Motor Megs out Bad, Then Runs?
Operator calls and is unable to start his machine.
Found: ABB Drive faulted with "Earth Leakage" Displayed.
Motor was 100 HP, Jacket cooled.
With Fluke 87 III Multi-meter I found a Slight reading to ground on one of the phases. (measured on Load side of VFD, Still connected)
I disconnected the Load side wires from VFD. (T1, T2, T3) and megged motor to ground. I found a short on all three Phases.
I disconnect wires at motor and megged at studs in motor junction box. Still reading all three shorted to ground. (Tested Meg out on another motor, work fine)
Tested another Identical machine (we have several). Disconnected from VFD and megged clear.
Plan was to pull motor on day shift ad send it out. Two electricians verified my readings. Motor was shorted on all three phases. They unbolted motor and removed cog belt. The rotated motor a few degrees and megged motor again, it was clear. 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.
Found: ABB Drive faulted with "Earth Leakage" Displayed.
Motor was 100 HP, Jacket cooled.
With Fluke 87 III Multi-meter I found a Slight reading to ground on one of the phases. (measured on Load side of VFD, Still connected)
I disconnected the Load side wires from VFD. (T1, T2, T3) and megged motor to ground. I found a short on all three Phases.
I disconnect wires at motor and megged at studs in motor junction box. Still reading all three shorted to ground. (Tested Meg out on another motor, work fine)
Tested another Identical machine (we have several). Disconnected from VFD and megged clear.
Plan was to pull motor on day shift ad send it out. Two electricians verified my readings. Motor was shorted on all three phases. They unbolted motor and removed cog belt. The rotated motor a few degrees and megged motor again, it was clear. 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.
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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?
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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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:
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
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
I pretwist and then use wire nuts. Solder pots rule.
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14,895 Posts
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.
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.
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.
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.
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)
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)
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.
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,
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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5,749 Posts
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.
Megged with a megohmmeter
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"
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"
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?
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.
Most likely force of habit.
I've known this for many years but still find myself hitting all 3 leads......
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.
Disconnected at the motor. All 6 or 12 leads disconnected from each other and separately megged.
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