Megger vs. Meter

are there any differences between checking to see if an AC motor is good using a megger or multimeter. Is one inherently better than the other? Will they ever have different results on the same motor?

thanks
Mike

If you use a multimeter and get a zero reading between the frame and the windings the motor is shorted, megger will not tell you anything different. Technically any zero reading between phases or phase to frame is a short.

A megger is meant for regularly testing motors etc to determine the breakdown in the insulation resistance. A new motor should be tested before installation, periodical tests using that baseline information and mfg info will tell you when the motor is reaching the end of its life cycle...its Preventative Maintenance.

rsdoran said:

If you use a multimeter and get a zero reading between the frame and the windings the motor is shorted, megger will not tell you anything different. Technically any zero reading between phases or phase to frame is a short.

A megger is meant for regularly testing motors etc to determine the breakdown in the insulation resistance. A new motor should be tested before installation, periodical tests using that baseline information and mfg info will tell you when the motor is reaching the end of its life cycle...its Preventative Maintenance.

Click to expand...

You also use the megger to test the cables.

Frankly I wouldn't just test with a multimeter. You'll end up having an explosion sometime in your career. A megger applies a much higher voltage that allows for real time simulation of potential short.

I stated "if you obtain a ZERO reading" it does not matter whether its a meter or a megger, the result is the same...its shorted. I hope that after obtaining that reading there would be no power applied ie no explosion can occur.

The question referenced motors so I did not go into all the details of what a megger or meter does.

IF the motor in question is having problems but a ZERO reading with a meter is NOT OBTAINED then you would use a megger to see if a breakdown is occuring under power...as mentioned the leads may need testing too.

My point was ..IT DEPENDS, in many cases a megger is not actually used properly nor the information obtained from the megger readings capable of giving a definitive answer. You need baseline information to interpret megger readings if they are not zero...if zero its a short and in many many cases a meter can tell you that....again IT DEPENDS.

Everything said so far is accurate. The difference is the test voltage.
A digital meter has .1 volts (100 millivolts) between it's leads on Ohms.
It has 1 Volt on Continuity or Diode test.

A Megger is much higher. It's been a while since I've used one, but I think 100 volts is typical. I believe you can select the voltage.

For a motor that's shorted, the results will be the same. You will get zero ohms regardless of the test voltage used. At this point, you already knew that because the the fuse blew or breaker tripped.

The simple answer is to use a Megger.

thanks for the replies. The reason for my question was because we had a small 1/2 horsepower motor(AC) go out the other night. I took it loose from the gearbox and watched the thing as we applied power to it. The motor never moved. So I assumed it was bad.

Well one of the electricians showed up later with his multimeter and checked it, and said the motor was good. However, I know he didn't check it with a megger. So I was wondering if there could be any differences.

One note, the motor does start up on a variable speed drive and was set low. Maybe the current was too low for this motor. Even though it usually works on this setting, it however did not work on the day we tested it.

Normally I would only use a megger to test in a troubleshooting scenario if a mulitmeter test looked good, and all the obvious stuff is ruled out first (mechanical binding, single-phase conditions, VFD problem). Then I will remove the motor leads from the power source. This is very important when dealing with a VFD (or any solid state) driven motor. Our megger has a 500v and a 1000v setting. I connect one lead of the megger to all the motor windings and the other to ground and test the wiring and motor all together for ground faults. Then, if indicated, you can isolate at the motor to determine if there is ground potential caused by a contactor, terminations or wiring, or the motor itself. Never connect the megger to any electronics. Contactor contacts are okay to megohm test, but not drives.

I have seen a regular Fluke 99 show 1 megohm, and a megger showed 2 kohm on the same motor. The high potential of the 1000v megger can reveal defects not possible with a standard multimieter.

We are supposed to be using the megger as part of our planned maintenance program to chart the readings of our larger motors. This info is useful to predict the end of life of windings. When done regularly, just before serious problems arise, there will often be a sharp drop in phase to ground resistance.

