How to measure insulation resistance of motor winding.

Winding insulation resistance checking of three-phase /  1 phase Motor

The motor is mostly used machine to drive loads in the industry, for pumping, elevators, conveyor belts. so its reliability must be maintained, for long life of motor its periodic inspection must be carried out without fail. one of the most important point of periodic checking is motor winding insulation testing this is done with the help of megger. Some electrical motors are operated in high humidity atmosphere, hence time to time motor winding insulation resistance measurement, motor Insulation condition checking must be done, for a longer life of motor.

If the motor is not put into operation immediately after arrival, it is important to protect motor against moisture, high temperature and impurities in order to avoid damage to the winding insulation. Before the motor is put into operation after a long period of storage, you have to measure the winding insulation resistance.

If the motor is stored in a place where humidity is high, periodical inspection of motor is necessary. It is practically impossible to determine the actual minimum winding insulation resistance value of a motor because resistance changes according to method of construction, condition of insulation material, rated voltage, size and type. In fact, it takes many years of experience to determine whether a motor is ready for operation or not. 

 A general rule-of-thumb is 10 Meg-Ω or more. The measurement of Insulation resistance is carried out by using Megger- high resistance range ohmmeter.

This is how the motor Insulation test works: 


DC voltage of 500V  or 1000 V is applied between the motor windings and the ground

Motor Winding Insulation resistance calculation:

insulation resistance of motor winding.
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 MΩ/kV. 

For example: 

If the rated voltage is 440 V = 0 .44 kV, 
the minimum insulation resistance is: 

0 .44 kV x 0 .5 Megohm/kV

=0 .22 Megohm Measurement 

Minimum motor insulation resistance of the winding to the ground is measured with 500 V DC . The winding temperature must be 25°C +/- 15°C . 

Maximum insulation resistance should be measured with 500 V DC with the windings at an operating temperature of 80-120°C depending on the motor type and efficiency. 

Checking 


If the insulation resistance of a new arrived, 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 due to moisture and need to be dried before putting in operation. 

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

Reference: Grundfos Motor Book

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