VFD Blew Up

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nuckythompson

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Nova Scotia
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Electrical
Hi there, so we had a VFD blow up and are now having troubles installing the new one (new VFD is a powerflex 753). The VFD drives 3 x 5HP motors that are connected to gearboxes that move a railed coal stacking system (example picture attached). 2 motors on the south rail and one on the north rail. When we start the system the motors spin to pick up any slack (flexible coupling) and then won't spin any further. Then the VFD amps just get higher and higher until it eventually trips on overcurrent. We were seeing upwards of 50A output. We clamped each motor and each motor was drawing 1/3 of the current. Thinking this is way too high and I don't want to blow up our replacement, we did not test any further with the motors coupled. The VFD parameters all matched the previous VFD which had been working for many years.

With the motors uncoupled, everything worked like a charm. All three motors spun and total amp draw was 5A (unloaded) and the VFD got up to the 60Hz speed. This led me to believe there is a mechanical issue. Also, one of the motor shafts spins easily by hand until it gets pulled back by whatever is keeping the stacker from moving, but you can feel the whole stacker move when turning the shaft by hand. The other two motor shafts will not spin by hand. Also, with the motors uncoupled, the mechanics took pipe wrenches to all 3 gear boxes at once and were able to move the stacker 6", but I am not sure how much force was required.

Also, we confirmed the directions of the motors, so it is not a case of one motor fighting the other.

I guess my question is, have I exhausted the electrical troubleshooting? Am i missing something? Is there somehow a way the motors are getting the current but not putting out enough torque or force to move the gears? I cannot remember what the voltage readings were but the motors appeared to be spinning at rated rpm when uncoupled.

There is also a start acceleration boost function on the VFD but I don't feel like this is the way to resolve the issue.

The mechanical team is reluctant to try anything, but our electrical team has run out of ideas. Any help or thoughts is greatly appreciated.

Thanks :)
 

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The difference between free hand turning and not is potentially worrisome. But if the two motors on the same rail go through independent gear boxes that do not transmit torque in the reverse flow (i.e. worm drive), then what you may be seeing is that all the slack is out in opposite directions and you cannot turn one motor shaft since the other gearbox is not able to turn the other motor shaft to go along.
But much more likely there is a mechanical binding problem.
All three motors are driven in parallel from one VFD output? That means that any one motor "sticking" will prevent the VFD from accelerating any of the other two motors.
Can you try decoupling the output of one or both gearboxes from the rail motion?
If you put a wrench on the motor shaft, can you turn it with a reasonable torque, something that the motor could be expected to develop from a locked rotor condition?
I hesitate to suggest for fear of burning out motors, but applying full line voltage to the motors (bypass) would tell you whether or not it is a mechanical problem.

The symptoms are consistent with a mechanical failure that burned out the first VFD and is keeping the new one from working.
 
Thanks for the reply. I really appreciate all your thoughts.

I am not exactly sure what you mean about the reverse flow and worm gear. Each gearbox is independent and we don’t suspect they are fighting each other because there was nothing changed with regards to motor leads or gear boxes since when it was operational.

Yes, all three motors are drive by one VFD, so if one is bad it affects the whole system.

The mechanics did take pipe wrenches to the gear box with the motor uncoupled and were able to move it but I don’t know how much force was required. The motors spin perfectly when uncoupled.

We can decouple the motors but I am told it is not easy to deal with the gearboxes. If we could remove the gearboxes we could try taking one out at a time. The stacker will move off two motors, maybe even 1. I will ask the mechanics about the gear boxes.
 
If it took significant torque from a wrench on the gearbox shaft which was uncoupled from the motor shaft, it is not likely that the motor would be able to turn it from a stopped condition, even with full locked rotor torque available.
It really sounds like a mechanical jam on that rail that is affecting both gearbox/motor combinations. Which might be hard to find without uncoupling both gearboxes, or might be obvious on thorough inspection..
 
Just throwing this out there, but perhaps it's jammed from foreign material blocking movement in the desired direction. If so, there could be an outside chance that programming the VFD to reverse the rotation and running it just a short time could free up the blockage. Before doing this, I would check with the mechanical guys whether they see a problem with this.
 
synchro said:
Just throwing this out there, but perhaps it's jammed from foreign material blocking movement in the desired direction. If so, there could be an outside chance that programming the VFD to reverse the rotation and running it just a short time could free up the blockage. Before doing this, I would check with the mechanical guys whether they see a problem with this.
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Thank you. The VFD is programming to run forward and reverse. We tried both directions. Same result.
 
The drive has to autotune to the motors. Normally a new drive will do this without you doing anything. It will usually autotune when parameters are changed, but I don't know how they do this with single drive multi motor load. Knowing the question but not the answer, I would call AB and ask.

