# Megging a 3 phase 480 volt motor



## jtashaffer

What should the ohms be going from ground to phase on a digital megger?
For a good motor should it read around what (400megaohms)? When the motor is bad it should read 0 ohms to ground? Need help clarifying this.


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## John Valdes

I found this in my documents. I think Zog posted it sometime back.


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## Deathyy

jtashaffer said:


> What should the ohms be going from ground to phase on a digital megger?
> For a good motor should it read around what (400megaohms)? When the motor is bad it should read 0 ohms to ground? Need help clarifying this.


I hope you figured our your voltage issue! :thumbsup:

Deathyy
http://www.acmotorcontrol.net


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## Zog

jtashaffer said:


> What should the ohms be going from ground to phase on a digital megger?
> For a good motor should it read around what (400megaohms)? When the motor is bad it should read 0 ohms to ground? Need help clarifying this.


It depends on the type of motor but more importantly the time of your test and the tempature of the windings at the time of test. You should apply your test voltage for 10 minutes (To allow time for absorbtion and polarization currents to decay) and take readings at 30 seconds, 60 seconds, and 10 minutes. Your 10 minute reading is your insulation resistance and needs to be tempature corrected to 40 degrees C to have any meaning. You should caluculate DAR and PI ratios as part of your test to really have a true indacation of the condition of your motor.


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## ajay34

480vac motor. easly word what ever above 150mg consider the motor are good 
normally you wont get any reading to ground that for sure.if you motor are running 
overheated or when the winding are giving away than you might find reading when measure to ground ..


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## SparkChaser

Fluke has a good app note on insulation resistance testing:
http://support.fluke.com/find-sales/Download/Asset/1579160_6115_ENG_C_W.PDF


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## yardbird

*megger question*

Hi.
In the Navy and at the shipyard that I work at, we have always used the rule that a motor must have at least 1 meg to ground to be safe, but this is the bare minimum. In a perfect world, it would read infinity. Most motors with some years on them would probably read any place between 40 to 20 megs, but that depends on things like exposure to dirt, water, and heat.
That is why we have had the program in place that all motors we had should be megged once or twice a year and the megs recorded on a card for that motor, so you can see if there is a trend . In normal use you will see a very gradual lowering as the motor ages, but if there is a big lowering of megs in a short period of time, you know that there may be trouble brewing inside, and the motor might need to be checked out or cleaned. Hope that helps.


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## mrmike

jtashaffer said:


> What should the ohms be going from ground to phase on a digital megger?
> For a good motor should it read around what (400megaohms)? When the motor is bad it should read 0 ohms to ground? Need help clarifying this.


When a motor reads 0 ohms this is a dead short or "grounded", "Fried". . We used a rule of thumb of anything under 20 megohms was bad. after that it was questionable until after 50 meg. When are readings went towards infinity we knew it to be good.
If you are meggaring it from the contactor or source of your voltage, don't forget that you are also meggaring the cable. So you want to unwire the motor & meggar it there to verify the motor or the cable.

Note* A motor that got wet may read bad, and can be saved if dried out & re-meggared ok.


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## Big John

yardbird said:


> ...That is why we have had the program in place that all motors we had should be megged once or twice a year and the megs recorded on a card for that motor, so you can see if there is a trend....


 They had that program when I started, and I axed it because it wasn't being done right to begin with, but I also think that an annual megger is a waste of time. Have you guys actually tracked and prevented failures with yearly meggers? 

-John


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## Wirenuting

yardbird said:


> Hi.
> In the Navy and at the shipyard that I work at, we have always used the rule that a motor must have at least 1 meg to ground to be safe, but this is the bare minimum. In a perfect world, it would read infinity. Most motors with some years on them would probably read any place between 40 to 20 megs, but that depends on things like exposure to dirt, water, and heat.
> That is why we have had the program in place that all motors we had should be megged once or twice a year and the megs recorded on a card for that motor, so you can see if there is a trend . In normal use you will see a very gradual lowering as the motor ages, but if there is a big lowering of megs in a short period of time, you know that there may be trouble brewing inside, and the motor might need to be checked out or cleaned. Hope that helps.


I almost feel bad referring to you as "YardBird". 
In my day Yardbird & Sand Crab were unflattering names we called the civilian workers. 

Megging a motor so often provided "Make Work" for underway. You could only do 8 hours worth of work using the MRC cards. 
But tracking the motors did provide valuable information. 
It gave the Yards time to locate a replacement. This might include pulling an old one from a mothballed ship and rewinding it for your use. 
I have heard that with a smaller mothballed fleet, some items are becoming scarce. The same can be said of shipyards. Fewer "Old" yards means we are fabricating "new", old parts at a higher cost. 
What class are you riding? Are you in pearl?


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## lefleuron

Big John said:


> They had that program when I started, and I axed it because it wasn't being done right to begin with, but I also think that an annual megger is a waste of time. Have you guys actually tracked and prevented failures with yearly meggers?
> 
> -John


 
I agree John. If you have hundreds of motors, or even thousands, doing a yearly Meg test is a waste of time and money. You figure that in a year out of a thousand motors, you will lose 20-40 ave. 

By Megging them and trending, you may catch a few that are trending badly and even then its a judgement call one whether to change it out or not.

Once you figure in the time it takes to un-wire, Meg, rewire each of those motors every year compared to swapping them as they go, you are money ahead to just let them be.

