Multiple motors fed from one VFD

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adamscb

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I have a colleague who's having trouble with multiple motors being fed from one VFD. I'm having him look to see if the drive is set up for volts/hz control, and not any form of vector control. Would that be a cause of the VFD burning up? There are separate overloads for each motor, and I believe that if one OL trips it takes out the VFD, so no worries there. Anything else I should consider as to why the VFD burnt up? Any help is appreciated, thanks
 
#1 problem: Collective motor amps is more than the VFD is rated for. People often go by "HP" and you ALWAYS must go by motor nameplate FLA. Then I always recommend adding an additional 10% to the amp rating, plus a load reactor, because the VFD is exposed to n times the number of circuit conductors and lengths (n = number of motors), so the risk of conductor capacitance / damage / failure increases by at least that much.

But honestly, a good VFD should have tripped on SOMETHING before "burning up" (assuming you mean failing catastrophically). Can you describe the failure better?
 
One 50 hp is feeding two 25 hp motors. I don't think these motors run at full load capacity, but I can check. I know there isn't a load filter, I'll recommend that. It ran a year and a half from what I hear. So I take it the motor control setting being on vector couldn't really cause a drive to burn up? "Burning up" is all I'm getting, this happened a while ago, so no chance of pictures. I'm assuming charring.
 
Haven’t done any multiple motors that size, but did do multiple Big Ass Fans off one VFD. Looked like they had a choke on the output. Factory sized everything.
 
It ran a year and a half.

Did only one drive burn up so far or is this a recurring issue?

As mentioned the motors likely didn't take out the drive, most drives are pretty good at protecting themselves and shutting down (leaving you with a fault code afterward) before they destroy themselves.

Issues with input voltage may be more likely than motor(s) causing problems with the drive, even with lost input phase it could still run on single phase input, but that makes the front end rectifier draw more on the remaining two lines than if it still had all three lines, and that could eventually lead to total failure to operate.
 
You are correct, Vector Control cannot be used on multiple motors because for it to function, you must have an accurate "model" of the motor equivalent circuit so that the drive can do the vector calculations. With multiple motors, the circuit is invalid for either motor individually. It has to be in V/Hz mode only. But that salone hould not have caused the drive to "burn up", it just wouldn't run the motors well, it would make them run "choppy" as the VFD kept trying to find the right output pattern to get it to respond correctly, which it could never do. It might have over loaded the drive, but it would have tripped.

Now that said, if it tripped, then they reset it, and it tripped, and they reset it, trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, .... that can kill the drive. But they shouldn't be surprised... I have seen this happen; some VFDs have,"hidden" records of faults and resets and in some cases. As a service tech years ago I had a 500HP drive fail "for no reason" and when we looked at this record, that's what we saw; over 100 OC trips and resets in the month leading up to the "failure for no reason", many in clusters within minutes of each other. They were using the VFD trip function to try to find the "sweet spot" in their operation of a high pressure blower.
 
Ballpark cable length from VFD to motors might also be helpful since you don’t have load reactor. I have several installs of (2) 50 HPs on a single 100HP VFD (ABB) with semi-longer motor cable length and no issues, BUT, we always used load reactor.

Lots of good insight here. I’ll also second kwired’s input voltage suspicion. Had a drive strangely fail a few years ago. Thankfully it had sent out a sort of SOS to the PLC about input voltage being above range right before it said goodnight IIRC.
 
Jraef said:
You are correct, Vector Control cannot be used on multiple motors because for it to function, you must have an accurate "model" of the motor equivalent circuit so that the drive can do the vector calculations. With multiple motors, the circuit is invalid for either motor individually. It has to be in V/Hz mode only. But that salone hould not have caused the drive to "burn up", it just wouldn't run the motors well, it would make them run "choppy" as the VFD kept trying to find the right output pattern to get it to respond correctly, which it could never do. It might have over loaded the drive, but it would have tripped.

Now that said, if it tripped, then they reset it, and it tripped, and they reset it, trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, , trip / reset, .... that can kill the drive. But they shouldn't be surprised... I have seen this happen; some VFDs have,"hidden" records of faults and resets and in some cases. As a service tech years ago I had a 500HP drive fail "for no reason" and when we looked at this record, that's what we saw; over 100 OC trips and resets in the month leading up to the "failure for no reason", many in clusters within minutes of each other. They were using the VFD trip function to try to find the "sweet spot" in their operation of a high pressure blower.
Click to expand...
Can possibly be worse yet if you have external reset enabled and set it up to automatically reset.

Finding that sweet spot would be easier to set it to display running amps and go by that vs wait and see when it trips:(
 
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