How to Megger a Motor

January 18th, 2021:
Technically speaking, you can’t "Megger" a motor. Megger is not a verb, but rather a registered trademark for a manufacturer of electrical test and measurement instruments, which coincidentally can be found on our website HERE. However, over the years, due to their strong reputation in the industry, the word Megger has been used to describe the act of taking insulation resistance measurements. Therefore, the real question is - how can you run an insulation resistance test on a motor?

Please Note: Information taken from a Megger application note and mainly pertains to products sold by Megger. However, most of the principles still apply.

So why would you need to Megger a motor? Or why should you be running insulation resistance tests on your motor?

When working with brand-new motors, the electrical insulation should be in tip top shape. However, despite major manufacturing improvements to motors over the years, insulation is still susceptible to classic wear and tear, as well as other villains like mechanical damage, vibration, excessive heat or cold, dirt, oil, corrosive vapors, moisture from processes, or just natural humidity, which can cause insulation failure.

Over time, these issues cause tiny holes and cracks, allowing moisture or foreign particles to leak into the surface of the insulation – giving way to a low resistance path for leakage current. And once that starts, there’s no turning back. Typically, the resistance drop is gradual though, which is where electrical testing comes in.

Checking the motor’s insulation periodically is key. Good insulation has a high resistance, whereas poor insulation has a relatively low resistance. The actual values might vary depending on the temperature or humidity, so keeping good records is a must.

With a preventative maintenance plan, complete service failure can be avoided by scheduling reconditioning or repairs ahead of time. This not only saves down time, but costly repairs as well! Furthermore, failure to check the motor’s insulation could result in dangerous conditions when voltage is applied and the motor could burn out entirely.

But how can you run an insulation test on a motor?

First you’re going to need an insulation tester, a megohmmeter, or an all-in-one rotating machine tester such as the Megger MTR105 - Multifunction Rotating Machine Tester, Insulation Tester, Ohmmeter & Multimeter, which will give you a measurement in ohms or megohms.

Keep in mind that this test is non-destructive, so you don’t have to worry about further damaging your motor’s insulation. Your instrument will simply apply a voltage and measure the resulting current over the insulation’s surface – giving you the resistance value.

*IMPORTANT* You should NEVER, under any circumstance, connect a Megger insulation tester (or any IR tester for that matter) to energized equipment!

Hooking Up the Test

For AC motors and starting equipment, check out the below diagram from Megger's "A Stitch in Time... The Complete Guide to Insulation Resistance Testing. Note that the starting equipment, connecting lines, and motor are in parallel, and the starter switch is set to “on”. It’s always better to disconnect component parts too and test them all separately, so you can know precisely where a weakness exists.

For DC generators and motors, you’ll need to raise the brushes, as shown in the figure below. You can also test the rigging and field coils separately, from the armature itself.

How do you interpret resistance readings?

For motors, it is always recommended to get a copy of the IEEE guide, "Recommended Practices for Testing Insulation Resistance of Rotating Machinery” as it is the most complete resource for dealing with the problem of interpreting insulation resistance measurements for motors.

However, in general, periodic testing is key!

While there are guides and rules for minimum values of insulation resistance, your best indication of trouble is a consistent downward trend in IR measurements. And this can only be achieved if you’re testing periodically and keeping good records.

Note: Information taken from an application note courtesy of Megger (via the Megger website). Ram Meter Inc. sells and stocks an assortment of Megger insulation resistance testers and other products, which can all be found on our website at You can also check out our full selection of other Insulation Resistance Testers.