19 Apr 2016

An instructional video on how to megger a motor and the proper use of a megohmmeter.

Have you ever wondered what a megger test does?  What does a megohmmeter do? It is a specific meter designed to measure mega ohms…. and beyond. What exactly is a mega ohm?

One Mega Ohm is equivalent to 1,000,000 Ohms. Why do we need to measure that high of resistance? Rather than thinking in terms of measuring resistance, think of this as a testing device. 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. We are testing the motor’s insulation, more specifically between the motor windings and the motor casing. If a fraction of current flows, it can be measured by these highly sensitive meters. Motor insulation is rated and built to withstand varying temperatures. Time and heat break the motor’s insulation down. Using a megohmmeter properly will test any motor insulation. Depending on what class of insulation you are testing will have an effect on the amount of ohms the meter should read. To test high class insulation you should use a megohmmeter capable of testing into tera ohms. Otherwise your test may not be reliable or accurate.

Triple Phase Training made the video above as a free tool for all to use. We hope this video will clear up how and why to use these meters. When a megohmmeter is used properly you will be able to easily identify insulation damage. Remember heat breaks down motor insulation, so if you are not sure about a motor you have just tested, try testing the motor again while it is still warm from use. If you get lower resistance readings each time, your motor should be taken out of service as soon as possible.

Most variable frequency drives and inverters are very sensitive to insulation loss, the drive may preemptively shut down the motor to save the drive from damage. Make sure the motor rating matches the environment, and check the rating for heat as well as service factor.

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