How to Measure Electric Motor Insulation Resistance

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To prolong the life of electrical systems and motors, regular insulation resistance testing is required. Over the years, after many cycles of operation, electric motors are exposed to environmental factors such as dirt, grease, temperature, stress, and vibration. These conditions can lead to insulation failure, resulting in loss of production or even fires.


An effective motor insulation resistance system has high resistance, usually (at an absolute minimum) greater than a few mega ohms (MΩ).  A poor insulation system has lower insulation resistance. The optimal insulation resistance for an electric motor is often determined by the manufacturer’s specifications, the criticality of the application where the motor is used, and the environment where it is located.

It is practically impossible to
determine rules for the actual minimum insulation resistance value of an electric motor because resistance varies according to method of construction, condition of insulation material used, rated voltage, size and type. A general rule-of-thumb is 10 Megohm or more. The insulation system of an electrical motor is said to be in good condition if:
Measured Insulation resistance is greater than or equal to 10MΩ

Typical Insulation Resistance Level for Electric Motors
There are no rules for determining the minimum insulation resistance value for a motor. Most data available are empirical. The ones listed below are from grundfos, a leading manufacturer of electrical motors:

Insulation Resistance Level

Insulation Level

2MΩ or Less

Bad
2 - 5MΩ 
Critical
5 - 10MΩ
Abnormal

10 - 50MΩ 

Good
50 - 100MΩ
Very Good
100MΩ or More 
Excellent

How to Measure Insulation Resistance of a Motor
The measurement of insulation resistance is carried out by means of a mega ohmmeter – a high resistance range ohmmeter. To measure insulation resistance, a DC voltage of 500V or 1000V is applied between the windings and the ground of the motor as shown below:

During the measurement and immediately afterwards, do not touch any terminals of the motor as some of them carry dangerous voltages that may be fatal.
The minimum insulation resistance of motor measured to ground at 500V can be measured at a winding temperature of  -15°C – 20°C. The maximum insulation resistance can be measured at 500V with operating windings temperature of 80-120°C depending on the motor type and efficiency

How to Calculate Minimum Insulation Resistance of Motors
The minimum insulation resistance of any motor, Rmin, can be calculated by multiplying the rated voltage, VR, with the constant factor 0.5 MΩ /kV:
Rmin = 0.5*VR


Regular Checks of Motor Insulation Resistance The key to prolonging the life of any electrical device is periodic checks & maintenance. The insulation resistance of stored and active motors should be checked regularly:
(a)  If the insulation resistance of a new, cleaned or repaired motor that has been stored for some time is less then 10 MΩ, the reason might be that the windings are humid and need to be dried.
(b) For a motor in operation, the minimum insulation resistance may drop to a critical level. If the measured value of insulation resistance is greater than 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 to prevent harm to personnel due to the high leakage voltage