. Capacitor-Start Capacitor-Run Motor
12

13. • Although the split-phase and capacitor-start
motors are designed to satisfy the rated load
requirements, they have low pf at the rated
speed
• The lower the power factor, the higher the power
input for the same power output
• Thus, the efficiency of a single-phase motor is
lower than that of a poly-phase induction motor
of the same size
• Since this motor requires two capacitors, it is also
known as the two-value capacitor motor
13

14. • The efficiency of a single-phase induction motor
can be improved by employing another capacitor
when the motor runs at the rated speed
• This led to the development of a capacitor-start
capacitor-run (CSCR) motor
• One capacitor is selected on the basis of starting
torque requirements (the start capacitor),
whereas the other capacitor is picked for the
running performance (the run capacitor)
14

15. • The start capacitor is of the ac electrolytic type,
whereas the run capacitor is of an ac oil type
rated for continuous operation
• Since both windings are active at the rated
speed, the run capacitor can be selected to make
the winding currents truly in quadrature with
each other
• Thus, a CSCR motor acts like a two-phase motor
both at the time of starting and at its rated speed
• Although the CSCR motor is more expensive
because it uses two different capacitors, it has
relatively high efficiency at full load compared
with a split-phase or capacitor-start motor
15

16. Permanent Split-Capacitor Motor(PSC)
16

17. • Is a less expensive version of a CSCR motor
• A PSC motor uses the same capacitor for both
starting and full load operation
• Since the auxiliary winding and the capacitor stay
in the circuit as long as the motor operates, there
is no need for a centrifugal switch
• For this reason, the motor length is smaller than
for the other types discussed above
• The capacitor is usually selected to obtain high
efficiency at the rated load
17

18. • Since the capacitor is not properly matched to
develop optimal starting torque, the starting
torque of a PSC motor is lower than that of a
CSCR motor
• PSC motors are, therefore, suitable for blower
applications with minimal starting torque
requirements
• These motors are also good candidates for
applications that require frequent starts
18

19. • Other types of motors discussed above tend to
overheat when started frequently, and this may
badly affect the reliability of the entire system
• With fewer rotating parts, a PSC motor is usually
quieter and has a high efficiency at full load
19

20. Shaded-Pole Motor
• When the auxiliary winding of a single-phase
induction motor is in the form of a copper ring, it
is called the shaded-pole motor
• The pole is physically divided into two sections
• A heavy, short-circuited copper ring, called the
shading coil, is placed around the smaller section
20

21. • shaded-pole motor is very simple in
construction and is the least expensive for
fractional horsepower applications
• Since it does not require a centrifugal switch, it
is not only rugged but also very reliable in its
operation
• Has low efficiency and low starting torque
21

22. Principle of Operation
• consider changes in the flux produced by the
main winding at three time intervals
a) When the flux is increasing from zero to maximum
b) When the flux is almost maximum
c) When the flux is decreasing from maximum to zero
• Any change in the flux in each pole of the motor
is responsible for an induced emf in the shading
coil in accordance with Faraday's law of induction
22

23. • Since the shading coil forms a closed loop having
a very small resistance, a large current is induced
in the shading coil
• The direction of the current is such that it always
creates a magnetic field that opposes the change
in the flux in the shaded region of the pole
• With this understanding, let us now analyze the
effect of the shading coils during the time
intervals mentioned above
23

24. Interval a:
• During this time interval the flux in the pole is
increasing and so is the current induced in the
shading coil
• The shading coil produces a flux that opposes the
increase in the flux linking the coil
• As a result, most of the flux flows through the
un-shaded part of the pole
• The magnetic axis of the flux is then the center of
the un-shaded section of the pole
24

25. Interval b:
• During this time interval the magnetic flux in the
pole is near its maximum value, therefore, the
rate of change of flux is almost zero
• Hence, the induced emf and the current in the
shading coil are zero, so, the flux distributes itself
uniformly through the entire pole
• The magnetic axis, therefore, moves to the
center of the pole
• This shift in magnetic axis has the same effect as
the physical motion (rotation) of the pole
25

26. Interval c:
• During this time interval the magnetic flux
produced by the main winding begins to
decrease, therefore, the current induced in the
shading coil reverses its direction in order to
oppose the decrease in the flux
• In other words, the shading coil produces the
flux that tends to prevent a decrease in the flux
produced by the main winding
• As a result, most of the flux is confined in the
shaded region of the pole
• The magnetic axis of the flux has now moved to
the center of the shaded region 26

27. shading-pole action during the positive half cycle of a flux
waveform
a) wt < π/2: Almost all the flux passing through unshaded
region;
b) wt = π /2: No shading action, flux is uniformly distributed
over the entire pole;
c) wt > π /2: Most of the flux is passing through the shaded
region. 27

28. • Note that without the shading coil, the center of
the magnetic axis would always be at the center
of the pole
• The presence of the shading coil forces the flux
to shift its magnetic axis from the unshaded
region to the shaded region
• The shift is gradual and has the effect of
revolving magnetic poles
• In other words, the magnetic field revolves from
the unshaded part toward the shaded part of
the motor
28

29. • The revolving field, however, is neither
continuous nor uniform
• Consequently, the torque developed by the
motor is not uniform but varies from instant to
instant
• Since the rotor follows the revolving field, the
direction of rotation of a shaded-pole motor
cannot be reversed once the motor is built
• To have a reversible motor, we must place two
shading coils on both sides of the pole and
selectively short one of them
29

