Identifying the Incorrect Statement for Squirrel Cage Induction Motors

Let's analyze each statement to determine which one is incorrect regarding squirrel cage induction motors.

Statement 1: It has fixed rotor resistance.

This statement is correct. The rotor of a squirrel cage motor consists of conductive bars (like aluminum or copper) inserted into slots in the rotor core and short-circuited at both ends by end rings. The resistance of this rotor circuit is determined by the design of these bars and end rings and is inherently fixed. It cannot be changed during operation like in a wound rotor motor.

Statement 2: It has rugged construction.

This statement is correct. The squirrel cage rotor design is very simple and robust. It lacks windings, slip rings, and brushes, making it highly durable, reliable, and requiring minimal maintenance. This ruggedness is a major advantage of this motor type.

Statement 3: The rotor acts as a short-circuited secondary of a transformer.

This statement is correct. The operation of an induction motor is often explained using a transformer analogy. The stator winding acts as the primary winding, and the rotor bars act as the secondary winding. The rotating magnetic field produced by the stator induces a voltage across the rotor bars, causing current to flow, just like in the secondary winding of a transformer. Since the rotor bars are short-circuited by the end rings, the rotor effectively behaves like a short-circuited secondary.

Statement 4: Brushes are required for transferring the power.

This statement is incorrect. Squirrel cage induction motors do not require brushes for power transfer to the rotor. Power is transferred from the stator to the rotor wirelessly via electromagnetic induction. The rotor circuit is self-contained and internally short-circuited. Brushes and slip rings are features found in wound rotor (slip ring) induction motors, used to connect external resistance to the rotor circuit for starting or speed control, but they are absent in squirrel cage designs.

Conclusion

Based on the analysis, the statement that is INCORRECT for squirrel cage induction motors is that brushes are required for transferring the power.