VFD / Motor / Breaker Failure

[drktmplr12]

Vertical turbine pump
250 HP, VFD driven
Motor is 285 amps, code G, design B
VFD inside of Eaton MCC
Circuit breaker, VFD and motor all failed
Motor megged at ~7 volts each leg to ground

HMCP600L6W digitrip RMS310 set to 'H' (6,000 Amps)
Altivar-61 6W12100081 with external DC choke

No power meters available
MCP was not fully tripped
Happened during thunderstorm, South Florida
No other drives connected to same MCC impacted
No SPD on upstream switchgear or MCC

My opinion is that the motor failed and the fault cascaded to the VFD, but the the VFD did not self-protect as I would expect. The circuit breaker operated above its rating but was not set to interrupt the arcing fault current. Evidence of arcing fault can be observed on the VFD DC choke terminals and above the circuit breaker protective shroud. I believe that the VFD circuit board failed when PA/+ arced to PC/- and this caused the power flow to the motor to stop.

Interested to hear others' thoughts.

Thanks for reading

 

[synchro]

It looks like there was an arc between the inner end of the lug on terminal PO and the VFD housing. How much of an air gap is between them? It looks like the shrink tubing would be the only insulation at that point.
Maybe a surge during the thunderstorm initiated an arc at that point, and then it cascaded from there. But that's just one posibility.

 

[drktmplr12]

It looks like there was an arc between the inner end of the lug on terminal PO and the VFD housing. How much of an air gap is between them? It looks like the shrink tubing would be the only insulation at that point.
Maybe a surge during the thunderstorm initiated an arc at that point, and then it cascaded from there. But that's just one posibility.

we noted this too, but also that there is no visible heat damage to the insulation or heat shrink tubing. curiously enough, the PO terminal was bent. I am thinking this was a result of the arc, but I have doubts.

 

[drktmplr12]

Rectifier board was fried as well

 

[ptonsparky]

Is there a reason that lightning could not have been the cause?

 

[drktmplr12]

Is there a reason that lightning could not have been the cause?

Surge is a possibility.

I was hoping some of the drive guys here would see and maybe they've observed similar failures

 

[rambojoe]

we noted this too, but also that there is no visible heat damage to the insulation or heat shrink tubing. curiously enough, the PO terminal was bent. I am thinking this was a result of the arc, but I have doubts.

View attachment 2561977

What was the volts? It looks like someone tried to mitigate the gap. Instant trouble, messing w/someones expensive equip...
Looks like tape melted and peeled, bridging the gap...
Hhmmm

 

[drktmplr12]

What was the volts? It looks like someone tried to mitigate the gap. Instant trouble, messing w/someones expensive equip...
Looks like tape melted and peeled, bridging the gap...
Hhmmm

No idea, the power to the VFD was off by the time anyone arrived.

 

[rambojoe]

Zoom in on the top pic, terminal pc/-...

 

[Jraef]

Much of the time a VFD’s “self protection” is too slow against short circuits in the motor to protect the transistors in the VFD from damage due to too rapid of a rise in current (referred to as “dI/dt” for delta (change) in I (current) over delta t (time) and the transistors fail. This by the way is the chief value in a load reactor, it slows down the rise time of any short on the motor circuit side so that the drive has time to react and shut down. It’s only a few more milliseconds, but that’s enough.

99.9% of the time though, a transistor will fail open, because they basically explode (internally). So although possible, it’s not probable that the motor failed first because it would have been really difficult for the damage to the transistors to have affected the DC bus and rectifier and breaker.

Your evidence points more to a nearby lightning strike and severe spike in voltage, which raised the DC bus voltage a lot, damaged the rectifier board, caused a flashover on those terminals, and if the motor was running at that moment, would have been passed down through the transistors (before they failed) and blew through the motor insulation. The entire even could have happened in milliseconds. A line reactor MAY have been able to slow it down enough to let the drive shut down and save the motor, but not likely the drive itself.

 

[drktmplr12]

Much of the time a VFD’s “self protection” is too slow against short circuits in the motor to protect the transistors in the VFD from damage due to too rapid of a rise in current (referred to as “dI/dt” for delta (change) in I (current) over delta t (time) and the transistors fail. This by the way is the chief value in a load reactor, it slows down the rise time of any short on the motor circuit side so that the drive has time to react and shut down. It’s only a few more milliseconds, but that’s enough.

99.9% of the time though, a transistor will fail open, because they basically explode (internally). So although possible, it’s not probable that the motor failed first because it would have been really difficult for the damage to the transistors to have affected the DC bus and rectifier and breaker.

Your evidence points more to a nearby lightning strike and severe spike in voltage, which raised the DC bus voltage a lot, damaged the rectifier board, caused a flashover on those terminals, and if the motor was running at that moment, would have been passed down through the transistors (before they failed) and blew through the motor insulation. The entire even could have happened in milliseconds. A line reactor MAY have been able to slow it down enough to let the drive shut down and save the motor, but not likely the drive itself.

Appreciate you sharing your opinion. I can certainly appreciate that if the motor was running, the motor is then susceptible to line side surges. It is also an old WP1 motor (indoors) in an environment with evidence of H2S corrosion. Likely the insulation isn't in great condition.

