Why do you Megger a motor?

Megger testing measures resistance between the phase windings of the motor under test and ground. If there is an insulation breakdown, the resistance is negatively impacted and there is a departure between phases or between one measurement and a previous one and a determination of motor health is made.

Why do we Megger a motor?

To measure the resistance in a three-phase motor, you will need a megohmmeter, otherwise known as a megger. Unlike an ohmmeter, a megger can measure high-voltage resistance values, which are generally the kinds of values you will encounter with three-phase motors.

Why do we need to check the insulation resistance of a motor?

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.

What does Megging a motor tell you?

It specifically tests the motor windings insulation. Depending on the insulation class of the motor, a good motor could measure well into Tera Ohms or 1,000,000,000,000 ohms! This is an insulation test of motor windings, commonly referred to as megging, to megger, or to meg a motor.

What voltage should you Megger a motor?

The rule may be stated: Insulation resistance should be approximately one megohm for each 1,000 volts of operating voltage, with a minimum value of one megohm. For example, a motor rated at 2,400 volts should have a minimum insulation resistance of 2.4 megohms.

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Can a Megger damage a motor?

Never use a megger if the motor windings are under a vacuum. … A high voltage for too long of a time may further weaken or fail motor windings and the winding insulation could be damaged by the testing procedure.

What is a good motor Megger reading?

This is how the test works: DC voltage of 500 or 1000 V is applied between the windings and the ground of the motor.



A general rule-of-thumb is 10 Megohm or more.

Insulation resistance value Insulation level
5-10 Megohm Abnormal
10-50 Megohm Good
50-100 Megohm Very good
100 Megohm or more Excellent

What is a bad Megger reading?

What is a bad Megger reading? If the megger reads a resistance under 1 (1,000 ohms) on your meter after the initial 60-second interval, the cable has failed and the cable should be removed. If the megger reads a resistance between 1-1.25 on your meter, then the cable passes.

What is a bad insulation resistance reading?

The purpose of the IR test is to check for damaged insulation, this can be mechanical damage or damage by heat, (overloaded cables), readings less than 2 Mohm indicate damaged insulation, readings of 2-50 Mohm are indicative of long circuit lengths, moisture and contamination and do not indicate the insulation quality, …

What could cause the insulation of a motor to fail?

Around 55% of insulating failures in motors occur due to overheating. Overheating can be caused by poor power quality, or a high temperature operating environment. For every 10oc that the temperature of a motor rises, the insulation life reduced by 50%.

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