# how to tell a motor is bad



## DannyC327 (Mar 23, 2016)

I work at an industrial facility with a lot of old equipment. I was always taught to meggar a motor to find out if it is bad. Is it best to meg between windings or meg to ground . thank you for your time


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## RePhase277 (Feb 5, 2008)

DannyC327 said:


> I work at an industrial facility with a lot of old equipment. I was always taught to meggar a motor to find out if it is bad. Is it best to meg between windings or meg to ground . thank you for your time


You can and should do both. Making sure the windings are disconnected from each other, of course:laughing:


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## danhasenauer (Jun 10, 2009)

DannyC327 said:


> I work at an industrial facility with a lot of old equipment. I was always taught to meggar a motor to find out if it is bad. Is it best to meg between windings or meg to ground . thank you for your time


Yes......


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## John Valdes (May 17, 2007)

All sound advice above. However, sometimes a megger is not enough. Especially if the motor is on a drive. Seems drives have better ability to verify motor health than anything else.
There were times at the motor shop when we just could not find the issue. Yet in a few cases, the manufacture did find the issue.

On small motors, you can read resistance and compare to the other coils.
On bigger motors you cannot as the resistance is to low to measure with a meter.
Megging is the very best way and will tell you the truth most of the time.
Now and again, you may need to ask for help from your motor repair shop.

Do you have and DC motors? These are fairly special and there really is not much you can do in house other than to keep them clean and check and change brushes regularly. Try to stay away from stones and learn how a healthy commutator looks and feels.


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## joebanana (Dec 21, 2010)

On a 3 phase motor you only need to meg one lead to the frame.


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## JRaef (Mar 23, 2009)

joebanana said:


> On a 3 phase motor you only need to meg one lead to the frame.


Well, I suppose if all you want to know it the health of that one leg with respect to losses to ground, then yes. How much that tells you about the rest of the motor is questionable. I suppose you are thinking that since all 3 legs will be, at some point, connected to each other, then yes, if one is leaking to ground (frame), then the megger will tell you that a problem exists, but now where. Won't tell you a thing about possible phase-to-phase shorts however.


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## telsa (May 22, 2015)

Any outfit with a lot of motors needs to maintain a database holding all of their readings -- as time goes by.

The sequential values become highly revealing.

You might take a peak at *Tribology* -- and the science of failure prediction.


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## emtnut (Mar 1, 2015)

DannyC327 said:


> I work at an industrial facility with a lot of old equipment. I was always taught to meggar a motor to find out if it is bad. Is it best to meg between windings or meg to ground . thank you for your time


As others have said .. Yes, Meg both.
And should be part of the PM routine, which over time will show degradation.


The sure way to tell if a motor is bad ...

If it has tats and rides a Harley :laughing:


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## joebanana (Dec 21, 2010)

JRaef said:


> Well, I suppose if all you want to know it the health of that one leg with respect to losses to ground, then yes. How much that tells you about the rest of the motor is questionable. I suppose you are thinking that since all 3 legs will be, at some point, connected to each other, then yes, if one is leaking to ground (frame), then the megger will tell you that a problem exists, but now where. Won't tell you a thing about possible phase-to-phase shorts however.


If you have a short/leakage to the frame does it really matter which leg it's on? A phase to phase short would be obvious without a megger, when you hit the start button. Besides megging phase to phase will indicate a short, regardless.


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## glen1971 (Oct 10, 2012)

telsa said:


> Any outfit with a lot of motors needs to maintain a database holding all of their readings -- as time goes by.
> 
> The sequential values become highly revealing.
> 
> You might take a peak at *Tribology* -- and the science of failure prediction.


And you might think a maintenance program for them... i.e.. checking oil/greasing, temperatures, overall state of repair, etc...

Almost to be proactive.. Or preventative... lol..


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## JRaef (Mar 23, 2009)

emtnut said:


> ... The sure way to tell if a motor is bad ...
> 
> If it has tats and rides a Harley :laughing:


Yes, but be careful TELLING the motor it's bad. Just offer it a suggestion of living a more genteel life...


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## Kellyetheredge (Jun 11, 2016)

I usally use my fluke..ohms setting..windings should match. A to b..b to c..a to c...etc. then to ground..all good? Good windings. Only use on 1 to 50 hp motors. I meg any motors above 50. Lots if data to sift thru. Youll know if motor is good/ bad.