 

[garbo]

At the large hospital/research center that I retired from had over 500 drives from 2 to 1,750 HP. All of these drives had fuses on both the input & output of VFD'S. Never had a grounded or shorted motor do damage to any drives that I can remember. One Sunday I got called in for a 75HP air handler that was down on ground fault. They had an open motor inside of the air handler that the humidifier was pouring out clouds if steam. Of course old motor windings were wet. I always removed the wires or on newer units that had a non fusible safety switch inside of air handler would turn off and run drive on hand for a few minutes starting at 15% to full speed to check drive. On this particular unit it was a 15 year old Danfoss VLT drive that had over 100,000 hours on it and still worked after motor was replaced.

 

[Jraef]

At the large hospital/research center that I retired from had over 500 drives from 2 to 1,750 HP. All of these drives had fuses on both the input & output of VFD'S. Never had a grounded or shorted motor do damage to any drives that I can remember. One Sunday I got called in for a 75HP air handler that was down on ground fault. They had an open motor inside of the air handler that the humidifier was pouring out clouds if steam. Of course old motor windings were wet. I always removed the wires or on newer units that had a non fusible safety switch inside of air handler would turn off and run drive on hand for a few minutes starting at 15% to full speed to check drive. On this particular unit it was a 15 year old Danfoss VLT drive that had over 100,000 hours on it and still worked after motor was replaced.

Wow, lucky. Fuses on the output of a VFD is a significant risk for blowing up the transistors. Fuses open by melting and the arc that forms when they do causes a brief but spectacular high dV/dt event that can then cause transistors to misfire and destroy themselves.

 

[mayanees]

My opinion is that the motor failed and the fault cascaded to the VFD, but the the VFD did not self-protect as I would expect. The circuit breaker operated above its rating but was not set to interrupt the arcing fault current. Evidence of arcing fault can be observed on the VFD DC choke terminals and above the circuit breaker protective shroud. I believe that the VFD circuit board failed when PA/+ arced to PC/- and this caused the power flow to the motor to stop.

I plotted out the circuit you described, because that's what I do for fun and because you theorized that the MCP didn't see the arcing fault current. I used 100-foot lengths on the two feeders. Depending on the let-through of the VFD, I show an arcing fault current of 15,776 amps at the motor, which is far less than the trip point of the HMCP at 6000 amps.
I think the storm is the wild card and deserves the blame.

 

[mayanees]

arcing fault current of 15,776 amps at the motor, which is far less than the trip point of the HMCP at 6000 amps.

...far MORE than the 6000-amp trip point!

 

[Jraef]

The MCP would not directly see anything that happens down stream if the VFD anyway. You have to think of the VFD as almost like an SDS. The OCPD on the line side of the VFD is only protecting the VFD input itself. The VFD is required to protect the load.

UL ceased allowing the use of MCPs ahead of VFDs for this reason starting in I think around 2012, with allowances for grandfathering of drives still in production under the old rules. but by now, every VFD mfr has released newer versions that would be listed under the newer UL rules. The latest change that just took place now will require high speed fuses on new VFD designs, even if you use a circuit breaker. So what you will start seeing is VFD packages that have a breaker sized for the real load, then high speed fuses in series, but over sized as much as possible so that they only blow if there is a catastrophic failure in the drive front-end. I just saw one for a 125HP VFD with a 200A breaker and 400A Class J fuses.

 

[garbo]

Wow, lucky. Fuses on the output of a VFD is a significant risk for blowing up the transistors. Fuses open by melting and the arc that forms when they do causes a brief but spectacular high dV/dt event that can then cause transistors to misfire and destroy themselves.

The drive manufacturer installed the fuses. Usually the input fuses blew rather then the output. Thing that I always raised concerned about was non fused safety switches installed next to a motor on a drive that did not have a NC micro switch that opened before the 480 volt blades to shut drive down. I did P touch some of these stating to only use safety switch to turn off drive supplied motor in an emergency. Had a contractor blow out cheap contactors on drives with bypasses by using the safety switch to turn motors on & off. Drive tech told me that opening up the load side of a drive might only cause damage one in 5 to 10 attempts but said it should never be done .

 

[paulengr]

On most recent VFDs UL has been cracking down on their extremely poor short circuit ratings. With normal breakers and fuses most have 35-65 kA ratings now. Most seem to have added front end reactors.

I disagree that a series inductance “slows” anything down. If we add 5% Z this adds 5% to the reactance of the transformer. Sure it shifts the phase angle slightly but the big deal is you are in most cases doubling the impedance seen by the VFD in exchange for cutting the input voltage by a like amount. So if in the above discussion we are at 15 kA adding the line reactor can cut it to 7.5 kA easily as seen by the VFD. That may not be enough for a cheap 5 kA VFD but it’s much closer.

 

[paulengr]

The drive manufacturer installed the fuses. Usually the input fuses blew rather then the output. Thing that I always raised concerned about was non fused safety switches installed next to a motor on a drive that did not have a NC micro switch that opened before the 480 volt blades to shut drive down. I did P touch some of these stating to only use safety switch to turn off drive supplied motor in an emergency. Had a contractor blow out cheap contactors on drives with bypasses by using the safety switch to turn motors on & off. Drive tech told me that opening up the load side of a drive might only cause damage one in 5 to 10 attempts but said it should never be done .

While I agree that this is supposed to be VERY rough on drives 3 contactor bypasses used extensively in water plants have no such protection either and slam the outputs in and out every time the system turns on and off which can be very frequent. I haven’t seen any evidence of more frequent replacement.