# Motor Megs out Bad, Then Runs?



## joe-nwt (Mar 28, 2019)

Tagged because of interest.


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## Navyguy (Mar 15, 2010)

Moisture, dust, carbon tracking, something loose in peckerhead; just a few of my finds over the years to which I have attributed the unexplainable.

Cheers
John


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## PSmitty (Aug 19, 2019)

No Moisture, The room is climate controlled.
Dust: Its a food process but power product. (there are three machines identical, two of them have been running for 5 plus years,with no problem)
This machine in question is 8 months old.
Checked all connections in pecker-head and all bolt were still torqued (Very Tight).
Also still torqued at ABB Brive.

John, What would you look at next?


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## dronai (Apr 11, 2011)

PSmitty said:


> *They connected the motor back up and tested. This time all three legs were clear. No shorts. The motor has been running for a month no problem?*
> 
> Try to explain? Ask me any questions.


Problem solved for now, next..... 

I wouldn't have been the one who energized a 100hp motor that faulted, without figuering out why first :surprise:


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## Navyguy (Mar 15, 2010)

On rare occasions, over greasing has been an issue too. I worked in food industry and all the grease had to be HAACP and food grade approved. Grease got washed out of the bearings a lot in some areas and motors needed to be greased often (well beyond the manufacturer's recommendation) but some of our guys thought all motors needed to be greased like that, so we lost a few motors because the grease contaminated the stator.

It initially showed as a ground fault on the VFD, which would "magically" clear. Once we tore it apart it was clear as to what was happening with the over greasing.

But quite frankly it simply could be a bad wind from the motor shop. Like anything else, even if they have 99% quality control, you still can get that odd 1%.

Cheers
John


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## Wirenuting (Sep 12, 2010)

Many years ago we installed a new dual Quincy air compressor.. 
Tripped o/l on one of the 10 hp motors within 5 minutes.. 
Checked, rang, looked at and found no fault. 
Reset o/l and restarted. 

Walked in the door the next morning and found same motor tripped on o/l

Looked and saw that the rear most winding had "Fallen off".

Replaced motor and 10 years later tore the 15 story building down.


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## paulengr (Oct 8, 2017)

Low insulation resistance doesn’t mean it can’t run just that there is a current leak from the coils to the frame. The obvious cause is damage to the ground wall insulation usually after it burns up but that’s not the only one. Any amount of moisture or contamination or scuffed or pinched insulation will do it too. Manufacturing errors are pretty common for instance. As an example I once went through 3 1 HP motors on a small fat pump in a row before we figured it out. Motors come in as standard bells. If customer wants C-face we knock the bell housing and bearing off and put on the C-face one. When we did the C-face fit differently. The coils were built sticking out too far so they were crushed and meggered bad and tripped the drive out once the C-face bell end was installed. Not your case but pointing out the kind of subtle stuff you can run into. Many times the only way to find it is a tear down at a motor shop.

Check ground current settings and actual reading at the drive because it’s not a “megger”. In fact it doesn’t even measure ground current. It simply sums the three phase currents together (as a vector) and trips if the resulting calculated current exceeds around 10-20% of the average. This is a much higher current than the micro amps that it takes to get a bad megger reading. Read the manual because there are lots of causes for unbalanced currents not just a bad motor or wiring.


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## gpop (May 14, 2018)

I worked in a wet food grade plant for years that shut down 4 months a year during the off season. During the shut down we would check all the starters and motors.

Most years we would end up with 90 to 120 motors that meged bad. After checking wiring and connections we would throw fire to the smaller motors that would cost more to pull for servicing than the cost of replacement (30hp or less). Most years we ended up with zero motor failures. The larger motors we would bag and heat them for a few days then test them. 

Your motor has been nice and its given you the head-up that it will fail in the future. If its critical then pull the motor and have it serviced. If you have a spare then leave it and run to failure as the motor shop will charge about the same amount either way. 

Ive personally seen a 75hp motor wipe out it 150amp fuses then pass all the test (hi-pot) then run for a month and repeat the exercise. We replaced the motor even though it tested good then stripped it to find chip marks in the coating. It took a while to find a balancing washer that had come loose and was rattling around the winding.

