1. AC Machines

2. 3 Phase Induction Motor Principles
At the end of this section you will be able to:
List the characteristics of the magnetic fields produced by a 3Ø windings
Calculate the speed of rotation of a rotating magnetic field
Describe the basic principles of operation of an induction motor
Reverse the direction of rotation of a 3 phase induction motor

3. So What Happens in the Stator?

4. A N N N C B

5. B = 87%
C = – 87%

6. B = 87%
= 150%
60
C = – 87%

7. C = – 87%
A = 87%

10. Rotating Magnetic Field
Synchronous Speed
The speed of the RMF is dependant on two factors:-
minutes
Frequency of the supply
Number of poles in the stator
seconds
There are always a multiple of 2 poles (North & South) per phase

11. Rotating Magnetic Field
Converts Seconds to minutes
Divides the poles by 2
Nsync = f= P=
Synchronous Speed (RPM)
Frequency of supply
Number of poles per phase

12. Rotating Magnetic Field
What happens if we increase the frequency of supply? 
Nsync
What happens if we increase the number of poles?
Nsync 

13. So What Happens in the Rotor?

14. A north & south pole are near a conductor

15. Flemings Left Hand Rule
Motion
Current
Field

16. Attraction
Repulsion

18. Magnetic field cuts conductor
Current is induced in conductor
A force acts on conductor
Conductor moves

19. What happens if the conductor is the same speed as the RMF?
Slip
What happens if the conductor is the same speed as the RMF?

20. Magnetic field does not cut conductor
No current is induced in conductor
Friction slows conductor
Magnetic field then cuts the conductor

21. DIFFERENCE between SYNCHRONOUS speed and ROTOR speed
Slip
Slip Speed
DIFFERENCE between SYNCHRONOUS speed and ROTOR speed
Normally expressed as a percentage
s% = n=
nsync=
Slip in %
Rotor speed (RPM)
Synchronous Speed (RPM)

22. Slip
Slip Speed
Slip
At start up
Running
Highest
Lowest

23. SLIP is the DIFFERENCE between SYNCHRONOUS speed and ROTOR Speed
Stator’s Magnetic Field
Rotor
SLIP is the DIFFERENCE between SYNCHRONOUS speed and ROTOR Speed

24. Rotor Frequency Rotor Stator RMF Stationary Locked
Cuts rotor a fast rate
Frequency in rotor highest

25. Rotor Frequency Rotor Stator RMF At normal speed With some slip
Cuts rotor a slow rate
Frequency in rotor lowest

26. Rotor Frequency fr = s = f = Frequency in rotor % slip
Supply frequency
Frequency in Rotor
% Slip
Rotor RPM

27. By changing the “firing order “ of the poles
Can we reverse a 3 Ø Motor?
By changing the “firing order “ of the poles

28. A N N N C B B C
Previous

29. Can we reverse a 3 Ø Motor?
By changing the “firing order “ of the poles
By reversing any two phase connections

30. U V W1
V W U2

31. U V W1
V W U2