Overvoltage during deceleration

OK guys, show me what you've got. One of our VFDs is starting to trip out a few times per hour with the message "Overvoltage during deceleration". This setup has been running this way since 1995, and I've been here 2 years and Haven't seen that one come up yet. Just wondering what you experienced guys have seen in the past when this type of problem arises. I'm sure the cause of the problem will rear it's ugly head soon, but if this is a warning of something I can help prevent, that would be better.

Thanks in advance,

Vetteboy

Vetteboy,

Since it has been running so long- my first thoughts are-

Is the incoming voltage higher? Maybe some machinery has
been taken offline.

Did someone decrease the accel time?

Is the load greater? trying to stop a larger mass?

I would say no to the first 2 questions. Is the load greater? Actually it is happening as we run smaller logs through the canter. My first instinct is that maybe something is binding somewhere, but I don't want to jump to conclusions. I would love to hear of other cases where this problem has occured long after the project was commissioned.

If the first two cases are no- then the drive must be in regen. mode- thus the bus voltage to going up.
Are you pulling the logs out with another mechanism?

Yes, this VFD runs the infeed chain on the bottom and the first 2 spiked rolls that feed the log forward by grabbing the sides of it. Right after this another VFD runs a second set of spiked rolls that run at the same speed as the first. These push the log through the side chipping heads.

I may have got on here with this too soon. It's looking like maybe we aren't getting lube to the bottom chain anymore. This added friction may be causing the problem. I will let you know how this pans out. Thanks for the quick reply JR. Does this sound like a probable cause to you?

I wonder if these speeds are still matched?
Does the first drive have a braking resistor?
If yes- is the chopper and resistor working?

The decel part is throwing me-
From the drive fault message - it sounds like it is tripping while the drive decels to a stop. Thats why it doesnt sound like a chain lube type problem.
Good luck.

I think JRW is right, check braking resistor is open or not, if you are using one.

Good Luck.

Remember also that when you say stop, the drive ramps down to zero speed in its decel time. If it is a high inertia load then the drive needs to pour power into the system to attain the decel time (if the drive is set st rsmp to stop). This I found is the main reason for that fault. Have you speed the drive up, increased the load or shortened the decel time.
I suppose also if you had a high inertial load on a binding chain, then a quick stop would be OK as the binding chain helps the braking. Lube the chain and the extra braking is not there anymore so the drive has to do it all. Just a few thoughts of mine, ie it could be many things. Regards Alan Case

Just concurring with the rest

Reducing friction will aggravate the problem. The fault indicates either that the decel is too fast or that the dynamic/regenerative braking is not functioning properly. The over-voltage trip is a mechanism for protecting the semiconductors in the drive and refers to the DC bus voltage. This typically rises on decel as the motor dumps energy back into the drive. The drive can dissipate the energy into a dynamic braking resistor or regeneratively into the mains. Over-voltage indicates that the braking energy is more than the drive is set up to handle or the drive is malfunctioning.

Try this analogy: you're cruising down the 401 in the 'vette at 100 mph and (inexplicably) slam it into 1st gear. What happens? motor loses some parts? clutch disintegrates?

A disintegrating clutch is probably a close analogy to over-voltage trip.
beerchug

I stand corrected. Could everyone read my last post as the "drive needs to soak up power/energy from the system". instead of pour power into the system. Thanks. Alan

Will the system allow you to increase the decel time. Even a small increase will help eliminate the overvoltage.

He says that his is using smaller logs. Perhaps larger logs helped the system stop. The smaller logs are making the drives absorb more of the stopping energy.

Vetteboy, does this drive have a separate fault message for high input voltage and overvoltage during decel? If it does not, your problem could be input power disturbances.

Assuming there are separate messages, it is clear that the problem is regenerative braking either when slowing down or when stopping if you are using ramp to stop.

If the original setup was very close to the "edge" on tripping, it is quite possible that obscure machine changes or process changes will have put the drive "over the edge" and caused a fault.

Changing the size of the logs, improving lubrication, machine becoming looser as it wears, higher power supply voltages, can all cause this fault to appear. Shortening the decel ramp time will surely get you into trouble if you are on the edge.

The solution may be just as simple. Run the decel ramp out a little bit longer if the process can stand it. Tap the input power down one tap if that's possible.

It will cost a little money but you may need to add a snubber brake chopper and resistor or increase the capacity of the one you already have.

The last thing I would suspect is that your drive has somehow changed to cause this fault.

I don't know if this is applicable to your situation but, I have found that reducing the driving sheave or sprocket diameter and running the motor into overspeed can improve the braking capacity without changing the drive hardware. The torque and cooling performance of the motor improves as well. You might want to consider this.