An eight pole wound rotor induction motor operating on 60 Hz supply is driven at 1800 rpm by a prime mover in the opposite direction of revolving magnetic field. The frequency of rotor current is

Option 1: 50 Hz

Option 2: 200 Hz

Option 3: 120 Hz

Option 4: 180 Hz

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Correct Option: 3

Explanation

Given that the Nr = -1800 rpm(rotating in opposite direction) N = 120×60 s = 900rpm 8 The slip s =   Ns−(−Nr ) 900−(−1800) = = 2700 = 3 Ns 900 900 fr = sf ⇒ fr = 3 × 60 = 180 Hz