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.