30. • To increase the starting torque, the leading edge
of the shaded-pole motor may have a wider air-
gap than the rest of the pole
• It has been found that if a part of the pole face
has a wider gap than the remainder of the pole,
the motor develops some starting torque
without the auxiliary winding
• Such a motor is called a reluctance start motor
• This feature is commonly employed in the design of a
shaded-pole motor to increase its starting torque
30

31. Speed-torque characteristic of a shaded-pole motor
• To cancel some of the third-harmonic effect, we
can use a relatively high-resistance rotor
• However, any increase in the rotor resistance is
accompanied not only by a decrease in the
operating speed of the motor but also by a drop
of motor efficiency
31

32. Universal Motor
• A universal motor is defined as a motor which
may be operated either on DC or 1-ph a.c.
supply at approximately the same speed and
output
• A universal motor is wound and connected just
like a dc series motor, i.e., the field winding is
connected in series with the armature winding
with some modifications
32

33. Principle of Operation
• When a series motor is operated from a dc
source, the current is unidirectional in both the
field and the armature windings
• Therefore, the flux produced by each pole and
the direction of the current in the armature
conductors under that pole remain in the same
direction at all times
• Hence, the torque developed by the motor is
constant
33

34. When a series motor is connected to an ac source,
• Current and flux directions in a universal motor
during (a) the positive and (b) the negative half
cycles
34

35. • During the positive half cycle the flux produced
by the field winding is from right to left
• For the marked direction of the current in the
armature conductors, the motor develops a
torque in the counterclockwise direction
• During the negative half cycle, the applied
voltage has reversed its polarity, consequently,
the current has reversed its direction
• As a result, the flux produced by the poles is now
directed from left to right
35

36. • Since the reversal in the current in the armature
conductors is also accompanied by reversal in
the direction of flux in the motor, the direction
of the torque developed by the motor remains
unchanged
• Hence, the motor continues its rotation in the
counterclockwise direction
• If Ka is the machine constant, ia is the current
through the field and the armature windings at
any instant, and ɸp is the flux per pole at that
instant, the instantaneous torque developed by
the motor is Kaiaɸp
36

37. • Thus, the instantaneous torque developed by the
motor is proportional to the square of the armature
current
• In other words, the average value of the torque
developed is proportional to the root-mean-square
(rms) value of the current
37

38. • It is obvious from the waveform above that the
torque developed by the universal motor varies
with twice the frequency of the ac source
• Such pulsations in torque cause vibrations and
make the motor noisy
38

39. The equivalent circuit, the phasor diagram, and the
speed-torque characteristics of a universal motor
39

40. • The back emf Ea, the winding current Ia, and the
flux per pole ɸp are in phase with each other as
shown
• Rs and Xs are the resistance and the reactance of
the series field winding. Ra and Xa are the
resistance and the reactance of the armature
winding
40

41. Design Considerations
1. When a series motor is to be designed as a
universal motor, its poles and yoke must be
laminated in order to minimize the core loss
produced in them by the alternating flux
• If a series motor with an unlaminated stator is
connected to an ac supply, it quickly overheats
owing to excessive core loss
41

42. 2. Under steady-state operation of a dc series
motor, the inductances of the series field and
armature windings have little effect on its
performance
• However, the motor exhibits reactive voltage
drops across these inductances when connected
to an ac source, which have a two-fold effect:
(a) reducing the current in the circuit for the same
applied voltage, and
(b) lowering the power factor of the motor. The
reactive voltage drop across the series field
winding is made small by using fewer series field
turns
42

43. 3. The decrease in the number of turns in the series
field winding reduces the flux in the motor. This
loss in flux is compensated by an increase in the
number of armature conductors
4. Under ac operation, an emf is induced by
transformer action in the coils undergoing
commutation. This induced emf (a) causes extra
sparking at the brushes, (b) reduces brush life,
and (c) results in more wear and tear of the
commutator. To reduce these harmful effects, the
number of commutator segments is increased
and high-resistance brushes are used in universal
motors 43

44. 5. The increase in the armature conductors results
in an increase in the armature reaction. The
armature reaction can, however, be reduced by
adding compensating windings in the motor as:
44

45. With all these drawbacks, why do we use a
universal motor?
1. A universal motor is needed when it is required
to operate with complete satisfaction on dc and
ac supply.
2. The universal motor satisfies the requirements
when we need a motor to operate on ac supply
at a speed in excess of 3600 rpm (2-pole
induction motor operating at 60 Hz). Since the
power developed is proportional to the motor
speed, a high-speed motor develops more power
for the same size than a low speed motor
45

46. 3. When we need a motor that automatically
adjusts its speed under load, the universal
motor is suitable for that purpose. Its speed is
high when the load is light and low when the
load is heavy.
Application
• Some applications that require variation in
speed with load are saws and routers, sewing
machines, portable machine tools, and vacuum
cleaners
46

47. • Example: A 120-V, 60-Hz, 2-pole, universal
motor operates at a speed of 8000 rpm on full
load and draws a current of 17.58 A at a lagging
power factor of 0.912. The impedance of the
series field winding is 0.65 + j1.2 Ω. The
impedance of the armature winding is 1.36 +
j1.6 Ω. Determine (a) the induced emf in the
armature, (b) the power output, (c) the shaft
torque, and (d) the efficiency if the rotational
loss is 80 W.
47

48. 48

49. Solution
(a) From the equivalent circuit of the motor we have
As expected, the induced emf is in phase with the
armature current
(b) The power developed by the motor is
The power output:
49

50. 50

51. Questions???
51