Single phase Induction Motor

1. Single phase Induction motor

2. Applications

9. Torque –
slip
characteri
stics

10. Double revolving field theory
• When the stator winding carries a sinusoidal current (being fed from a
single-phase supply), a sinusoidal space distributed mmf, whose peak
or maximum value pulsates (alternates) with time, is produced in the
air gap.
• This sinusoidally varying flux (φ ) is the sum of two rotating fluxes or
fields, the magnitude of which is equal to half the value of the
alternating flux (φ 2/ ), and both the fluxes rotating synchronously at
the speed, ( s = ⋅ /)2( Pfn ) in opposite directions.

11. Understanding Double revolving field theory
• The current flows in the rotor conductors, and the electromagnetic
torque is produced in the same direction as given above, which is
termed as positive (+ve).
• The other part of flux or field rotates at the same speed in the
opposite (clockwise) direction, taken as negative.
• So, the torque produced by this field is negative (-ve), as it is in the
clockwise direction, same as that of the direction of rotation of this
field.
• Two torques are in the opposite direction, and the resultant (total)
torque is the difference of the two torques produced

12. • If the rotor is stationary (ωr = 0), the slip due to forward
(anticlockwise) rotating field is sf= 1.
• Similarly, the slip due to backward rotating field is also Sb=1.
• The two torques are equal and opposite, and the resultant torque is
zero.
• So, there is no starting torque in a single-phase IM.

13. Understanding Double revolving field theory
• if the motor (rotor) is started or rotated somehow, say in the
anticlockwise (forward) direction, the forward torque is more than
the backward torque, with the resultant torque now being positive.
• The motor accelerates in the forward direction, with the forward
torque being more than the backward torque.
• The resultant torque is thus positive as the motor rotates in the
forward direction.
• The motor speed is decided by the load torque supplied, including the
losses (specially mechanical loss).

14. Understanding double revolving field theory

15. Equivalent circuit

16. Torque speed characteristics of single winding
single phase IM

17. Important
• The Single Phase Motor is not self starting and hence needs an
auxiliary means or equipment to start it.
• Mechanical methods are impractical and, therefore the motor is
started temporarily converting it into two phase motor.
• Single phase Induction motors are usually classified according to the
auxiliary means used to start the motor.

18. How to make it
starting?

20. Split phase induction motor
• The Split Phase Motor is also known as a Resistance Start
Motor.
• It has a single cage rotor and its stator has two windings
known as main winding and starting winding.
• Both the windings are displaced 90 degrees in space.
• The main winding has very low resistance and a high
inductive reactance whereas the starting winding has high
resistance and low inductive reactance.

21. Split phase IM

22. Operation
• The starting torque is small, of the order of 1.5 to 2 times of the
started running torque.
• At the starting of the motor both the windings are connected in
parallel.
• As soon as the motor reaches the speed of about 70 to 80 % of the
synchronous speed the starting winding is disconnected automatically
from the supply mains.

24. Phasor diagram
and
Torque speed
characteristics

25. Application
• Washing machine
• Air conditioning fans.
• Mixer grinder
• Floor polishers
• Blowers
• Centrifugal pumps
• Drilling and lathe machine.

26. Capacitor start IM
• A Capacitor Start Motors are a single phase Induction Motor that
employs a capacitor in the auxiliary winding circuit to produce a
greater phase difference between the current in the main and the
auxiliary windings.

27. Construction
• The capacitor start motor has a cage rotor and has two windings on
the stator.
• They are known as the main winding and the auxiliary or the starting
winding.
• The two windings are placed 90 degrees apart.
• A capacitor CS is connected in series with the starting winding.
• A centrifugal switch SC is also connected in the circuit.

28. Features
• The capacitor starts motor develops a much higher starting torque of
about 3 to 4.5 times of the full load torque.
• To obtain a high starting torque, the two conditions are essential.
They are as follows:-
 The Starting capacitor value must be large.
 The valve of the starting winding resistance must be low.
• The electrolytic capacitors of the order of the 250 µF are normally
used because of the high Var rating of the capacitor requirement.

29. Phasor diagram and Torque speed
characteristics

30. Applications
• These motors are used for the loads of higher inertia where frequent
starting is required.
• Used in pumps and compressors
• Used in the refrigerator and air conditioner compressors.
• They are also used for conveyors and machine tools.

31. Permanent Split Capacitor (PSC) Induction
Motor
• The Permanent Split Capacitor motor also has a cage rotor and the
two windings named as main and auxiliary windings similar to that of
a Capacitor Start and Capacitor Start Capacitor Run Motor.
• It has only one capacitor connected in series with the starting
winding.
• The capacitor C is permanently connected in the circuit both at the
starting and the running conditions.

