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
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
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.