Question

Consider the following statements related to induction motor drives:

1. Power to weight ratio is high.

2. Suitable for operation at high voltage.

3. Suitable for high speed operation.

4. Power converter is simple and economical.

5. Speed control is easy and of low cost.

6. Reliability is good.

Out of these statements
1. 1, 2, 4 and 6 are correct
2. 2, 3, 5 and 6 are correct
3. 1, 2, 3 and 5 are correct
4. 1, 2, 3 and 6 are correct

Answer 1

Correct Answer - Option 4 : 1, 2, 3 and 6 are correct

Advantages of Induction Motor:

• Construction is quite simple in nature.
• Induction motor is Robust and mechanically strong, so working of the motor is independent of the environmental condition.
• A Squirrel cage induction motor does not contain Brushes, Slip rings, and Commutators. Due to this reason, the cost of the motor is quite low and also the power to weight ratio is high.
• However, Slip Rings are used in Wound type induction motor to add external resistance to the rotor winding.
• Due to the absence of Brushes, there are no sparks in the motor. It can also be operated in hazardous conditions.
• Unlike synchronous motors, a 3-phase induction motor has a high starting torque, good speed regulation, and reasonable overload capacity.
• An induction motor is a highly efficient machine with full load efficiency varying from 85 to 97 percent, reliability is good and suitable to operate at high voltages.

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Disadvantages of Induction Motor:

• A single-phase induction motor, unlike a 3 phase induction motor, does not have a self-starting torque. Auxiliaries are required to start a single-phase motor.
• During light load conditions, the power factor of the motor drops to a very low value. This is because, during the start, the motor draws a large magnetizing current to overcome the reluctance offered by the air gap between the Stator and the Rotor.
• Due to the high magnetizing current, the copper losses of the motor increase. This in turn leads to a decrease in the efficiency of the motor.
• Speed control of an induction motor is very difficult to attain. This is because a 3 phase induction motor is a constant speed motor and for the entire loading range, the change in the speed of the motor is very low. The power converter is complex and expensive.
• Induction motors have high input surge currents, which are referred to as Magnetizing Inrush currents. This causes a reduction in voltage at the time of starting the motor.
• Due to poor starting torque, the motor cannot be used for applications that require high starting torque.