1. Induction Motors Lecture 4
Abdul Afram
BEE1-ABC

2. Effect of Rotor Resistance on Torque-Speed Characteristics of Induction Motor
Increasing rotor resistance
Increases starting torque
Reduces Starting current
Increases Slip
Reduces Motor Efficiency
Increasing slip reduces converted mechanical power, lowering motor’s efficiency

3. Variations in Induction Motors Torque-Speed Characteristics
Design Requirements for Induction Motors
Higher starting torque
Lower slip at normal speed (good efficiency)
Lower starting current
How to achieve these goals
Higher starting torque is achieved by increasing rotor resistance
Lower slip at normal speed is achieved by decreasing rotor resistance
Lower starting current is achieved by increasing rotor resistance
“Design requirements are conflicting”

4. How to Overcome these Conflicting Requirements?
Use a wound rotor motor
Insert resistance in the rotor circuit at startup
Gives higher torque and lower current
Remove the resistance when motor reaches normal speed
Decreases the slip and increases the efficiency
Drawbacks of Wound Rotor Motors
More expensive
Need more maintenance (slip rings, brushes)
Require a more complex automatic control circuit than cage rotor motors
Then what should one do?
Figure out a way to add extra rotor resistance at start and remove it during normal running without slip rings and without operator or control circuit intervention

5. Desired Motor Characteristics
It is possible to accomplish desired motor characteristics by taking advantage of leakage reactance in induction motor rotor design.

6. Leakage Reactance
Reactance due to the rotor flux lines that do not also couple with the stator windings
Reactance 𝑋 2 in an induction motor equivalent circuit represents the referred form of the leakage reactance
The farther away from the stator a rotor bar or part of a bar is, the greater its leakage reactance, since a smaller percentage of the rotor flux will reach the stator windings

7. Rotor Bars Depth Controls Reactance 𝑋 2
If the bars of the cage rotor are placed near the surface of the rotor, they will have small leakage flux and 𝑿 𝟐 will be small
If the bars of the cage rotor are placed deeper into the rotor surface, there will be more leakage and 𝑿 𝟐 will be larger

8. Control of Motor Characteristics by Cage Rotor Design
Motor with Large bars near the surface
Low resistance due to the large cross section
Low leakage reactance and 𝑋 2
Pullout torque will be quite near sync. Speed - Good
The motor will be efficient - Good
Since R is small
The starting torque is small - Bad
The starting current is high - Bad
NEMA Design Class A motor
NEMA – National Electrical Manufacturers Association
Small fixed Rotor Resistance
Small Rotor Leakage Reactance

9. Torque-Speed Curves for Different Rotor Designs
Class A Motor

10. Motor with Small Bars near the Surface
Rotor resistance is high as cross section of bars is relatively small
Leakage reactance is small as the bars are near the surface
Like a wound rotor with extra resistance inserted in rotor
Starting torque is quite high - Good
Starting current is low – Good
Pullout torque occurs at high slip - Bad
Low Efficiency - Bad
NEMA Design Class D motor
Large fixed Rotor Resistance
Small Rotor Leakage Reactance

11. Torque-Speed Curves for Different Rotor Designs
Class D Motor

12. Variable Rotor Resistance
Deep rotor bars or double cage rotors produce variable rotor resistance
Current flowing through upper part of a deep rotor bar
Flux is tightly coupled
Leakage reactance is low
Current flowing through lower part of a deep rotor bar
Flux is loosely coupled
Leakage reactance is high

13. Resulting Equivalent Circuit of the Rotor Bar as a Function of Depth in the Rotor
A series of parallel electrical circuits
Upper ones having smaller inductance
Lower ones having larger inductance
Large cross sectional area means small resistance
At low slip (normal operating speed)
Small rotor frequency
Reactances are small compared to resistances
Impedances of all paths are almost same
Current flows through all parts of bar equally
Large cross sectional area makes resistance small
Good efficiency - Good
At high slip (starting condition)
Large rotor frequency
Reactances are large compared to resistances
All current flows in low reactance part near the top of bar
Effective cross section is lower and the rotor resistance is higher than before
Starting torque is higher - Good
Starting current is lower - Good
Class B Motor

14. Torque-Speed Curves for Different Rotor Designs
Class B Motor

15. Double Cage Rotor Similar to deep-bar rotor
Produces variable resistance
Large bars – low resistance
Buried deep in rotor
Small bars – high resistance
At the rotor surface
Difference b/w low slip and high slip is exaggerated
At high slip (starting condition)
Only small bar is effective
The rotor resistance is quite high
At low slip (normal operating speeds)
Both bars are effective
Resistance is as low as in deep-bar rotor
Design Class B and Class C Motors

16. Torque-Speed Curves for Different Rotor Designs
Class C Motor

17. Comparison of Rotor Bars
Large bars near the surface
Deep Rotor Bars
Double-cage rotor bars for
Class B
Double-cage rotor bars for
Class C
Small bars near the surface

18. Advantages and Disadvantages of Double-cage Rotor Bars
They are cheaper than wound rotor
High starting torque
Low starting current
Good efficiency at normal speed
Reduced maintenance: No need of slip rings and brushes
Disadvantages
They are more expensive than other type of cage rotors