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## Kellyetheredge (Jun 11, 2016)

We took monthly winding readings on our fanuc servo motors...5hp motors reading .9 ohms between phases. Low ohm readings common to servo motors...just know by experience...not sure why.


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## Kellyetheredge (Jun 11, 2016)

If i had a common 5 hp motor that read that between a phase it would get my attention


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## backstay (Feb 3, 2011)

Motors will either have a winding problem or a bearing problem(alignment can look like bearing). We used vibration analysis with history databases for mechanical trouble. But electrically we didn't have histories, even though some of our motors were 60-70 years old. Meggers were it, each phase to ground, and it should be infinite. Each phase to the other two. Those need to match. If not, you had turn to turn shorting. How the motor is acting is also part of the trouble shooting. Blowing fuses, all or some, overloads, current readings.


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## MechanicalDVR (Dec 29, 2007)

As others have said meg both ways. I also liked to keep a chart of amp draw and approximate front and rear bearing temps on motors that needed to run uninterrupted.


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## Kellyetheredge (Jun 11, 2016)

MechanicalDVR. Yes...when allowed the time that is an ideal and actionable histogram/PM


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## Big John (May 23, 2010)

If you're plotting resistance data you must record temperature and humidity.

Temperature can cause drastic swings in value, so if you're not correcting to 40°C you have no idea whether your trend is reliable.

There isn't any way I'm familiar with to correct for humidity, but it does allow for an educated judgement: If the corrected IR is much lower today at 85%H than it was 1 year ago at 20%H then maybe a retest is in order when environmental conditions match.


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## Bootss (Dec 30, 2011)

" I love the smell of a burned-out motor in the morning"
:laughing::laughing:


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## micromind (Aug 11, 2007)

How to tell if a motor is good or bad is much simpler than all these silly tests.......just apply rated voltage and it, like every other electrical apparatus will do one of 3 things;

1) It'll do what it's supposed to do.

2) It'll do nothing.

3) It'll blow up. 

See how easy it is!!

Seriously, as noted above, meggering and winding resistance are pretty good indicators of a motors health.


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## John Valdes (May 17, 2007)

Lep said:


> " I love the smell of a burned-out motor in the morning"
> :laughing::laughing:


Unless its a 500HP DC motor in a bad spot at 3:30 on a Friday afternoon!


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## Moonshot180 (Apr 1, 2012)

For 3 phase motors, first bridge test it. This will make sure each phase winding is sound, and in tact.

It's been said already in this thread, but if you have an open phase winding, then your meggar test will not be accurate.

The bridge/resistance test will give you a good idea of the health of the phase windings. Should see consistent & balanced ohm readings across all 3 phases.

Only after you verify the windings are good, should you proceed to megger the motor.


Motor should also be checked for mechanical binding w/ bearings etc..if the motor passes bridge and megger tests.


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## Big John (May 23, 2010)

Since it can be hard to do accurate winding resistance on motors, a good check is to do an impedance test with an AC supply.

If you can break the windings apart, ideally you do each winding separately, but in theory you could do phase-to-phase: 

Put a variac on the windings and slowly ramp up to a stable voltage, staying below the nameplate FLA. Measure your current (for small motors you may have to use a series ammeter to get a value precise enough to be helpful). Compare measurements between each winding.

Serious turn-to-turn failures or problems with the backiron will show up as a current imbalance on that winding.


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## chrisfnl (Sep 13, 2010)

Just to note... Motor windings don't always fail with a ground... I had one fail recently, it was a tube axial fan motor. 

The drive showed a short circuit fault, measuring the resistance at the terminals showed the same between phases within an ohm or so (there was a slight imbalance, but I've seen far worse on functional motors). 

The megger showed infinite at 1000v. Once we disassembled the ductwork, the paint was clearly discoloured from over heating, the motor cooling fan had melted onto the cowling. 

Come to think of it, does anyone make a decent Lcr meter in a portable package?


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## gnuuser (Jan 13, 2013)

Lep said:


> " I love the smell of a burned-out motor in the morning"
> :laughing::laughing:


megging is a good way but follow the first rule with motors
If it smells like burnt $#!t on a stick dont bother megging it:laughing:


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## John Valdes (May 17, 2007)

I once had an idea I never put to use or tried out.
What if you used a small DC power supply connected to the individual coils with your meter in series and reading amps.
Would this not be a way to check winding health? You could even use an AC supply and measure that current.
I'm talking very low voltage like in the range of 0-24 ac or dc volts.
But we would concern ourselves only with the current making sure we had the exact same voltage applied to each coil.

