# Can a motor that is turning be tested??



## tHeKiNgMaN (Sep 4, 2010)

I have a 600V 250HP motor that is coupled to a Cooling Tower fan and is turning due to wind passing through the blades. The 1336 drive that it's connected to has a F-41 Trip alarm (phase to phase short). I meggered it and it's 26 MegaOhms on all three phases to ground. Winding test is impossible since it's turning. I don't think the motor turning will affect the megger results but don't they seem low? Anyone know of another test I can do before calling in Millwrites to decouple?


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## Jlarson (Jun 28, 2009)

tHeKiNgMaN said:


> I have a 600V 250HP motor that is coupled to a Cooling Tower fan and is turning due to wind passing through the blades. The 1336 drive that it's connected to has a F-41 Trip alarm (phase to phase short). I meggered it and it's 26 MegaOhms on all three phases to ground. Winding test is impossible since it's turning. I don't think the motor turning will affect the megger results but don't they seem low? Anyone know of another test I can do before calling in Millwrites to decouple?


Did you disconnect the feeder at the drive and motor and megger that yet?


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## tHeKiNgMaN (Sep 4, 2010)

Jlarson said:


> Did you disconnect the feeder at the drive and motor and megger that yet?


Yes I have eliminated the Drive and the feeder cables.


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## MDShunk (Jan 7, 2007)

I wouldn't _necessarily _expect a phase to phase short to show up on a megger test that was performed phase to ground. 

The only way I can think of to test this motor phase to phase would be to decouple or stop the fan blades and test the winding leads (assuming all the winding's lead ends were brought out to the peckerhead). 

If you're going to have to go to the trouble of getting millwrights involved, it's probably best to put the motor through the whole megger, PI test, HiPot, and the whole nine yards.


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## Jlarson (Jun 28, 2009)

tHeKiNgMaN said:


> Yes I have eliminated the Drive and the feeder cables.


How did you eliminate the drive, did you put a spare motor one it an run it?

I ask because the last thing I would want to do is take a big azz fan blade off and find the motor was fine and the drive was really the problem.


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## tHeKiNgMaN (Sep 4, 2010)

Jlarson said:


> How did you eliminate the drive, did you put a spare motor one it an run it?
> 
> I ask because the last thing I would want to do is take a big azz fan blade off and find the motor was fine and the drive was really the problem.


I test ran the drive without the motor connected, VFDs are pretty smart, if they have specific alarms, like phase to phase short, you can almost always believe them.


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## Jlarson (Jun 28, 2009)

tHeKiNgMaN said:


> I test ran the drive without the motor connected, VFDs are pretty smart, if they have specific alarms, like phase to phase short, you can almost always believe them.


What brand drive?


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## MDShunk (Jan 7, 2007)

He said it was a 1336. That's A-B, if I'm not mistaken. I don't think they even still make them.


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## Jlarson (Jun 28, 2009)

MDShunk said:


> He said it was a 1336. That's A-B, if I'm not mistaken. I don't think they even still make them.


That's what I thought too. I haven't ever been this exact position before where I couldn't stop the motor. Thinking out loud here, there should be a small voltage being generated right?


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## tHeKiNgMaN (Sep 4, 2010)

MDShunk said:


> I wouldn't _necessarily _expect a phase to phase short to show up on a megger test that was performed phase to ground.
> 
> 
> If you're going to have to go to the trouble of getting millwrights involved, it's probably best to put the motor through the whole megger, PI test, HiPot, and the whole nine yards.


I partially agree with your statement but only on a new motor. An older motor in a humid environment that scored poorly on a megger test would prove insulation breakdown, which is suspected here. In that case it would be whichever failed first that would alarm in on the drive.

I'm not sure I would goto the trouble of using a hi-pot to check a used 600V motor, if it fails the windings test with a low resistance meter, it gets shipped off to be rewound. 

Thanks for the posts, thought maybe someone had an old trick for me to try before we had to call in support trades.


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## MDShunk (Jan 7, 2007)

If your drive showed a UV, UW, or VW shorting fault, you could just about say that the drive was your test instrument for the motor, couldn't you?


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## tHeKiNgMaN (Sep 4, 2010)

Jlarson said:


> That's what I thought too. I haven't ever been this exact position before where I couldn't stop the motor. Thinking out loud here, there should be a small voltage being generated right?


