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The insulation resistance of motor shall be measured between _________ by using megger.
1. Winding to rotor
2. Winding of machine and its frame
3. Between two windings
4. Winding and direct earth

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Correct Answer - Option 2 : Winding of machine and its frame

Measurement of Winding insulation resistance:

  • The measurement of insulation resistance is carried out by means Megger (500 V or 1000 V) normally at 160 RPM.
  • The insulation resistance of each phase winding against the frame and between the winding shall be measured.
  • The minimum acceptable value of the insulation resistance varies with the rated power and rated voltage of the motor.

 

The following equation is used while measured the insulation resistance.

 \(IR = \frac{{20 \times E}}{{1000 + 2P}} \)

Where,

IR is the insulation resistance in MΩ at 25° C

E is rated phase voltage and,
P is rated power in KW

  • If the resistance is measured at a temperature different from 25° C, the value shall be corrected to 25° C.
  • The insulation resistance is measured at ambient temperature does not always give a reliable value because of environmental condition.

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