With a meter in series we can measure very low current. Milliamps and compare readings.


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## macmikeman (Jan 23, 2007)

I do a quick markup and profit calculation at the red light intersections on my way over there before I ''find'' that the motor is bad............:thumbsup:

usually it is....................


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## chrisfnl (Sep 13, 2010)

John Valdes said:


> I once had an idea I never put to use or tried out. What if you used a small DC power supply connected to the individual coils with your meter in series and reading amps. Would this not be a way to check winding health? You could even use an AC supply and measure that current. I'm talking very low voltage like in the range of 0-24 ac or dc volts.
> 
> But we would concern ourselves only with the current making sure we had the exact same voltage applied to each coil. With a meter in series we can measure very low current. Milliamps and compare readings.


That's how a ohmmeter measures resistance... A small dc voltage, calculates resistance based on current.

An Lcr meter measures impedance with an ac voltage more or less the same way.


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## KennyW (Aug 31, 2013)

I hate to say it but if it doesn't measure a short to ground or open circuit with a regular meter, the chances a megger will find anything conclusive are slim. In my mind an impulse tester like a baker tester or sweep frequency analyzer is really the only thing that can conclusively tell you a motor is healthy. These sorts of tests are probably not worth doing on motors less than 50 or 100hp though.


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## Flyingsod (Jul 11, 2013)

KennyW said:


> I hate to say it but if it doesn't measure a short to ground or open circuit with a regular meter, the chances a megger will find anything conclusive are slim. In my mind an impulse tester like a baker tester or sweep frequency analyzer is really the only thing that can conclusively tell you a motor is healthy. These sorts of tests are probably not worth doing on motors less than 50 or 100hp though.


That's been my experience. Some people here disagree with that though.

Sent from my C6725 using Tapatalk


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## John Valdes (May 17, 2007)

chrisfnl said:


> That's how a ohmmeter measures resistance... A small dc voltage, calculates resistance based on current.
> 
> An Lcr meter measures impedance with an ac voltage more or less the same way.


But an ohm meter cannot read resistance on a bigger motor because the windings resistance is to low.
Putting a power supply on the winding should allow for precise current measurements. Not resistance. Current.
I do understand the meter does apply a voltage to measure the resistance. But the meter has limited power and limited current capabilities.
I have not tried doing this. Just something that I should have done to see if it would provide a better idea as to motor winding health.



KennyW said:


> I hate to say it but if it doesn't measure a short to ground or open circuit with a regular meter, the chances a megger will find anything conclusive are slim. In my mind an impulse tester like a baker tester or sweep frequency analyzer is really the only thing that can conclusively tell you a motor is healthy. These sorts of tests are probably not worth doing on motors less than 50 or 100hp though.


Baker surge testers at one time were used exclusively by motor shops. It is how they determine motor winding health.
Today, some companies are actually investing in them for plant use and training employees to use them.


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## Moonshot180 (Apr 1, 2012)

DLROs are what we use to perfrom what John Valdes is talking about I believe.

It injects current through the circuit, and based on the result, an ohm value is given.

We use DLROs quite often, ours can inject up to 10 amperes of current throught the circuit being tested.


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## MDShunk (Jan 7, 2007)

In this age where damn near everything installed new is on a VFD, the VFD is a fine troubleshooting tool in itself. If it says current limit, overload, ground fault, etc.... believe it. Pretty good chance (if it's not mechanically bound) it's the motor or the wire between the drive and the motor. Can't tell you how many "bad drives" I know people have replaced for these "faults", when the fault was actually diagnosing the motor. Nothing wrong with the drive. 

While the tests can get exotic if you like, the old sniff test in the peckerhead will sort out the lion's share of suspect bad motors. No need to make it harder than it needs to be. 