1336 is now PF70/700, or at least that's what we replace them with when they goto heaven. 

I'm sure there'd be something there, it spinning at about 300 RPM... nothing I could read with a normal multi-meter and I didn't get blasted when I was working on it. We've had check valves go on 4160v motors that have genrated dangerous voltages though.


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## tHeKiNgMaN (Sep 4, 2010)

MDShunk said:


> If your drive showed a UV, UW, or VW shorting fault, you could just about say that the drive was your test instrument for the motor, couldn't you?


Lol, wish you were the specialist... That was my justification, but they asked me to see if I could get more proof. I'm gonna say I called Baldor and they think it's the motor. :laughing:


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## tHeKiNgMaN (Sep 4, 2010)

Hey while some veterans are on here, do you guys know of a table that shows acceptable limits for testing motors to ground?


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## Jlarson (Jun 28, 2009)

tHeKiNgMaN said:


> Hey while some veterans are on here, do you guys know of a table that shows acceptable limits for testing motors to ground?


See my full table further down.


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## MDShunk (Jan 7, 2007)

tHeKiNgMaN said:


> Hey while some veterans are on here, do you guys know of a table that shows acceptable limits for testing motors to ground?


Nothing hard and fast, unless you have historical data that shows a dropoff. Since yours has been getting misted in the cooling tower for Lord knows how long, your values would be expected to be low, even if the thing is good.

I'd personally condemn anything less than 20 megohm. Below 100, and it would definitely be on my watch list. At 26 megohm in a damp environment... I dunno. If it was mine, I'd want to see higher values that to feel good about it lasting.


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## tHeKiNgMaN (Sep 4, 2010)

Jlarson said:


> I would have meggered at 1,000 volts and looked for 100 mega ohms minimum.


I'm getting 26 megohm... the motor beside it is just as old (8 years) and it's reading 825 megohms...



MDShunk said:


> Nothing hard and fast, unless you have historical data that shows a dropoff. Since yours has been getting misted in the cooling tower for Lord knows how long, your values would be expected to be low, even if the thing is good.
> 
> I'd personally condemn anything less than 20 megohm. Below 100, and it would definitely be on my watch list.


Is that 20 megohms for a 480 V motor? would your minimum value change if the motor was 600V?


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## MDShunk (Jan 7, 2007)

tHeKiNgMaN said:


> I'm getting 26 megohm... the motor beside it is just as old (8 years) and it's reading 825 megohms...
> 
> 
> 
> Is that 20 megohms for a 480 V motor? would your minimum value change if the motor was 600V?


No, not likely, since the test voltage would not change. 600 is close enough to 480.

Besides, if the companion motor in that cooling tower is reading 825 megohms, that's good enough to say your motor is "done". That 26 megohm reading you got would be enough for me to send that motor out to be tested and probably rewound.


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## Jlarson (Jun 28, 2009)

My full table for 600 volt tested @1000 volts

100 megs + ok, If pm is in place continue as normal

100 to 70 megs - needs to be checked routinely for decline 

40 to 60 megs - needs to be addressed soon

30 or less - replace or rebuild

all based on personal history


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## MDShunk (Jan 7, 2007)

Jlarson said:


> My full table for 600 volt tested @1000 volts
> 
> 100 megs + ok, If pm is in place continue as normal
> 
> ...


That squares pretty well with what I do also, except that between 20 and 30, depending on the application, I may or may not worry about it.


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## tHeKiNgMaN (Sep 4, 2010)

MDShunk said:


> No, not likely, since the test voltage would not change. 600 is close enough to 480.
> 
> Besides, if the companion motor in that cooling tower is reading 825 megohms, that's good enough to say your motor is "done". That 26 megohm reading you got would be enough for me to send that motor out to be tested and probably rewound.





Jlarson said:


> My full table for 600 volt tested @1000 volts
> 
> 100 megs + ok, If pm is in place continue as normal
> 
> ...


Thanks guys, I put the call in for support trades. I'm gonna watch some TV till they get here!


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## Jlarson (Jun 28, 2009)

MDShunk said:


> That squares pretty well with what I do also, except that between 20 and 30, depending on the application, I may or may not worry about it.


I also base what I do when it gets that low based off of stuff like leakage current.