If you have 3023 wiring fault enabled on the ABB drive the drive will do a basic meg test while the motor is off and alert you to a problem. (it can give false alarms if you have another drive in the same conduit)


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## tmessner (Apr 1, 2013)

I had a 200 hp centrifuge motor that faulted the drive. I could not find anything wrong. I did not megger it, only used a vom. It showed no gfaults. When I took the cover off the peckerhead I "think" I found a very small burn through on one of the insulated connections. I installed a much larger jbox in place of the original box on the motor 16 x 16 x 8 so there was lots of room to fold wires in . The motor has been running fine now 9 months.


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## PSmitty (Aug 19, 2019)

*Thanks Paulengr*

Paulengr,
Thanks for your insight. This was an interesting problem it was breaking all my rules according to troubleshooting. I appreciate your knowledge on drives and suggestion of possibilities. Keep commenting no matter where we are in our careers we must be open to learn. Thanks for that,


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## JRaef (Mar 23, 2009)

paulengr said:


> Low insulation resistance doesn’t mean it can’t run just that there is a current leak from the coils to the frame. The obvious cause is damage to the ground wall insulation usually after it burns up but that’s not the only one. Any amount of moisture or contamination or scuffed or pinched insulation will do it too. Manufacturing errors are pretty common for instance. As an example I once went through 3 1 HP motors on a small fat pump in a row before we figured it out. Motors come in as standard bells. If customer wants C-face we knock the bell housing and bearing off and put on the C-face one. When we did the C-face fit differently. The coils were built sticking out too far so they were crushed and meggered bad and tripped the drive out once the C-face bell end was installed. Not your case but pointing out the kind of subtle stuff you can run into. Many times the only way to find it is a tear down at a motor shop.
> 
> Check ground current settings and actual reading at the drive because it’s not a “megger”. In fact it doesn’t even measure ground current. It simply sums the three phase currents together (as a vector) and trips if the resulting calculated current exceeds around 10-20% of the average. This is a much higher current than the micro amps that it takes to get a bad megger reading. Read the manual because there are lots of causes for unbalanced currents not just a bad motor or wiring.


Also worth noting, a Fluke 87 is a DMM, not a megger, so what it is reading is resistance to ground based on the energy in a 9V battery. A 480V VFD is sending DC pulses at roughly 75 times that potential into the windings, so it finds faults that a meter might miss. I always recommend using a 1,000V megger when testing motors run by VFDs, anything else is not truly useful information.


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## PSmitty (Aug 19, 2019)

*Megged with a megohmmeter*



JRaef said:


> Also worth noting, a Fluke 87 is a DMM, not a megger, so what it is reading is resistance to ground based on the energy in a 9V battery. A 480V VFD is sending DC pulses at roughly 75 times that potential into the windings, so it finds faults that a meter might miss. I always recommend using a 1,000V megger when testing motors run by VFDs, anything else is not truly useful information.


JRaef
I initially took readings with Fluke 87 when called. Its just something I carry with me in the Plant for breakdowns. I took it for granted when I said megged motor that there would be an understanding I used a megohmmeter. (I'll take the hit, my fault). 

But JRaef I like your tenacity. I megged at 1000 V, found short on all three legs. (measured at studs in pecker-head with leads feeding motor disconnected).

My question for you JRaef. What would you do next?

"Keep the comments coming"


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## gpop (May 14, 2018)

PSmitty said:


> JRaef
> I initially took readings with Fluke 87 when called. Its just something I carry with me in the Plant for breakdowns. I took it for granted when I said megged motor that there would be an understanding I used a megohmmeter. (I'll take the hit, my fault).
> 
> But JRaef I like your tenacity. I megged at 1000 V, found short on all three legs. (measured at studs in pecker-head with leads feeding motor disconnected).
> ...


Large motors can give you a false reading on a megger unless you leave the megger connected for a period of time. 100hp isnt that big so the megger probably needs 5-10 seconds. 

"Still reading all three shorted to ground" is not a measurement. 0.1 Mohm at 56v (if you have a fluke megger it shows the voltage) would give us a better idea because some people would class 1.0 Mohm as shorted so real life numbers saves a lot of guessing. 

After the motor was rotated and megged again what was the reading on the megger?


