Wait, Rewind

19 September 2019


Another week, another blog. Welcome back. Negative Nancy’s (or is it Nellie’s?), this one is just for you. Because sometimes, things go wrong. Unfortunately, we are not talking about break-ups, recipe fails, or when you accidentally wash a bright red shirt with your brand-new white clothes. No, this week we are talking about static diagnostic testing of electric motors, specifically, high voltage testing. Sorry low voltage, we’ll get to you another week.

Oh, before we get started, you might want to head over here for a quick review of static motor testing.

So, what can go wrong?

Well, if you are performing a diagnostic test, chances are that something probably already went wrong. So, it’s a little late for that question. However, when we say, “things go wrong”, we are not referring directly to the diagnostic test itself. Rather, we are talking about the results the static analyzer is shooting back at you. It’s not the test that’s going wrong, it’s what’s happening inside your motor. Make sense?

Getting results is one thing. Knowing what those results mean though, that’s a whole different ball game.

Thus, today’s initiative. We are going to look at 4 potential indications (or results) that your static analyzer might throw your way when you are performing a high voltage diagnostic test.

1. Low MegOhm reading

2. MegOhm Failure (over current)

3. Low PI (polarization index)

4. High PI

Before we get started though, it’s time for a quick visualization exercise. If you’ve ever wanted to be an actor, now is your time to shine. Put on your work boots (and hard hat) and envision that you are a motor repair technician. Maybe, you are a top-of-the-line, distinguished, award winning engineer. Or perhaps, this is the first day on the job, you’re twenty years old, full of caffeine, and you have no idea what’s going on. Pick your fate!

For each of the following indications, imagine you just performed a static diagnostic test with the stated result. We will walk you through a basic background understanding of the results, as well as identify probable causes and next steps. Does that sound like fun or what? Everyone is learning something new today. Let’s get started.

Low MegOhm reading

If you get this result, this is a good sign that your motor’s insulation is struggling. There is contamination, or perhaps, deterioration of its ground wall insulation. Don’t panic though, let’s talk about the next steps. The simplest explanation may be due to moisture in the motor, so heating could potentially bring up these low readings by driving off the moisture. If that doesn’t work, we recommend following up with a polarization index (PI) and dielectric absorption (DA) test.

A PI test is designed to address specific issues related to moisture, motor health, and insulation deterioration by determining the environmental factors present. If you’re thinking, “Well, I don’t want to buy another piece of equipment. These things are kind of pricey.” Stop that! A PI test doesn’t require additional test equipment, since it is included in the suite of tests in most static motor analyzers. You are set.

Like the PI test, the DA test is also an extension of your classic insulation resistance test. While the polarization index is a ratio of the insulation resistance at 1 minute and 10 minutes, the dielectric absorption ratio is calculated by comparing the IR measurements at 30 and 180 seconds (or sometimes 60 seconds). We are going to get in to more specifics of PI testing later on, so hang tight. For now, let’s look at the next steps.

After you’ve completed the PI and DA test, it is common to then set a minimum threshold value to reflect the nature of your motor’s operating environment. For example, many technicians will set a threshold of 100 MegOhm at 40°C for their industrial motors.

Okay, almost there. You’ve cleaned your motor. You’ve inspected every inch of that pesky thing. You are about to lose your mind because the insulation resistance readings are still low. But, there’s still one last resort – complete motor overhaul. In case you are not familiar with the cool motor lingo, a motor overhaul encompasses the total removal, disassembly, cleaning, inspection and the resulting necessary repairs.

MegOhmm Failure (over current)

Next up, MegOhm failure. Basically, a step worse than what we previously discussed. If you’re not hopping on that predictive motor maintenance bus yet, just you wait.

We really do hate to break it to you, but this is not great news. Your motor’s ground wall insulation has failed. Do not pass go, do not collect $200, and do not start your electric motor, please. Unfortunately, at this point, your only option is to identify and isolate the location of the ground wall insulation failure and then rewind and repair. In the worst cases, a replacement motor may also be necessary. We are sorry.

Low PI

Ah, the polarization index test is back again. This time, we are looking at low PI values. These are an indication that you have contamination or insulation deterioration, similar to a low MegOhm reading. If you are one of those people that actually enjoys cleaning, then you’re going to love this. The first step is to make sure that your motor’s insulation system is clean and dry. In severe cases, a motor overhaul may also be necessary.

If your motor’s insulation is clean and dry, then you should be getting PI values greater than 2. Remember that the polarization index is a ratio, so there are no units involved. If you do the math, a PI of 2 (or more) indicates that the measured leakage current has been reduced by half in the 10-minute time frame. This value is susceptible to contamination and moisture, particularly in high-humidity environments, as parallel leakage paths are more prevalent.

There’s one last thing to mention with low PI values. Let’s say you are working with a small motor (<100 Hp). It may be more practical to use a DA (dielectric absorption) test. Small motors have very little detectable polarization, so a DA test may be more representative of your motor’s ability to polarize. This makes sense, right? The dielectric absorption test gives you a value using a ratio with a smaller timeframe (30 seconds and 3 minutes), compared to the 10-minute PI test. Smaller time frame, smaller motor.

High PI

You made it. This is our last indication that something is wrong with your motor. First, a word of caution. If you are working with a modern insulation system, a high PI value is not a red flag. This is normal.

However, if you are working with a motor with an asphaltic insulation system, dating back to before the year 1970, this should raise some concern. Congrats to you though, by the way! Way to keep that motor running all these years. We are beyond impressed.

Unfortunately, it is likely that the insulation has delaminated, which can cause increased air pockets in the system. Delamination is just a fancy way of saying that the insulation has fractured into separate layers. Who knew? Not us. This can cause elevated PI values (anything greater than 6), since the capacitance of the circuit has increased.

The clear, maybe even obvious, solution is to rewind your motor, if it is older than 49 years. Otherwise, don’t do anything. Drop everything and walk away. Just kidding. Don’t drop anything. Take care of your test equipment, folks.


-Meredith Kenton, Digital Marketing Assistant, Megger – Valley Forge