Polyphase Induction Motors MCQ Questions & Answers | Electrical Engineering

The slip in an induction motor is the difference between the synchronous speed and the actual rotor speed, expressed as a percentage of the synchronous speed. The frequency of the rotor emf is related to the slip and the supply frequency. For a 3-phase induction motor:

Supply Frequency (f_s): 50 Hz
Slip (s): 4% or 0.04
The frequency of the rotor emf (f_r) can be calculated using the formula:

f_r = s × f_s

Substituting the values:

f_r = 0.04 × 50 Hz = 2 Hz

Therefore, the frequency of the rotor emf is 2 Hz.

Percentage slip in an induction motor is given by the formula:
slip (%) = [(Ns - Nr) / Ns] * 100
where Ns is the synchronous speed and Nr is the rotor speed.

Synchronous speed (Ns) is determined by the supply frequency (f) and the number of poles (P) of the motor, given by:
Ns = (120 * f) / P

Thus, slip depends on:
1. Supply frequency: Changes in supply frequency affect the synchronous speed, which in turn affects the slip percentage. Higher frequency results in higher synchronous speed and can reduce the slip if rotor speed remains constant.

Other options are incorrect because:
Option B: Supply voltage: While supply voltage affects the torque and current characteristics of the motor, it does not directly determine the slip percentage.

Option C: Copper losses in motor: Copper losses affect the efficiency and heating of the motor but do not directly influence the slip percentage.

Option D: None of the above: This is incorrect because supply frequency directly affects the slip percentage.

Therefore, the correct answer is Option A: Supply frequency.