Deterioration Dilemma: Why Electric Motors Fail

28 February 2020


As a loyal blog reader, you already know that when we are talking about motor failure – 60% of insulation failures are caused by excessive heat, since you’ve already read Motor Mayhem – of course. 

What about the other 40% of insulation failures?

You’re in the right place.

Before you jump right into testing your motor, you need to understand what’s happening inside of motor, specifically – the insulation aging process. Once you’ve got that covered, we’ll give you the go-ahead to grab your trusty tester.

What does my motor’s insulation system look like?

Inside your motor, you’ll find groundwall, phase-to-phase, and turn-to-turn insulation.

In a typical induction motor, the groundwall insulation is a bodyguard to the insulated copper to ground – made of slot liner paper. Phase-to-phase insulation is usually a sheet of insulation paper – you guessed it – squished between the phases. Finally, your turn-to-turn insulation is going to protect the copper on your motor’s windings or coils. Unfortunately, this is the weak link in an insulation system (usually). Both turn-to-turn and phase-phase insulation fall under the category of copper-to-copper insulation, for the record.

Here’s another bummer for your Friday morning. You can’t just test the entire insulation system at once. You’re looking at several different tests for each of our various types of insulation.

What tests do you recommend for a motor’s insulation system?

Let’s break it down by insulation type. For groundwall insulation, you can use a megohmmeter (an insulation resistance tester) to determine the insulation resistance. A polarization index test is also recommended to evaluate the elasticity of your insulation, as well as detect any moisture or contamination that may be present in the windings. You can also use a  DC hipot to verify the insulation’s dielectric strength.  

For both turn-to-turn and phase-to-phase insulation, surge testing is the answer. The surge test is not for testing the groundwall insulation though. We’ll get into more details of surge testing later on.

Now that we’ve got that covered…

Why is my insulation deteriorating, anyways?

Well, there’s five factors at play, and one – or more – of these is likely to blame.

  • Contamination – If a chemical deposit gets into the windings, deterioration is very possible. Makes sense though, right? Chemical spills are usually not a great sign, and a good reason to panic. Unless – of course – you dropped a gallon of H2O or a pound of C12H22O11 on the floor. No need to panic, just grab a paper towel (or 50) or call your dog into the kitchen.
  • Mechanical – Over time, vibration or movement in a motor’s windings (or the motor itself) will wear down the insulation system.
  • Normal Thermal Aging – With normal operation, a winding’s insulation will naturally deteriorate – albeit slowly – throughout its lifetime. That’s just normal (expected) wear and tear, folks.
  • Overvoltage Spikes – Switching, lighting, and VFD designs can all cause high voltage surges, which can lead to insulation aging.
  • Early Thermal Aging – Ah, we are back to the topic of excessive heat. A hot motor is not a happy motor. If your motor’s winding temperatures are on the rise, premature motor failure is right around the corner too. Of course, you already know this because it was the very first sentence of this very blog.

Now what?

Maybe, you’re wondering how a motor goes from insulation deterioration to complete motor failure?

Well, it’s the combination of weak insulation, along with steep-front surges, that will accelerate deterioration and lead to a motor’s ultimate downfall – electric failure. Ugh.

By the way, steep-fronted surges commonly occur when switching a motor on and off.

Mechanical abrasion in a motor’s winding is another cause for concern – speeding up insulation degradation, specifically in the turn-to-turn insulation. 

Okay, you’ve gotten all the background information.

Let’s get back to that surge test we mentioned before.

What’s a surge test?

Surge testing has been around for almost 100 years but compared to the classical suite of motor tests – it’s practically new! Unlike those tests, the surge test can detect an insulation defect in turn-to-turn, coil-to-coil, and phase-to-phase insulation. Pretty handy.

Surge testing is based on the assumption that in a stator with absolutely no winding defects, all three-phase windings are identical. However, not all motors are designed to be perfectly balanced between phases. Most are, but not all. Then, each phase is tested against each other – A to B, B to C, and A to C.

Additionally, a Pulse to Pulse Error Area Ratio (P-P EAR) assessment can evaluate each phase - unto itself - as the amplitude of the test increases to the target test voltage. This test parameter provides a reliable assessment in the event that the motor being tested is unbalanced by design or when testing an assembled motor that may appear to be artificially unbalanced due to rotor position.

Get it?

For each of the pairs being tested, the test instrument will send out a quick voltage pulse, and the reflected pulses are displayed on the instrument’s oscilloscope screen. Check out the image below for an example, if you’re confused.


If the windings are identical – as they would be in a healthy motor – you’d see a single trace. If you see separation between the two waves, as the test voltage is increased – you’re looking at weakened insulation in your motor.

Why should you do a surge test?

Because we said so! Just kidding.

Surge testing is an imperative component of a predictive motor maintenance program. Since all motors will eventually fail – without proper maintenance and repair – a comprehensive maintenance program is a no-brainer (or at least it should be).

Because the surge test can predict insulation failure in copper-to-copper insulation - as we’ve now mentioned at least ten times – there’s no doubt that this test has earned a place in your maintenance program.

Of course, the surge test is not the be-all end-all of motor testing, as it should be used in conjunction with those other tests we mentioned earlier – like insulation resistance and polarization index.

If you like what you just read, check out these 20+ Reasons to Start Testing Your Motor Today or share this post by clicking the share link at the top!