32. Operation
• As the capacitor is always in the circuit and thus this type of motor
does not contain any starting switch.
• The auxiliary winding is always there in the circuit.
• Therefore, the motor operates as the balanced two-phase motor.
• The motor produces a uniform torque and has noise free operation.

33. Advantages
• No centrifugal switch is required.
• Efficiency is high.
• As the capacitor is connected permanently in the circuit, the power
factor is high.
• It has a higher pullout torque.

34. Limitations
• The paper capacitor is used in the motor as an Electrolytic capacitor
cannot be used for continuous running.
• The cost of the paper capacitor is higher, and size is also large as
compared to the electrolytic capacitor of the same ratings.
• It has low starting torque, less than full load torque.

35. Application
• Used in fans and blowers in heaters and air conditioners.
• Used in refrigerator compressors.
• Used in office machinery.

36. Shaded pole IM
• Shaded pole motor is a split phase type single phase induction motor.
• The shaded pole motor is very popular for ratings below 0.05 HP (~
40 W) because of its extremely simple construction.
• It has salient poles on the stator excited by single- phase supply and
a squirrel cage rotor.
• A portion of each pole is surrounded by a short- circuited turn of
copper strip called shading coil.

38. Construction
• The stator has salient poles.
• Usually 2 to 4 poles are used.
• Each of the poles has its own exciting coil.
• A part of each pole is wrapped by a copper coil.
• The copper coil forms a closed loop across each pole.
• This loop is known as the shading coil.
• The poles are laminated. A slot is cut across the lamination of the
pole.
• The slot is approximately one third distance from the edge of the
pole.

39. Contd…
• The short circuited copper coil described above is placed in the slot.
• So we can call this part as the shaded part and other part of the pole
as unshaded part.
• Selecting a 2 poled stator gives a synchronous speed of 3000 rpm
while a 4 poled stator speed will be 1500rpm for 50Hz supply.

40. Rotor construction
• The rotor of shaded pole induction motors is Squirrel Cage type rotor.
• The rotor bars are provided with a 60 degree skew.
• This is to obtain an optimum starting torque.

42. During the portion OA
• During the portion OA of the alternating-current cycle [Fig 1], the flux
begins to increase and an e.m.f. is induced in the shading coil.
• The resulting current in the shading coil will be in such a direction
(Lenz’s law) so as to oppose the change in flux.
• Thus the flux in the shaded portion of the pole is weakened while that
in the unshaded portion is strengthened as shown in figure 2.

43. During the portion AB
During the portion AB of the alternating-current cycle, the flux has
reached almost maximum value and is not changing.
Consequently, the flux distribution across the pole is uniform [See Fig
3] since no current is flowing in the shading coil.

44. During the portion BC
• As the flux decreases (portion BC of the alternating current cycle),
current is induced in the shading coil so as to oppose the decrease in
current.
• Thus the flux in the shaded portion of the pole is strengthened while
that in the unshaded portion is weakened as shown in Fig 4.

45. Effect
• The effect of the shading coil is to cause the field flux to shift across
the pole face from the unshaded to the shaded portion.
• This shifting flux is like a rotating weak field moving in the direction
from unshaded portion to the shaded portion of the pole.
• The rotor is of the squirrel-cage type and is under the influence of this
moving field.

46. Effects
• Consequently, a small starting torque is developed.
• As soon as this torque starts to revolve the rotor, additional torque is
produced by single-phase induction-motor action.
• The motor accelerates to a speed slightly below the synchronous
speed and runs as a single-phase induction motor.

47. Capacitor start capacitor run motor

48. Operation
• There are two capacitors in this motor
represented by CS and CR. At the
starting, the two capacitors are
connected in parallel.
• As the motor reaches the synchronous
speed, the starting capacitor Cs is
disconnected from the circuit by a
centrifugal switch Sc.

49. Capacitor Start Capacitor Run Motor
Auxiliary winding and capacitor remain connected in the circuit at all
time.
Continue duty capacitor is used.
CR – small value of oil impregnated paper continues rating capacitor.
CS – Larger value electrolytic short duty capacitor.
Centrifugal switch is connected in series with electrolytic capacitor.
Cs is about 10-15 times as large as the running capacitor
Efficiency – 55-65 %
Power factor – 0.8 -0.95

50. Torque-Speed Characteristics
Application
• Compressors
• Refrigerators
• Pump
• Conveyers
• Air conditioners