Polyphase Induction Motors MCQ Questions & Answers | Electrical Engineering

Slip rings in polyphase induction motors are typically made of phosphor bronze. This material is chosen because it has excellent wear resistance and conductivity. Phosphor bronze also provides a good balance between conductivity and durability, which is essential for the efficient operation of the slip rings in the motor.

Slip in an induction motor is the difference between the synchronous speed (the speed of the rotating magnetic field) and the rotor speed (the actual speed of the motor shaft). It's usually expressed as a percentage.

* Option A (0.04 %): This is too low. A slip this small would mean the rotor is almost running at the same speed as the rotating magnetic field, which isn't typical for medium-sized induction motors under load.
* Option B (0.4 %): This is also quite low. It's more typical of very large induction motors or motors operating with very light loads.
* Option C (4 %): This is the most common slip value for medium-sized induction motors under normal operating conditions. It provides a good balance between torque production and efficiency.
* Option D (14 %): This is a very high slip value. Such high slip is not ideal because it causes more losses in the motor, creates excessive heat, and reduces efficiency. It might be found in specific applications requiring high starting torque.

In summary: For medium-sized induction motors, a slip of around 4% is the most realistic and commonly encountered value.