Which of the following is true, considering the rotor of a three-phase induction motor in the blocked state?

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MPPGCL JE Electrical 28 April 2023 Shift 1 Official Paper
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  1. Supply frequency > Rotor frequency
  2. Supply frequency = Rotor frequency 
  3. Supply frequency < Rotor frequency
  4. Rotor frequency = 0 (Always)

Answer (Detailed Solution Below)

Option 2 : Supply frequency = Rotor frequency 
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Detailed Solution

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Concept

The slip of an induction motor is given by:

where, Ns = Synchronous speed

Nr = Rotor speed 

Explanation

The rotor frequency of a 3ϕ induction motor is given by:

When the rotor of a three-phase induction motor is in the blocked state (not rotating), the rotor speed, i.e. Nr = 0

s = 1

The rotor induced frequency is:

Supply frequency = Rotor frequency 

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