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## joebanana (Dec 21, 2010)

I just don't understand why people megger all 3 leads on a 3 phase motor. If one phase is grounded, all 3 will be. If one phase is good, all 3 will be.


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## gpop (May 14, 2018)

joebanana said:


> I just don't understand why people megger all 3 leads on a 3 phase motor. If one phase is grounded, all 3 will be. If one phase is good, all 3 will be.


In a perfect world that is true. Then one day you find only one phase grounded and you realize that the vfd only lists one fault when it dam well knows that the motor has both a short to ground and a open phase.


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## micromind (Aug 11, 2007)

joebanana said:


> I just don't understand why people megger all 3 leads on a 3 phase motor. If one phase is grounded, all 3 will be. If one phase is good, all 3 will be.


Most likely force of habit. 

I've known this for many years but still find myself hitting all 3 leads......


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## paulengr (Oct 8, 2017)

joebanana said:


> I just don't understand why people megger all 3 leads on a 3 phase motor. If one phase is grounded, all 3 will be. If one phase is good, all 3 will be.



Six or 12 lead motors tested at the motor. Testing one lead only works if all the groups are wired together.

Also if you run into a 6 or 12 lead wye when the neutral is grounded you have to lift the ground first in addition to disconnecting VFDs and sift starts. I see these once in a while with CTs (differential protection) on large motors or goofy remote overload wiring when engineers overthink it.


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## glen1971 (Oct 10, 2012)

paulengr said:


> Six or 12 lead motors tested at the motor. Testing one lead only works if all the groups are wired together.
> 
> Also if you run into a 6 or 12 lead wye when the neutral is grounded you have to lift the ground first in addition to disconnecting VFDs and sift starts. I see these once in a while with CTs (differential protection) on large motors or goofy remote overload wiring when engineers overthink it.


I've read over this a few times, and I'm still mildly confused by this. After disconnecting the "T" leads at a VFD or Soft Start, if you are meggering them to ground from the starter, VFD, Soft Start or whatever runs the 3 phase motor, if you see a connection to ground there is a problem. Delta or Wye, it shouldn't matter, as you are meggering from one line through the cabling, the motor and back on the other two leads. A grounded winding or conductor anywhere along the system will show, unless there is a broken connection. Typically, before I megger I do a quick ohm reading between all 3 conductors to see if there is a notable difference indicating a broken wire.
On any motor, I would be meggering all of the conductors that go to the motor, a two speed motor comes to mind really quickly.
If you are meggering with the VFD or Soft Start connected, you may end up chasing your tail on electronics and the inner workings of the controller. If they are unhooked, there is no doubt in what you are checking. 
Perhaps I'm way off, as I've only had one coffee so far this morning.


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## CoolWill (Jan 5, 2019)

glen1971 said:


> I've read over this a few times, and I'm still mildly confused by this. After disconnecting the "T" leads at a VFD or Soft Start, if you are meggering them to ground from the starter, VFD, Soft Start or whatever runs the 3 phase motor, if you see a connection to ground there is a problem. Delta or Wye, it shouldn't matter, as you are meggering from one line through the cabling, the motor and back on the other two leads. A grounded winding or conductor anywhere along the system will show, unless there is a broken connection. Typically, before I megger I do a quick ohm reading between all 3 conductors to see if there is a notable difference indicating a broken wire.
> On any motor, I would be meggering all of the conductors that go to the motor, a two speed motor comes to mind really quickly.
> If you are meggering with the VFD or Soft Start connected, you may end up chasing your tail on electronics and the inner workings of the controller. If they are unhooked, there is no doubt in what you are checking.
> Perhaps I'm way off, as I've only had one coffee so far this morning.


Disconnected at the motor. All 6 or 12 leads disconnected from each other and separately megged.


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## paulengr (Oct 8, 2017)

CoolWill said:


> Disconnected at the motor. All 6 or 12 leads disconnected from each other and separately megged.



Yep. Meggering on a wye-delta or about inner delta soft starter if done at the starter. Both will trip up first years every time.

If you know how something is wired you can definitely cut down on the Megger tests. But if you do not then you either test every leads or bolt/jumper them all together and test just one time. Usually multiple tests is faster.