You should not need pipe wrenches on the motor shaft for anything. If it has a bad bearing, somewhere in the mechanical load drive system, turning it with a pipe wrench is not giving the information you want.

Possibly usually the easiest thing to try is to pull the fan cover off and turn the fan by hand, being careful the fan may be plastic and attached very fragilely, but that is all that may be needed to get shaft movement through the gearbox and move the trolley. Probably all three at the same time, sounds like that may be an easy test you can perform. I won't say should, but it could move the trolley just by turning the fan blade by hand, without applying enough force to break the fan. If you try that try it carefully. It's one of the easiest things to try unless you break the fan blade.

If the drive it set up properly for the multimotor load, the current draw means the motors are trying the turn the load. It would indicate mechanical binding if it does not move with motor power applied normally. I am wondering if there is some system to keep the three drive gearboxes synched somehow so the drive system is always square and parallel to the rails and not racked out of square. I don't see it moving if the mechanical drive system gets racked out of square. I would want to know how that's usually accomplished, letting the individual motors just pick up slack, would not be my first guess. That machine manufacturer must know what they expect for that.

The final drive system should be able to move several feet just by turning the fan blades by hand. Don't try to apply so much force the plastic fan blades get damaged.
 
__dan said:
The drive has to autotune to the motors. Normally a new drive will do this without you doing anything. It will usually autotune when parameters are changed, but I don't know how they do this with single drive multi motor load. Knowing the question but not the answer, I would call AB and ask.

You should not need pipe wrenches on the motor shaft for anything. If it has a bad bearing, somewhere in the mechanical load drive system, turning it with a pipe wrench is not giving the information you want.

Possibly usually the easiest thing to try is to pull the fan cover off and turn the fan by hand, being careful the fan may be plastic and attached very fragilely, but that is all that may be needed to get shaft movement through the gearbox and move the trolley. Probably all three at the same time, sounds like that may be an easy test you can perform. I won't say should, but it could move the trolley just by turning the fan blade by hand, without applying enough force to break the fan. If you try that try it carefully. It's one of the easiest things to try unless you break the fan blade.

If the drive it set up properly for the multimotor load, the current draw means the motors are trying the turn the load. It would indicate mechanical binding if it does not move with motor power applied normally. I am wondering if there is some system to keep the three drive gearboxes synched somehow so the drive system is always square and parallel to the rails and not racked out of square. I don't see it moving if the mechanical drive system gets racked out of square. I would want to know how that's usually accomplished, letting the individual motors just pick up slack, would not be my first guess. That machine manufacturer must know what they expect for that.

The final drive system should be able to move several feet just by turning the fan blades by hand. Don't try to apply so much force the plastic fan blades get damaged.
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Interesting, thanks. The drive is set to auto tune “static tune” and does so on first startup. As for the machine staying square, I do not know how this is done. I will look into it. The wheels are slightly offset on the rail, maybe it needs to be centered. To my knowledge there is nothing electrical in the system that would accomplish this. I will also mention the fan idea and be careful about it. Much appreciated!
 
If the mechanical team is reluctant and your end checks out fine, check the start up sheets on the rail.. You may find there is safety locks and a start up and unlock procedure for your rails.
Its worth a drive to chat with a vet or a older operator. Do not rely on the younger facility crew- you may find that they just want you to figure it out the hard and expensive way... I hate to say. Report back!
 
nuckythompson said:
Interesting, thanks. The drive is set to auto tune “static tune” and does so on first startup. As for the machine staying square, I do not know how this is done. I will look into it. The wheels are slightly offset on the rail, maybe it needs to be centered. To my knowledge there is nothing electrical in the system that would accomplish this. I will also mention the fan idea and be careful about it. Much appreciated!
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If the drive is not able to get the motors to move at all, auto tune will inevitably fail. But exactly replicating the settings of the old drive should work in the absence of external problems.
 
GoldDigger said:
If the drive is not able to get the motors to move at all, auto tune will inevitably fail. But exactly replicating the settings of the old drive should work in the absence of external problems.
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they move the tiniest bit to pick up slack. Do you recommend any alternative?
 

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zbang said:
I've always understood that one VFD/one motor was the preferred setup. When is it acceptable, or even preferred, to have multiple motors (assuming they're identical, different motors sounds like courting disaster).
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I hear that, The last company I worked at bought equipment from a manufacturer that did this a lot for motors that were doing the same thing like conveyors that ran at the same time or 2 edge trimmers that would be on at the same time. Made it a pain to trouble shoot but saved a little money and cabinet space I guess?
 
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