How-to Meg a Motor

17 December 2020


Well, if we’re nit-picking, technically speaking, you can’t ‘Megger’ a motor. Megger is a registered trademark, not a verb, but we get it – old habits die hard. Plus, we’re flattered that you’re throwing around our name so much, so we can’t complain.

But what you’re really asking is – how can you run an insulation resistance test on a motor? And that is a question we can definitely help you with, regardless of whether you say Megger, megging, or meg a motor. To each their own, right? Plus, this is a blog, not a lecture hall.

So, let’s start with the why.

Why would you need to Megger a motor? Or better yet…

Why should you be running insulation resistance tests on your motor?

If you’re working with brand-new, shiny motors in your plant, then your 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 rascals 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 your motor’s insulation periodically is key. By the way, 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 make sure you’re keeping good records.

With a preventative maintenance plan, you can schedule reconditioning or repairs before complete service failure. If you like saving money and preventing down time, then this one is for you!

Plus, failure to check your motor’s insulation could result in dangerous conditions when you apply voltage, or your motor could burn out entirely.

Now, for the main event.

How can you run an insulation test on a motor?

First things first, you’re going to need an insulation tester, a megohmmeter, or an all-in-one rotating machine tester (if you’re tired of lugging multiple test instruments around your plant), 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 (thanks Ohm’s Law).

Also, it’s really important to remember that you should never, under any circumstance, ever connect a Megger insulation tester (or any IR tester for that matter) to energized equipment. Now that that’s covered, let’s talk about hooking up the test.

For AC motors and starting equipment, check out the below diagram from A Stitch in Time – our 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.

So, you ran your test, now what? Let’s chat about your results.

How do you interpret resistance readings?

Well, for motors, we always recommend you grab a copy of IEEE’s 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.

But the biggest recommendation we can give you is as follows…

Periodic testing is key.

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

If you’ve already grabbed your copy of a Stitch in Time, our complete guide to electrical insulation testing, then you’re all set – for now. Just hang tight though because we’ll have some more tricks of the trade for megging a motor coming out on the blog soon. Specifically, if you’ve been looking for a step-by-step procedure for running various insulation tests, you won’t want to miss it.