Not to mention mistakes like wiring a motor backwards, or worse is Megging through a VFD, I don't think its worth while.

IMO a megger is a trouble shooting tool or a initial check out tool. Its not much use for PM work.

Now of coarse, if this was a ship that is going to be a long way from spares, absolutely meg the motors. But for plants or mills, I see no benefit.


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## Big John

lefleuron said:


> ...By Megging them and trending, you may catch a few that are trending badly and even then its a judgement call one whether to change it out or not....


 My thinking also is that it's hard to get any sort of trend with only a yearly megger. There are usually too many variables involved that can change the reading: What's the humidity and temperature of the air, what's the winding temperature, how long has it been at that temperature? 

If I was really concerned about trending a motor, I'd probably want to see a megger every five or six months at a minimum and they'd have to record all that info, too.

-John


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## Motorwinder

Megger trending is common practice in the motor industry. It is usually done only on the large key motors. PI test (polarization index) is very effective for trending.

When we do on-site PI testing, we do it at the MC. This way we test the cables also. If the megs are low, _then _we un-terminate the motor. I found many ground faults in the cables. Usually moisture.

Our minimum meg readings are 1.5 megs for 480 volt motors at 1000 volt meg. (That would be 1.5 million ohms to ground resistance.) 1.5 to 20 is fair. 20 to 50 is good. 50 megs and above is excellent.

I've seen 4160 volt motors running fine at .2 megs at a 5000 volt meg test. 

Most motor failures are from mechanical issues.IE: bearing failures, misalignment, loose connections.


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## Jefro

Don't forget "A Stitch in Time."


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## Zog

Big John said:


> My thinking also is that it's hard to get any sort of trend with only a yearly megger. There are usually too many variables involved that can change the reading: What's the humidity and temperature of the air, what's the winding temperature, how long has it been at that temperature?
> 
> If I was really concerned about trending a motor, I'd probably want to see a megger every five or six months at a minimum and they'd have to record all that info, too.
> 
> -John


That is why it is important to tempature correct readings to 40 degrees C, so you eliminate the variable and trending can be done.


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## yardbird

*motor megging blues*

Perhaps there are a few things to piont out here. First, while it may seem a waste of time, in the long run having that kind of data can be helpful to a degree. When a motor is first put in and it is recorded, you have a record of the motor at the best shape that it will be in: it will give you a baseline to go by when you start to notice what looks like low readings, because you can see just how low they have dropped. many companies take the approach that it is not worth the time to do that, but for me anyway, it is just some good info to refer to.
You probably will have many electricians taking a reading on that motor from time to time for troubleshooting purposes. And as long as you have a standard to go by, saying that 1 or 1.5 megs are good for example, as long as the motor makes that reading, yes, it is good. But having that little bit of extra info to refer to helps to make up your mind if the motor may need to be watched for a possible failure later.

Say for example, a motor is put in and reads 40 megs or so for the first 3 years it is operating, and then you suddenly see a drop to say 20 megs, the motor is still in great shape, but that is a very significant drop. Some shops may say that the motor should be megged again at a later time to see how much it changes. Perhaps the drop is because the motor has some moisture in it, or is getting dirty inside, ect. But if you never knew what the motor read before you took your last megger reading, then how would you know that it had taken such a large drop?

without that extra info to refer to, all you would know is that the motor reads within the range you can accept, and you would never see that there was a big change in resistance from a year ago. while yes it is not 100% nessesary, you can see that perhaps the shop head might wish that the motor be blown out or have a cleanning if it continues to drop from the last reading a few months ago. And cleanning the motor at that time may well stop or reverse that downward trend, saving the motor from doing something bad later.

Perhaps on small motors this would be too much trouble. But for the big expensive ones ( Kind of like the big 2400 volt motors that we use to run the drydock pumps, for example ) that could mean a very big savings to try to catch a problem before it gets too big, rather than letting it grow too big unknowingly.

Oh yes, and I did forget to tell him that for general troubleshooting, it is a good idea to first take motor megger readings from the t-leads at the bottom of the controller, so that you also check the wire going to the motor from the controller at the same time. If you get strange readings from there, then disconnect them from the motor and see what they read seperatly, to find out if the problem is in the motor or the cables going to it. just my two cents, it has been great discussing this topic with you all. :thumbsup:


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## Big John

Thanks for the clarification _yardbird.

_I agree on medium-voltage or very sizable LV motors that a megger would be a good check.  I thought we were talking about double-digit HP motors. We were originally checking_ all_ our motors (but without the environmental data, so it was useless) and I didn't see the point in dedicating the man-power to continue that program for 15 and 25HP motors.

We do in fact megger in addition to Doble and winding checks on our MV equipment.

-John


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## yardbird

Hey no problem Big John.
I can see with the smaller motors that you can just call Grainger to drop off a new one in a day or so that would be a bit of work. One of the neatest things that I saw not long ago on one of the newer class of roll on, roll off ships that carry tanks and stuff around in was that most every motor control panel had its very own little meter mounted in it that read motor megs by itself. You can set an alarm (a little beeper or a small flashing lite if I remember) that will go off at a meg level that you can set.

I thought that was the neatest thing I had seen in years, they are just small little units that you put in the controller box, then mount the little panel. The ship's electrician says it speeds things up greatly. If I still have the paperwork on that item that he gave me perhaps I will post it for you if you are interested. Have a great one, John Cantrell.


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