INDUCTION MOTORS AND THEIR USES

[seminar class]

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Induction Motors
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
1. Higher starting torque
2. Lower slip at normal speed (good efficiency)
3. Lower starting current
 How to achieve these goals
1. Higher starting torque is achieved by increasing rotor resistance
2. Lower slip at normal speed is achieved by decreasing rotor resistance
3. 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
Leakage Reactance