Ground fault on VFD

rejoe.koshy

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Dec 2011
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kolkata
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Hi guys,
I am facing weird issue of my VFD tripping on ground fault.I have a Sinamics drive & a siemens servo-motor installed.A couple of days back the drive started tripping of ground fault repeatedly .I checked the cable & motor for existing fault conditions but couldnt find any.Hence I conclude that the drive might at fault & replaced it by a new one.To my dismay, the fault reappeared.

I really at a loss as whats going on.I have checked the 3 phases of the motor to find equal resistances on all three phases with good values for phase to ground values on the megger.The cable was also subjected to megger test but seemed fine.

What else could trigger a fault like this?

Pls help

Note; The fault appears as soon as I issue the servo-on command.
 
Sorry, didn't see you had already megged it. What do you consider "good values?" Have you megged the wires feeding the motor? Often times they can get easily nicked going into the peckerhead.

Also, if that is the case you can try a different drive and see if the problem persists.

Generally I will open the local disconnect or unload the wires from TUV and start with the wiring all the way through to the motor. If I get a bad reading I will go straight to the motor. If that is good, segment and work my way through the feeders on the load side of the VFD.
 
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I megged the cable , where in I have got a phase to ground values of 4 Megaohms which I am assuming is good.The motor phases also throw up the same megger values.

& Yes, tried with another drive & the same fault appears.

By the way how does the drive detect the fault.Does it see the difference between the current flowing in positive & negative link of the DC bus or does it have a balance zero core current transformer to measure the imbalance in its 3 phases??
 
I'm not sure how the drive senses a ground fault. If you tried with a different drive and are getting the ground fault, I would bet that there is some condition when you hook everything back up causing there to be an actual fault to ground, you just have no found it yet.

What voltage are you testing at? What voltage is the motor? Most generally, depending on the voltage and the size of the motor I would want to see resistance reading approaching closer to a a gigaohm unless you are using very high voltages.

Other than that, you will have to wait for someone that knows more to chime in. Also, if you do figure it out please post back here so that it may help someone else.
 
I'm not sure how the drive senses a ground fault. If you tried with a different drive and are getting the ground fault, I would bet that there is some condition when you hook everything back up causing there to be an actual fault to ground, you just have no found it yet.

What voltage are you testing at? What voltage is the motor? Most generally, depending on the voltage and the size of the motor I would want to see resistance reading approaching closer to a a gigaohm unless you are using very high voltages.

Other than that, you will have to wait for someone that knows more to chime in. Also, if you do figure it out please post back here so that it may help someone else.

I am testing it with a 500V megger.The rated voltage is 440 V, 50 Hz.
 
When was the last time you ran these drives. If over a year ago then maybe the capacitors got drained and are now shorting out causing this fault.
 
500V Megger is not good enough for a 440V Motor. You need to exceed the bus voltage (~650v).

1000V is the standard, and IMO, it's lower than you really need. We should be testing 400v class systems at 2000-2500v.
 
500V Megger is not good enough for a 440V Motor. You need to exceed the bus voltage (~650v).

1000V is the standard, and IMO, it's lower than you really need. We should be testing 400v class systems at 2000-2500v.
Beat me to it. 🍻

VFDs sense ground faults using a Residual Current Detection method with current sensors on the output conductors, usually looking for 100mA or more. Small differences in cable capacitance can sometimes trigger it, but that generally shows up immediately at commissioning, not later on. The fact that the output has high frequency common mode noise and standing wave spikes can also cause GF trips if the cables are not properly shielded, so check your cable shield connections. One thing that can happen if you used steel conduit instead of shielded cable, is that over time, corrosion interferes with the conduit's ground path and that HF noise finds a different more resistive path back into the drive, which means losses and thereby an imbalanced return current.
 