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## Jlarson (Jun 28, 2009)

tHeKiNgMaN said:


> Thanks guys, I put the call in for support trades. I'm gonna watch some TV till they get here!


Your working night shift?


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## MDShunk (Jan 7, 2007)

Curious what your liquid line temps are up to with being down a fan? I'm guessing this is an R717 system.


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## tHeKiNgMaN (Sep 4, 2010)

Jlarson said:


> Your working night shift?


Yup, coverage, it pays good an is really easy. to bad I only get to do it once every few months. More work in the winter though.


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## tHeKiNgMaN (Sep 4, 2010)

MDShunk said:


> Curious what your liquid line temps are up to with being down a fan? I'm guessing this is an R717 system.


Not sure what an R717 system is... it's a ten fan cooling tower.


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## Jlarson (Jun 28, 2009)

I'm working too ,rushing to get stuff done for tomorrow. Mostly CAD work for asbuilts so I can turn over a SCADA system.


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## tHeKiNgMaN (Sep 4, 2010)

Jlarson said:


> I'm working too ,rushing to get stuff done for tomorrow. Mostly CAD work for asbuilts so I can turn over a SCADA system.


Ah sounds fun... We have a SCADA for our river pumphouse. It messes up all the time! lol


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## Jlarson (Jun 28, 2009)

tHeKiNgMaN said:


> Ah sounds fun... We have a SCADA for our river pumphouse. It messes up all the time! lol


Actually the only thing I have really got done it rack up like 100 posts in like 2 hours.


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## John Valdes (May 17, 2007)

The drive has told you that it is a phase to phase short. This does not have to be a shorted motor. It could very well be the motor leads or the conductors feeding the motor. 
When you say you ran the VFD with the motor disconnected did you disconnect at the drive or at the motor?
If the drive operates with the motor leads disconnected you already have your answer. Now, is it the motor or the conductors?
Also, Why could you not block the fan from turning? I know 250 HP is a big motor, but a 2X4 should have wedged it good enough.

Below is a table for megger limits courtesy of our esteemed member "Zog".


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## MDShunk (Jan 7, 2007)

John Valdes said:


> Also, Why could you not block the fan from turning? I know 250 HP is a big motor, but a 2X4 should have wedged it good enough.


That could be a scary and expensive proposition. Many cooling tower fan blades nowadays are fiberglass or carbon fiber, and are finely engineered pieces that look a lot like the prop of a huge ship.


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## John Valdes (May 17, 2007)

MDShunk said:


> That could be a scary and expensive proposition. Many cooling tower fan blades nowadays are fiberglass or carbon fiber, and are finely engineered pieces that look a lot like the prop of a huge ship.


That's true Marc. If I could not get to the motor shaft, I would not try and wedge the blade. They can cost a small fortune.
Sounds like the OP was just waiting for the buzzer anyway.


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## tHeKiNgMaN (Sep 4, 2010)

John Valdes said:


> The drive has told you that it is a phase to phase short. This does not have to be a shorted motor. It could very well be the motor leads or the conductors feeding the motor.
> When you say you ran the VFD with the motor disconnected did you disconnect at the drive or at the motor?
> If the drive operates with the motor leads disconnected you already have your answer. Now, is it the motor or the conductors?
> Also, Why could you not block the fan from turning? I know 250 HP is a big motor, but a 2X4 should have wedged it good enough.
> ...


As stated earlier I ran the drive without the motor and yes I also disconnected the feeder cable. For some reason our specialist is not an Electrical Engineer or Electrician. I actually think he's an accountant. Support trades arrived and barred the shaft and after testing the motor windings I have submitted my request to send the motor in for repair. 
Using a piece of wood to stop fan blades from spinning would go against many safe work practices here, including line of fire hazzards and falls from heights. Also to use a piece of wood here would require Engineers approval, a request that would surely send me out the gate. Your advice could have been quite dangerous to someone less experienced...

Thank you for the chart, any idea as to where the information came from. If the source is from a reputable vendor I would love to post this on our info wall.


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## Jlarson (Jun 28, 2009)

That table came from NETA the National Electrical Testing Association, I think they call themselves InterNational they just didn't change the acronym.

I based my personal table for 600v off of their minimum. Anything above 100 megs is ok, below I look at it based off my table, but I wouldn't just take a motor out of service cause it is 99 megs. That in my book is spending money for no reason and wasting resources.


