1. SMJE 2103
Induction Motor

2. Induction Motor Scope of discussion Structure Basic concept
Equivalent Circuit
Power and Torque
Speed Control
Testing

3. Induction Motor - Structure -
Squirrel Cage – no winding and slip ring

4. Wound Rotor – the winding connected to slip rings

5. Induction Motor - Basic Concept -
Induced Torque:
Speed of magnetic field
Induced Voltage
Induced Torque

6. Induction Motor - Basic Concept -
Rotor Slip:

7. Induction Motor - Basic Concept -
Rotor Frequency

8. Example
A 208V, 10hp, 4 ploe, 50 Hz, Y-connected induction motor has a full-load slip of 5%.
What is the synchronous speed of this motor?
What is the rotor speed of this motor at the rated load?
What is the rotor frequency of this motor at the rated load?
What is the shaft torque of this motor at the rated load?

9. Induction Motor - Equivalent Circuit -
Rotor voltage – Locked-rotor voltage
Rotor frequency – Frequency of the induced voltage

10. Induction Motor – Equi. Circuit
RR is constant but XR depend on S
Rotor current flow
Overall rotor impedance considered S
Referring to stator side, considered turn ratio – new Rotor voltage

11. Induction Motor – Basic Concept
Rotor current
Rotor impedance
If,
Final per-phase equi circuit

12. Induction Motor - Power and Torque -

13. Induction Motor - Power and Torque -
No output voltage instead of mechanical force.
Higher speed, higher friction, windage and stray losses.
But component losses change inverse.

14. Example/Tutorial
A 480V, 60Hz, 50hp, 3 phase induction motor is drawing 60A at 0.85 Pf lagging. The stator copper losses are 2 kW, and the rotor losses are 700W. The friction and windage losses re 600W, the core losses are 180W, and the stray losses are negligible. Find:
The air gap power PAG
The power converted Pconv
The output power Pout
The efficiency of the motor

15. Induction Motor - Power and Torque -

16. Induction Motor - Power and Torque -

17. Induction Motor - Power and Torque -

18. Induction Motor - Power and Torque -

19. Induction Motor - Power and Torque -

20. Induction Motor - Power and Torque -
Air gap needs R2/S and rotor copper loss needs R2

21. Example/Tutorial
A 460V, 25hp, 50Hz, 4 pole, Y-connected induction motor has the following impedances in ohms per phase referred to stator circuit:
The total rotational losses are 1100W and are assumed to be constant. The core loss is lumped in with the rotational losses. For a rotor slip of 2.2% at rated voltage and rated frequency, find the motor’s
speed d) Pconv and Pout
stator current e) ind and load
power factor f ) efficiency

22. Induction Motor - Speed Control -
Two methods
Varying stator and rotor magnetic field speed
- electrical frequency or changing the number of poles
Varying slip
- varying rotor resistance or terminal voltage

23. Induction Motor - Speed Control (pole changing)-
Created two poles, N and S
Created an extra poles by changing current polarity
(only 2 speeds)

24. Induction Motor - Speed Control (Frequency) -
With or without adjustment to the terminal voltage:
Vary frequency, stator voltage adjusted – generally vary speed and maintain operating torque.
Vary frequency, stator voltage maintained – able to achieve higher speeds but a reduction of torque.
Could be combined together, easy way by introducing power electronic fields.

25. Induction Motor - Speed Control (terminal voltage) -

26. Induction Motor - Speed Control (rotor resistance) -

27. Induction Motor - Testing -
There are basically 3 types of test in modeling induction motor parameters:
No-load test
DC test
Locked rotor test or blocked rotor test
Equivalent circuit element – R1, R2, X1, X2 and XM

28. Induction Motor - Testing (no load) -
Measuring rotational losses and providing info about its magnetizing current (X1 + XM)

29. Due to the rotor speed close to sync speed, S small and higher rotational losses

30. Induction Motor - Testing (no load) -

31. Induction Motor - Testing (DC test) -
To find R1 (stator resistance).
Dc voltage is applied and adjusted to rated condition.
Voltage and current flow are recorded.

32. Induction Motor - Testing (Locked rotor)-
Steps:
1- locked
2- AC voltage is applied and currents flow is adjusted to full load condition
3- Measure voltage, current and power flow

33. Induction Motor - Testing (Locked rotor)-

34. Induction Motor - Testing (Locked rotor)-

35. Example/Tutorial
The following test data were taken on a 7.5 hp, 4-pole, 208V, 50Hz, design delta-star connected induction motor having a rated current of 28A
Sketch the per-phase equivalent circuit for this motor
Find the slip at the pullout torque, and find the value of the pullout torque