28 December 2012

Induction Motor Interview Questions Answers

What are the advantages and disadvantages of Induction Motors?
  • It is simple and rugged in construction
  • It is relatively cheap
  • Induction motors require less maintenance
  • Induction motor has high efficiency and reasonably good power factor
  • 3-phase induction machines are self starting
  • Induction motors are essentially a constant speed motor and its speed cannot be changed easily
  • Its starting torque is inferior to dc shunt motor

2. What is the condition for maximum torque in induction motor?
Answer: Starting torque will be maximum when the rotor resistance / phase is equal to standstill rotor reactance / phase

3. Slip ring induction motor advantages and disadvantages compared to squirrel cage motors?
  • High starting torque with low starting current
  • Smooth acceleration under heavy loads
  • No abnormal heating during starting
  • Good running characteristics after external rotor resistances are cut out
  • Adjustable speed
  • The initial and maintenance costs are greater than those of squirrel cage motors
  • The speed regulation is poor when run with resistance in the rotor circuit

4. Methods to control speed of Wound Rotor Motors?
Answer: The speed of wound rotor motors are changed by changing the slip of the motor. This can be achieved by:
  • Varying the stator line voltage
  • Varying the resistance in the rotor circuit
  • Inserting and varying a foreign voltage source in the rotor circuit

5. Explain how Torque-Slip Characteristics vary when adding resistance to rotor circuit?
Answer: The addition of resistance to the rotor circuit does not change the value of maximum torque but it only changes the value of the slip at which the maximum torque occurs

6. Disadvantages of Star-Delta Starting of Induction motor?
Answer: In Star-Delta starting induction motor stator is connected in star connection for starting after picking up speed it is connected to delta connection. When induction motor is connected in star connection stator phase voltage reduced by 1/(31/2 ) times the line voltage. This also results in reduced starting torque (1/3 times compared to delta connection). 

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