I've done a lot of work in food and pharma factories, where hundreds and hundreds of washdown rated motors are subjected to daily chemical and hot water washdown. EVERY motor and every local disconnect in such plants will meg "bad". By the way, "washdown rated" and "IP67" are pure marketing terms that means that it takes a little bit longer for water to get inside. :laughing:


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## John Valdes (May 17, 2007)

MDShunk said:


> In this age where damn near everything installed new is on a VFD, the VFD is a fine troubleshooting tool in itself. If it says current limit, overload, ground fault, etc.... believe it. Pretty good chance (if it's not mechanically bound) it's the motor or the wire between the drive and the motor. Can't tell you how many "bad drives" I know people have replaced for these "faults", when the fault was actually diagnosing the motor. Nothing wrong with the drive.
> 
> While the tests can get exotic if you like, the old sniff test in the peckerhead will sort out the lion's share of suspect bad motors. No need to make it harder than it needs to be.
> 
> I've done a lot of work in food and pharma factories, where hundreds and hundreds of washdown rated motors are subjected to daily chemical and hot water washdown. EVERY motor and every local disconnect in such plants will meg "bad". *By the way, "washdown rated" and "IP67" are pure marketing terms that means that it takes a little bit longer for water to get inside. :laughing:*


And has weep holes that allow water to drain from the motor. We loved those facilities as a motor shop.
Make sure the correct plugs are removed depending on mounting position.

Marc I totally agree with you on drives being a very good piece of test equipment. However, a drive is smarter than most noses and will see the tiniest problem.
We got the occasional motor in shop faulting on winding failure/short. In several instances we could not find the problem and sent the motor back. Then they send it back with the same report.
Luckily in most of those cases the motors were still in warrantee and the manufacturer replaced them.
I would imagine many motors get rewound and even the shop cannot pin point (no pun intended) the exact cause.


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## sbrn33 (Mar 15, 2007)

DannyC327 said:


> I work at an industrial facility with a lot of old equipment. I was always taught to meggar a motor to find out if it is bad. Is it best to meg between windings or meg to ground . thank you for your time


Supro 500 baby....


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## Flyingsod (Jul 11, 2013)

sbrn33 said:


> Supro 500 baby....


Omfg, you people own stock in that company or something?

Sent from my C6725 using Tapatalk


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## sbrn33 (Mar 15, 2007)

Be careful. it is a trick. It only puts out 400 volts....


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## JRaef (Mar 23, 2009)

MDShunk said:


> ... the old sniff test in the peckerhead ...


Be careful that this is not taken out of context! :whistling2:

I think I posted this some time ago, but I have it on my "permanent record" with HR that I had been using "inappropriate terminology" when doing training classes on VFDs, because I called the motor termination box a "peckerhead". I was asked to provide proof in writing that the term came from some meaning other than what it appears to come from, and I could not find anything about the origin. HR on the other hand found something in writing that claimed it IS what it sounds like, and ordered me to change all of my presentations that used that term. I missed one and got written up after someone else used it.

I did, by the way, find evidence to what I was told in my youth was the origin: that it stemmed from "Picker Head", a protrusion on the drum of a cucumber picking rig that pulled the cukes off of the vine when the horse drawn tractor went over it. The way it came off of the drum looked a lot like a motor termination box, which came about a lot later, so the motor people referred to it as a Picker Head, and in the South, that became "pecker head", a term also used for dumb people, derived from the same source. But what I found was not definitive enough for the HR department here where I work now.


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## Moonshot180 (Apr 1, 2012)

JRaef said:


> Be careful that this is not taken out of context! :whistling2:
> 
> I think I posted this some time ago, but I have it on my "permanent record" with HR that I had been using "inappropriate terminology" when doing training classes on VFDs, because I called the motor termination box a "peckerhead". I was asked to provide proof in writing that the term came from some meaning other than what it appears to come from, and I could not find anything about the origin. HR on the other hand found something in writing that claimed it IS what it sounds like, and ordered me to change all of my presentations that used that term. I missed one and got written up after someone else used it.


 
have that same kind of lawl laid down here also.

We can only say motor termination box now. sheesh. Folks that automatically think of the other term for peckerhead need to get their mind out of the gutter and leave us alone. ha


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## chrisfnl (Sep 13, 2010)

No one up here uses the term "pecker head", *except* in the offshore oil industry, and I assume that's because of mixing of professionals from the gulf if Mexico.

That being said... Was on a conference call a while back with a handful of electricians on a drilling rig on one end, and myself and several women from various office jobs on the other end....

The conference call was about developing a training package, and the term pecker head came up several times (in context, but it was a totally foreign word in that context to the women with me)

Eventually I had a chuckle, and said "I'm here with a group of non technical people, could you explain what a pecker head is?"

No one was offended, but up to that point there were few very embarrassed smiles.


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