In an electronic drive the electronics is a semiconductor so by nature you should normally just see the “off” resistance of the electronics which is much less than a “good” insulation resistance.

But here’s a common mistake. So quite often I hear they meggered all three leads after say unwiring from a VFD. Readings were say 15 megs, 18 megs, and 21 megs. Different readings but as you said wired together. Why? Residual charge and polarization. The same guys saw you do it and only tested for about 10 seconds and didn’t ground for 3 times longer afterwards. So really you are just seeing the results after around a 30 second megger test. It’s funny to hear them chase their tails on “one phase failed”. We charge them for a diagnostic on a good motor and send it right back.


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## glen1971 (Oct 10, 2012)

CoolWill said:


> Disconnected at the motor. All 6 or 12 leads disconnected from each other and separately megged.


Yup.. When you've narrowed it down to the motor. I personally don't like disconnecting the motor leads first as there is a greater chance of messing something up when it is reconncected. Meggering from the starter out, as most know, checks the cabling, connections and motor in one shot. If there is a fault, then further work is needed to narrow it down and the next step I'd do is to look at the existing connections in the peckerhead and see if there is a smoking gun in there. If not, then and as you've said breaking apart each connection will narrow it down to which winding has an issue. For me, if the motor has a failed winding I don't care which one it is, as a motor shop will repair it as needed. If the motor checks out, then I'd test the cabling, and see if it is clear of faults. And as Paul said grounding in between each test is also key.


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## paulengr (Oct 8, 2017)

One other thing. You can do a surge or LCR test, resistance test (with a milliohm meter not a multimeter), and an insulation resistance, PI, and graphical PI test on a motor and it can still be bad. This is every diagnostic test an advanced motor tester runs. It is the list of tests that every motor shop runs on incoming motors before they tear them down and recommended by IEEE, NETA, and EASA. This set of tests does not tell anything about the rotor, bearings, or the fan. But most failures are the load, the power source, the bearings, or the stator. Rotor failures aren’t very often. So killing it with stator tests covers about 95% of motor problems with the motor itself. Just because it passes all of these tests or especially just one (megger) doesn’t mean the motor is good. Just that it is less likely the motor is bad and you should probably look for something else first. Again another rookie mistake...motor meggers fine so it must be good. It’s an easy one to make though because most problems are in the load in most plants and the starter trips on overload just like it should. But operators assume it’s a motor problem because the motor won’t turn when they hit the start button no matter how big of a hammer they use on the button or how long they hold it down.

Yes you cannot fix stupid. You just promote them to management so they can do less damage.


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## gpop (May 14, 2018)

The "all-test pro" and meters like it are finally coming down in price. They are also getting simpler so you don't have to understand the readings just look for the good, warning or bad. 
Add that to a good recorder that can show you what really happens on start up/events and you now have a tool box that's capable of diagnosing 99% of the problems or at least avoid sending of a good motor to the shop.


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## paulengr (Oct 8, 2017)

gpop said:


> The "all-test pro" and meters like it are finally coming down in price. They are also getting simpler so you don't have to understand the readings just look for the good, warning or bad.
> Add that to a good recorder that can show you what really happens on start up/events and you now have a tool box that's capable of diagnosing 99% of the problems or at least avoid sending of a good motor to the shop.



That one in particular is very questionable. See this report. Baker online and All Test are questionable. The others produce similar results. I would not say prices are coming down though. Except the ATP none of them have changed much in the last 10 years.

https://www.epri.com/#/pages/product/000000000001008377/?lang=en-US


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## joebanana (Dec 21, 2010)

paulengr said:


> Six or 12 lead motors tested at the motor. Testing one lead only works if all the groups are wired together.
> 
> Also if you run into a 6 or 12 lead wye when the neutral is grounded you have to lift the ground first in addition to disconnecting VFDs and sift starts. I see these once in a while with CTs (differential protection) on large motors or goofy remote overload wiring when engineers overthink it.


 All good points to consider but..........I was referring to your basic 3 lead, 3 phase induction motor. So, thanks for playing, and we have some lovely parting gifts for you, Don, tell him what he's won........Well you can't have that, but, if you're an American citizen you are entitled to a heated kidney shaped pool.
~Fee Waybill~


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