Like Ron said, if it checks bad with a multimeter, then you don't need the megger. I check for ground faults with a meter in the same way. One lead on a good ground, and all motor winding leads tied together on the other lead. Usually 1 megohm or higher means it's okay. Also check resistance of each winding. They should be balanced, but the resistance can vary greatly from one motor to the next, or even between different sets of windings of multi-speed motors. If in doubt, compare with specs, or another known good motor.

In my experience, more than 90% of the time, if it checks good with a multimeter for both of those tests, then it should run when powered correctly.

Being VFD driven the best way to check the voltage and current output of the drive is with the drives keypad/display or analog output if available. It's not practical to measure voltage of a VFD output with a multimeter. Even the best Fluke meter we have is inaccurate when hit with the carrier frequencies present on VFD power signals.

Paul C.

As stated before the big difference is the test voltage. You can check it & read good with a standard ohm meter and still have a bad motor. The higher test voltage will help detect faults in the insulation. Also you can have a short in the windings that wont show up on ground, but show from winding to winding. Simply checking for a ground with an ohm meter means nothing unless it actually shows a ground.

Added Info

I find meggers very usefull when working on motors that are powered by VFD's. One thing to keep in mind is that most new drives have very sensitve GFI's built into them. I have been told by multiple drive vendors that if you receive a reading of 200 M ohms or less you should go ahead and start looking at getting the motor rebuilt. The 1/2 Hp motor you mentioned could have been fine if you ran the unit in bypass but as soon as you put it in auto the GFI will trip out and cause a fault. In my experience if the motor is running straight power a standard multimeter will work fine when looking for a short, but when working with drives it pays to use a megger.

Hope this helps,
R.

Simple in my opinion, if asked to check a motor you do need a megger.
Any "megger" I ever had also had an ohms scale so that did the job of the multimeter.
Naturally all leads should be disconnected from the motor.

Nice graveyard, zombies rising up and all...

TurpoUrpo said:

Nice graveyard, zombies rising up and all...

Click to expand...

Yep this megger was just released around the time of the original post.

Faulty Insulation resistance may pass with a multi meter - 9v battery typical.
But with a potential voltage of up to 1000v for testing the megger will highlight the fault.
Low resistance inductive appliances need this type of test.

Pablox said:

Yep this megger was just released around the time of the original post.

Click to expand...

Well you are about an old guy aren't you? Have one similar early 1950's.

While you can check insulation resistance with a VOM - analog ie Simpson are a bit better than digital they are nowhere near as good as a megger. IF very good ie 10 meg or better I would be fairly comfortable with just VOM. If 100 K or less I would megger to ensure - anything less I would tend to trust the VOM but if high dollar cost then would megger.

Equipment you can megger
transformers, any distribution wire and equipment, motors almost any kind, heaters.
BUT NOT SOLID STATE and NOT ENERGIZED.

Biddle has good info - well worth looking at.

Dan Bentler

The whole idea behind testing insulation with a megger is based on the principle that insulation should be tested at or a little above its normal operating voltage. So, on a 480VAC motor, that's a peak voltage of 660V twice each cycle. You would want to use a 1000V megger. On 230VAC, a 500V megger. On 120VAC, a 250V megger.

It is very common to find insulation checking good at 1.5VDC (the output of a VOM) and totally shorted at 1000V. That is because insulation, dirt, and moisture change as the voltage goes up due to arcing and ionization. At some high voltage, all insulation will flash over giving you essentially zero ohms.

As to insulation resistance values at the reject point, I've always heard 1Meg as being the pass/fail point. I've seen AC motors running at 200K but that's just asking for a flash and probably destroying the motor. Incidently, motor insulation that tests unacceptably low will often go back up to very good values once the windings are steam cleaned and then baked dry in an oven.

ANSI has guidelines for testing power line insulation and it is half of the voltage listed above. I've always thought that they were too low on motors, especially motors running on inverter power.

Going beyond megger testing, motor rewinders use another method to test insulation involving placing a high voltage on the wiring and leaving it there for (as I recall) around 10 minutes. It seems that insulation that initially tests good can start to leak and fail after a few minutes of stress. If memory serves, that is called a PI or potential index test.