Hi, I was taught back in the day that 1 Megaohm per volt was the MINIMUM you could get away with for resistance from phase to ground. Therefore, about 500 megaohms to ground would be bare minimum on a 480 V motor voltage. I like to see at least a gig on all conductors. And,
BTW, as Gene says, test voltage is important. I was taught 3 X line voltage for megging and Hi-pot testing. So at 480, approx. 1,500 V. We used a Hi-pot machine a lot on medium voltage motor feeders at a utility I worked for and sent about 15kv on a 4160 V. motor feeder.
 
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Hi, I was taught back in the day that 1 Megaohm per volt was the MINIMUM you could get away with for resistance from phase to ground.

That would be nice to see, but realistically it is 1 Megohm per KV. Minimum 1 Megohm.

Worked in underground mines with very wet conditions and ground fault protected motors often tested close to this minimum without failure.
 
interesting note the difference between HiPot and Megger testing
HiPot is destructive testing that is when it fails the part you are testing is destroyed in the test
Megger is non destructive if the part fails it is not damaged during the test but you should still not use it

I was thought way back when to test a 480VAC motor or line you need a 5,000V megger otherwise your reading can not be relied on.

I hope you didn't meg or hipot the motor while it was connected to the drive that could blow the drive

most drives look at the current on the output for a imbalance, current going out = current coming in the difference is a fault.

look for moisture or dirt in the motor that can cause a ground fault trip.
 
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We had a similar issue with a Yaskawa drive. It would keep tripping on ground fault even though the motor meggered fine. We ended up replacing the motor (150HP 460V 3Phase). Had it sent out to a motor shop and they found water damage on the stator of the motor.

If you've already replaced the drive and you've checked your feeders, motor cables and ground connections then it is most likely the motor.
 
Just a thought...

Check your motor lead connections at the drive. A "wild hair" on a lead touching a ground will certainly give you a ground fault.
 
PITA motor

Looks like you got a lot of advise here. One thing no one has mentioned yet is.... TURNING the motor WHILE watching for a ground. I've encountered this problem and when my PITA (Pain In The A**) motor spun to a stop for Meg testing, "what do you know.... no ground again."
Turned out that only at a certain rotation did the ground appear. With it happening in only 5 degrees of a certain position, I guess the chances of it stopping on the ground was 1 in 72.
I have a few more suggestions, but give this a try 1st if no one else gotcha ya going.(y)
 
Looks like you got a lot of advise here. One thing no one has mentioned yet is.... TURNING the motor WHILE watching for a ground. I've encountered this problem and when my PITA (Pain In The A**) motor spun to a stop for Meg testing, "what do you know.... no ground again."
Turned out that only at a certain rotation did the ground appear. With it happening in only 5 degrees of a certain position, I guess the chances of it stopping on the ground was 1 in 72.
I have a few more suggestions, but give this a try 1st if no one else gotcha ya going.(y)

This happened to me once. Motor would only fail a megger test (or fault the VFD) if it was stopped at one particular angle. My PITA motor was difficult to replace so we marked the shaft and housing with a paint pen. If it faulted in that spot, we would power down, turn it a couple of degrees to get off that spot, then fire it back up. This machine ran almost all the time, so we only had to do this somewhat boneheaded maneuver a few times to buy us a couple of weeks until we could gather the materials and manpower to swap the thing.

I was always taught to use 500v test on 480 DOL motors and 1000V test on VFD (480v) driven motors.

Trivia: I learned that the old A/B 160 series VFDs could tolerate megger readings as low as 2 to 3 megohms, but a Toshiba drive we had needed at least 80 megohms to keep from faulting. I bet one of the drive gurus could explain in detail the reasons why this was the case, all I cared about was what to tell my technicians when they were doing testing or complaining about why this drive versus that drive gave them variable results. Eight years in a food plant gave me a lot of practice running megger tests and replacing motors. Every morning after sanitation it was a roll of the dice whether you would be replacing one to five motors and whether they would be 1/2 or 100 HP.
 

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