Polyphase Induction Motors - Study Mode

[#46] A 3-phase 440 V, 50 Hz induction motor has 4% slip. The frequency of rotor e.m.f. will be
Correct Answer

(C) 2 Hz

Explanation

Solution: 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.

[#47] DOL starting of induction motors is usually restricted to
Correct Answer

(A) Low horsepower motors

[#48] In induction motor, percentage slip depends on
Correct Answer

(A) Supply frequency

Explanation

Solution: 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.

[#49] Reduced voltage starter can be used with
Correct Answer

(C) Squirrel cage as well as slip ring induction motor

[#50] The speed of a squirrel-cage induction motor can be controlled by all of the following except
Correct Answer

(C) Changing winding resistance