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Old May 16th, 2018, 04:41 PM   #1
lesmar96
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overvoltage fault on vfd

I am having some issue with two vfds installed at the same location. They are tripping out on overvoltage faults very randomly. I checked the incoming voltage, and at 245, it is pretty high, but not high enough to get the utility company to look at it.

The one drive is running a bandsaw. We have plenty of decel time programmed in. It seems it faults out when the the load changes very rapidly. Like it tends to trip out, right when they hit a knot in the log etc. It is a bit puzzling to me why rapid fluctuation on the load would create an overvoltage condition. The only thing I can think of is if the motor is somehow creating a regen when the load suddenly lessens. But I wasn't sure what to change in the programming to get around it.

I would appreciate any thoughts you would have on this.
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Old May 16th, 2018, 04:52 PM   #2
James Mcquade
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Please post the brand and model of the vfd units so we can better assist you.

james
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Old May 16th, 2018, 04:53 PM   #3
jraef
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Common cause:
1) Regeneration takes place when you have the motor energized (excited), AND the output frequency is lower than the load speed. Lengthening the decel time can help dampen a rapid change in speed to help the load dissipate more of it's kinetic energy via friction and other load losses, but it is not a "cure".

2) If you have the VFD set to current limit rather than trip on a high change in load, you can get yourself in trouble. What a VFD will do to limit the current is to override your commanded speed and lower the output frequency in an attempt to shed load on the motor. This also overrides your decel time so if the change in load is too fast, the rapid drop in output frequency becomes an issue allowing for regen as described above.

One thing to consider: why do you have decel enabled at all on a band saw? Decel is used to PREVENT a load from stopping faster than it would naturally, for example a pump to avoid creating water hammer. I see no benefit in having decel enabled on a band saw. Just turn it off. Usually you do that by setting the "Stop mode" to "Coast". If you want to stop the band saw FASTER than it will coast, then you want BRAKING, not decel. Decel is the OPPOSITE of braking.
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Old May 16th, 2018, 05:49 PM   #4
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Quote:
Originally Posted by jraef View Post
Decel is used to PREVENT a load from stopping faster than it would naturally...Decel is the OPPOSITE of braking.
I've never thought about it in that depth. I've learned my new thing for the day!
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Old May 17th, 2018, 05:26 AM   #5
Gene Bond
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Quote:
The only thing I can think of is if the motor is somehow creating a regen when the load suddenly lessens. But I wasn't sure what to change in the programming to get around it.
I've seen this many times. Basically it comes down to either a poor tuning, or just the load dynamics being too much for the drive to handle without some persuasion.
Most medium to high performance drives have current limits for both motoring and braking/regenerating. By limiting the braking torque through current limit, the drive can ignore some of the regenerated energy an use it later.
The alternative is to waste the energy into a shunt/DB resistor to regulate the speed.

It just depends on what hardware you have, how precise speed regulation is, and how much you want to spend.
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Old May 17th, 2018, 05:41 AM   #6
bitmonkey
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Check your mains voltage. I once had an overvoltage fault because the AC supply voltage was too high.
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Old May 17th, 2018, 06:02 AM   #7
drforsythe
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Overvoltage faults can be caused by a few things. The most common is minor, quick fluctuations in the incoming power to your drive. At the plant I work at, we have fluctuations all the time. The power supplied is within the power company's specifications, but on the high side of the specification. That means that any increase in voltage could put us over the drive's incoming voltage spec. We have eliminated the problem by installing line reactors on the incoming power leads to the drives. Reactors do a great job of smoothing those fluctuations.
Another is regeneration, as mention previously. This is not likely the case on a band saw if it is happening during a cut.
Third, poor grounding of the motor and drive can cause the drive to believe it is in a regenerative mode and cause trips.
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Old May 17th, 2018, 06:49 AM   #8
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In this case it is not because of trying to stop too fast or because of incoming supply voltage fluctuations.
Quote:
The one drive is running a bandsaw. We have plenty of decel time programmed in. It seems it faults out when the the load changes very rapidly. Like it tends to trip out, right when they hit a knot in the log etc.
The problem is when some of the masses in the machine changes from being driven by the drive, to sending energy back to the drive, you get the overvoltage.

The remedy is to use a brake chopper (sonetimes integrated into the drive) and a brake resistor (usually an option that has to be installed outside the drive).
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Old May 17th, 2018, 06:55 AM   #9
JesperMP
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Quote:
Originally Posted by drforsythe View Post
Another is regeneration, as mention previously. This is not likely the case on a band saw if it is happening during a cut.
Why not ? I imagine that of passing a knot and there may be some kind of mechanical slack, you can get the effect that the mechanism oscillates in a way that changes from being driven to regenerating.
If he says that it happens when the load changes, then that is an indicator that it has to do with the load.
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Old May 17th, 2018, 07:16 AM   #10
lesmar96
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Quote:
Originally Posted by jraef View Post

One thing to consider: why do you have decel enabled at all on a band saw? Decel is used to PREVENT a load from stopping faster than it would naturally, for example a pump to avoid creating water hammer. I see no benefit in having decel enabled on a band saw. Just turn it off. Usually you do that by setting the "Stop mode" to "Coast". If you want to stop the band saw FASTER than it will coast, then you want BRAKING, not decel. Decel is the OPPOSITE of braking.
The drive defaults at 20 seconds decel and that is where I left it. I have not set it to coast to stop. I do have the DC injection breaking turned on so that it does not coast so long for safety.

I thank you all for your replies. Sounds like a resistor might be the best solution? It is a Fuji Electric, Mega series. I will have to check about the current limiting.
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Old May 17th, 2018, 07:17 AM   #11
lesmar96
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Quote:
Originally Posted by drforsythe View Post
Third, poor grounding of the motor and drive can cause the drive to believe it is in a regenerative mode and cause trips.
Can you please explain this comment a bit more? I'm quite certain grounding is not a problem. Just wondering about the logic behind this.
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Old May 17th, 2018, 07:20 AM   #12
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This regen voltage problem is only made alot worse by our high incoming voltage? The drive trips out at 400V on the DC bus. In the trip history, it registered at 406VDC when it tripped.
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Old May 17th, 2018, 07:52 AM   #13
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I used to see this a lot when I worked for a company that supported one of the Japanese drive manufacturers. The circuitry that would monitor for regenerative overvoltages was really sensitive. When grounding was the issue it would trip even when the motor shaft was sitting still and there was no commanded motion (and therefore no chance of regen). It seemed to occur mostly on older machine retrofits where the original wiring was questionable or on monorail drives where the drive ground came through a brush system. The manual for the drive even mentioned to check motor and drive grounding when that error occurred.
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Old May 17th, 2018, 11:51 AM   #14
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Quote:
Originally Posted by lesmar96 View Post
Can you please explain this comment a bit more? I'm quite certain grounding is not a problem. Just wondering about the logic behind this.
Most drives have some form of input line filtering, from very basic to pretty substantial. When you don't have a good ground, the capacitance of the filtering can charge up to a point that it will cause a high voltage trip on the DC bus. Normally, the caps would shunt the high frequency noise to ground.
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Old May 17th, 2018, 01:57 PM   #15
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Is it a 240V 3 phase delta supply then? If so, and it is an ungrounded delta or one phase is center tapped (high leg delta), many Asian drive designs have trouble with delta power systems, because are expecting the supply to be referenced to ground, which is not if it's delta. Then as Gene Bond explained, that can lead to the common mode noise having nowhere to go and building up on the DC bus.
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