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Measurement Of Insulation Resistance
Insulation resistance is the direct current resistance of the insulation which corresponds to leakage current flowing inside and on the surface of the insulation. Decrease in insulation resistance is caused by deterioration or absorption of moisture of the insulation, adhesion of dirt, and temperature rise. The values of insulation resistance obtained by measurement decreases as the test voltage increases and as the duration of the test voltage application is shortened. Since insulation resistance is a useful measure for determining the condition of insulation, it should be measured periodically, while the motor is not in operation, and records of the measuring apparatus used, coil temperature, ambient temperature, relative humidity, and other conditions (for example, the time elapse after motor stoppage) are to be kept.
The motor, if contaminated, is to be cleaned in the aforementioned manner, the insulating resistance before and after cleaning is to be recorded. Either 500V rotary megger or the-electron tube megger may be used. With a rotary megger, rotate it continuously for at least one minute. With an electron tube megger, apply voltage for 10 minutes. Record insulation resistance values for 30 seconds, 1 minute, and 10 minutes. Recording of time is necessary since insulation resistance varies with the duration of the test because of charge current or absorbed current. Insulation resistance varies with changes in temperature and humidity, but the pressure of salty substance affects the value greatly. If this -happened, consult Mitsubishi Electric Corporation.
To be continued ..... soon