In a single phase induction motor: 1) The stator generates a pulsating magnetic field rather than a rotating magnetic field, as there is only one winding supplied by a single phase power source. 2) The magnetic field strength varies sinusoidally with time but remains stationary in space. 3) Without additional starting mechanisms, the rotor cannot start rotating due to the lack of a rotating magnetic field and resulting starting torque.

1. Operation
Single phase induction motor

2. ILOs – Day2
 Summarize the conditions for generating RMF in poly-phase IM
 Explain the generation of pulsating magnetic field in 1-ph IM

3. Recapitulation of 3-phase IM
•Three coils in stator (space difference 1200)
•Three supply voltages (time difference 1200)
•Thus, we have 3 magnetic fields at time and space difference
•Resultant RMF is produced
•Rotor start & continue to rotate in the same direction as RMF
Operation of 2-phase IM
•Two coils in stator (space difference 900)
•Two supply voltages (time difference 900)
•Thus, we have 2 magnetic fields at time and space difference
• Resultant RMF is produced
•Rotor start & continue to rotate in the same direction as RMF
Single phase Induction motor…

4. RMF in poly-phase IMs
 The basic requirement for generating a rotating magnetic
field (RMF) in the air gap of a poly-phase motor is that
 There must be more than one magnetic fields present
 with certain time-phase difference
 and space-phase difference between themselves
 In 3-phase and 2-phase induction motors, this is achieved by
 putting more than one winding in the stator
 Space difference
 supplying those windings with a poly-phase supply
 Time difference

5. RMF in poly-phase IMs
 Under action of this RMF, the rotor in 3-phase and 2-phase
induction motors start to rotate in the same direction as the
RMF
 Thus, a poly-phase IM has both:
 Starting torque
 Running torque

6. Stator magnetic field in Single phase induction
motor
 In single phase induction motors
 there is only one main winding in the stator
 So, no space difference
 it is supplied from a single phaseAC source
 So, no time difference
 Thus, there is no so-called rotating magnetic field (no RMF)
 Rather the magnetic field developed by stator is pulsating
(oscillating) in nature.
 As the supply current magnitude varies sinusoidally:
 strength of the magnetic field developed by the stator coil also varies
in magnitude with time
 but remains stationary in space (no RMF)

7. Activity
 Fill in the blanks
 RMF can be generated when _____ _____ at least
______ magnetic _______ present in the air gap of a
poly-phase motor
 These magnetic fields must have ____-____ time
________ difference as well as _______ - phase
_________ between __________
there are
two fields
non-zero
phase space
difference themselves

8. Rotating magnetic field in 3-phase motors

9. Pulsating magnetic field in single phase induction
motor
Pulsating magnetic field in single phase motor

10. Pulsating magnetic field
Single phase supply given to a single winding in stator

11. Instant (1)
Supply current zero
So magnetic field is also zero

12. Instant (2)
• Supply current positive 0.5Im
• A is dot
• A’ is cross
So magnetic field is downwards and increases from 0 to 0.5Bm

13. Instant (3)
• Supply current positive Im
• A is dot (bigger)
• A’ is cross (Bigger)
So magnetic field is downwards and increases from 0.5 Bm to Bm

14. Instant (4)
• Supply current positive 0.5Im
• A is dot (smaller)
• A’ is cross (smaller)
So magnetic field is downwards but reduces from Bm to 0.5Bm

15. Instant (5)
Supply current zero
So magnetic field comes back from 0.5 Bm to zero

16. Instant (6)
• Supply current negative 0.5Im
• A is cross
• A’ is dot
So magnetic field is upwards and increases from 0 to 0.5Bm

17. So…
This process continues to generate a pulsating field
Stator magnetic field
• Changes magnitude with time
• But physically stationery (does not rotate)

18. Pulsating field
• The resultant MMF in a 1-ph IM is thus pulsating in nature
• Variation in its magnitude being sinusoidal with respect to
time
• Time period of pulsation being same as that of the
sinusoidal supply

19. 1-phase induction motor can’t start to rotate by itself
Due to the absence of any rotating magnetic field (RMF)
• The rotor cannot start to rotate since there is no
starting torque
• The rotor thus simply tends to vibrate
• Some external auxiliary arrangements are thus necessary to start
a single phase induction motor and bring it to running condition
• Once the motor can be started by some external methods,in
any direction,it can continue rotation in that direction.

20. Summary
1) .
2) .
3) .
4) .
5) .
6) .

21. Activity
 Frame 3 MCQs on topics that we have discussed so far on
single phase induction motor..