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## Introyble (Jul 10, 2010)

tHeKiNgMaN said:


> I have a 600V 250HP motor that is coupled to a Cooling Tower fan and is turning due to wind passing through the blades. The 1336 drive that it's connected to has a F-41 Trip alarm (phase to phase short). I meggered it and it's 26 MegaOhms on all three phases to ground. Winding test is impossible since it's turning. I don't think the motor turning will affect the megger results but don't they seem low? Anyone know of another test I can do before calling in Millwrites to decouple?


UM , one way to test a motor , since after all it's also a generator, is to test the mV output of the motor while turning it by hand.


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## Big John (May 23, 2010)

Introyble said:


> UM , one way to test a motor , since after all it's also a generator, is to test the mV output of the motor while turning it by hand.


 What would they be looking for with that? Just a few millivolts per phase to show that the coils weren't open?

I only ask because I think that reading will gradually reduce down to nothing the longer the motor sits without power.

-John


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## Introyble (Jul 10, 2010)

Big John said:


> What would they be looking for with that? Just a few millivolts per phase to show that the coils weren't open?
> 
> I only ask because I think that reading will gradually reduce down to nothing the longer the motor sits without power.
> 
> -John


Provides low voltage output tests for turn to turn shorts or soft shorts


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## John Valdes (May 17, 2007)

tHeKiNgMaN said:


> As stated earlier I ran the drive without the motor and yes I also disconnected the feeder cable. For some reason our specialist is not an Electrical Engineer or Electrician. I actually think he's an accountant. Support trades arrived and barred the shaft and after testing the motor windings I have submitted my request to send the motor in for repair.
> Using a piece of wood to stop fan blades from spinning would go against many safe work practices here, including line of fire hazzards and falls from heights. Also to use a piece of wood here would require Engineers approval, a request that would surely send me out the gate. Your advice could have been quite dangerous to someone less experienced...
> 
> Thank you for the chart, any idea as to where the information came from. If the source is from a reputable vendor I would love to post this on our info wall.


This is a professional site and is intended for professionals only. I would never advise anyone to perform a dangerous task or one that is against company policy. But your point is well taken.


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## Introyble (Jul 10, 2010)

John Valdes said:


> The drive has told you that it is a phase to phase short. This does not have to be a shorted motor. It could very well be the motor leads or the conductors feeding the motor.
> When you say you ran the VFD with the motor disconnected did you disconnect at the drive or at the motor?
> If the drive operates with the motor leads disconnected you already have your answer. Now, is it the motor or the conductors?
> Also, Why could you not block the fan from turning? I know 250 HP is a big motor, but a 2X4 should have wedged it good enough.
> ...


Please explain the importance of disconnecting all conductors from a VFD before meg testing. Also, never test on high range unless absolutely necessary


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## tHeKiNgMaN (Sep 4, 2010)

Introyble said:


> Please explain the importance of disconnecting all conductors from a VFD before meg testing. Also, never test on high range unless absolutely necessary


Meggering a motor that is connected to a drive can give you skewed readings especially on larger drives, something to do with the drives voltage measuring circuits. You also run the risk of damaging electronic parts of the drive (so I'm told, I have never done this and have meggered a few):whistling2:.


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## Shorty Circuit (Jun 26, 2010)

MDShunk said:


> I wouldn't _necessarily _expect a phase to phase short to show up on a megger test that was performed phase to ground.
> 
> The only way I can think of to test this motor phase to phase would be to decouple or stop the fan blades and test the winding leads (assuming all the winding's lead ends were brought out to the peckerhead).
> 
> If you're going to have to go to the trouble of getting millwrights involved, it's probably best to put the motor through the whole megger, PI test, HiPot, and the whole nine yards.


I'd expect on a phase to phase short, the shorted windings would meggar much lower to ground than the unshorted winding because their impedence to ground would be in parallel. Not definitive one way or the other but suggestive. I'd also expect a phase to phase short to trip out instantly on overload on being energized. With a VFD I'd expect it to shut down on sensing a load fault.

Since the fan blades are spinning the motor should be acting as a generator. I'd disconnect it from the VFD and look at the phase to phase output voltage to see if they are all the same or close. Just an idea.


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