Winding Insulation Resistance

If the motor is not put into operation immediately upon arrival, it is important to protect it against external factors like moisture, high temperature and impurities in order to avoid damage to the insulation.

Before the motor is put into operation after a long period of storage, you have to measure the winding insulation resistance .

Measurement of Insulation Resistance

Measurement of Insulation resistance is done using an MEGOHMETER/ MEGGER (high resistance range ohmeter)

How to perform the IR testing

1. Calibrate the Megger to appropriate test voltage.
2. Connect the line probe to the motor terminal (U-V-W)
3. Connect the earth probe to the case or ground.
4. Press TEST and read the insulation Resistance, in megaohms.
   

Interpreting the measure values by comparing to the NOMINAL VALUES:

INSULATION RESISTANCE
The minimum insulation resistance of new, cleaned or repaired windings with respect to ground is 10 Megohm or more.
The minimum insulation resistance, R, is calculated by multiplying the rated voltage Un, with the constant factor 0.5 Megohm/kV.

For example:
If the rated voltage is 690 V = 0.69 kV,
the minimum insulation resistance is: 0.69 kV x 0.5 Megohm/kV = 0.35 Megohm

CHECKING
If the insulation resistance of a new, cleaned or repaired motor that has been stored for some time is less then 10 Mohm, the reason might be that the windings are humid and need to be dried.

If the motor has been operating for a long period of time, the minimum insulation resistance may drop to a critical level. As long as the measured value does not fall below the calculated value of minimum insulation resistance, the motor can continue to run. However, if it drops below this limit, the motor has to be stopped immediately, in order to avoid that people get hurt due to the high leakage voltage.

#ELECTRICAL 101
#STEEMIT # MOTOR TESTING
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