Induction Motors Lecture 4 Abdul Afram BEE1-ABC

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

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”

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

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

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

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

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

Torque-Speed Curves for Different Rotor Designs Class A Motor

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

Torque-Speed Curves for Different Rotor Designs Class D Motor

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

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

Torque-Speed Curves for Different Rotor Designs Class B Motor

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

Torque-Speed Curves for Different Rotor Designs Class C Motor

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

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