DC Bus Overvoltage on 20hp VFD 3

[rockman7892]

I'm having an issue with a 480V 25hp vfd tripping on overvoltage. The drive is a Siemens MM440. The application is as follows.

We have a reclaimer that reclaims material and moves back and forth on a set of rails with two wheels. Both of these wheels have a seperate 25hp VFD and run at the same speed to avoid skewing on the reclaimer. If one wheel falls behind for some reason the drive of that wheel compensates by speeding up and trying to keep the reclaimer from skewing. The two wheel assemblies have skew detection switches to detect skewing.

We recently have an issue where one of the rails that these wheels ride on appears to be misalighen in a certain spot and therefore causing the wheel and drive to get caught and have to try to drive harder to overcome the misalignment. While this drive tries to speed up and ovecome this area the drive is tripping on an overvoltage fault. After tripping and resetting we try to start the reclaimer again and while the drive is starting and attempting to re-align with the other drive it goes down on an overvolatge fault again.

I cannot seem to understand why this drive is tripping on an overvoltage fault. From my experience, I always thought that overvoltage on the DC bus could be caused by one of two reasons, either a high voltage on the line side of the drive, or regeneration from the motor due to the load causing the motor to turn faster than the synchronous speed that the motor is being driven at. These drives are only being run very slowly at around 15hz and do not move the reclaimer very fast. Therefore I cannot see this motor causing any regeneration onto the drive. Even when the drive is stopped on other parts of the rail we do not have a problem with bus overvoltage, only at this certain spot on the rail that appears to be warped and causing the drives to get hung up.

Does anyone know what other than re-generation from the motor can cause this bus overvolate especially when the drive would have an increaed toruqe requirement to overcome this area of the rail?

 

[Phaneas]

A load drop can also cause a short voltage peak.

Do you have no brake resistors at the drive?

 

[itsmoked]

You can have one drive actually driving the other which leads to the driven one going high on the DC bus.

I can see this happening during the gyrations caused by the alignment issue. They both experience the drag and both increase due to this bind. Then the bind passes and both are over driving and as they are commanded back to nominal levels the one finds itself in regen from inertia and the other drive perhaps, still providing push.

What you should probably do is fix the rails. Head out with sledge hammers, torches, jacks, and some strong backs.

I would not start dorking with the drives to try to compensate for mechanical issues as they will come back worse and worse. You may be seen as, "never getting it right". Also the misalignment could show a serious defect brewing that you don't want to gloss over with electrical smoke and mirrors.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

I agree with itsmoked.
Fix the problem, not the symptoms.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DickDV]

It probably wouldn't be a bad idea to tie the two DC bus's together as in a common DC bus application. That way, if one regenerates, the other simply uses the excess energy for driving.

 

[rockman7892]

I agree with the posts above that the mechanical problem should be fixed, and I have gotten maintnance involved to fix this issue. I am simply just trying to look at this situation to try and disect what is happening electrically.

There are brake resistors on each of these drives (13.5kW) however the dynamic braking duty cycle was set at 50%. We went ahead and increased this duty cycle to 100% and it seems to have eliminated the issue, so maybe these brakes are being more effective.

So it sounds like there is a possibility that as the on drive is pushing hard to get thrugh the warped area, it may speed up and then when it finally brakes through this are the speed is decreased but the wheel is still moving due to the drive pushing it through, as well as the other drive moving he other side, and this combined movement my cause regen on the drive as it is slowing the output speed?

Phaneas

You mentioned that a load drop can cause a short voltage peak. What typically happens in this case to cause the voltage to go high?

DickDV

I like your idea of tying these buses together. Not sure that I know enough to attempt it but out of curiosisty what is generally involved with doing this? Can you cable them together? Any settings or parameter changes?

 

[waross]

The motor doesn't have to change speed to go into regen. If the controls decide that the positions are equal and drop the set point on a motor below the motor speed the drive will go into regen.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[DickDV]

The two DC bus's are wired together + to + and - to -. You need fuses in those leads too. Check with mfgr for sizing.

Do a search in this forum for common bus. You need to change a few parameters as well. You can leave the brake resistors connected.

 

[itsmoked]

Why would you bother to do this? The system worked fine before the mechanicals went south. I can't see how this would benefit your company or operations at all.

It's not like these two VFDs are expected to be in opposite quadrants on a regular basis. You have two electrically independent systems and now you want to intertie them for no obvious gain. System complexity is going to go up. What you see on the DC buss is not going to make sense during future troubleshooting, making that more problematic.

You might not have discovered the mechanical problem if you'd tied them together previously. How is that an improvement?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rockman7892]

Itsmoked

I should have further clarified in my last statement that I do not intend to try to tie these buses together. I was simply wanting to gain a better understanding of this practice and how it would be implemented so that I would be equippped with this knowledge in the future.

After changing the duty cycle on the dynamic braking cycle as I mentioned above these drives seem to be working fine.

 

[itsmoked]

rockman7892; Good to hear. I certainly have no problem with wanting to understand things. That's great.

Glad it's working for you too!

Keith Cress
kcress -

http://www.flaminsystems.com