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Discussion Starter · #1 ·
I have a 5.5kW motor & VFD combo. The VFD and motor can take voltages from 400 to 500V, 3-phase. My voltage is 470, 471, 472 phase to phase. Right after I apply power to the combo unit I get the DC bus overvoltage fault. I'm not able to enable the drive because it faults right away. I check the DC bus voltage on the display an it is showing 980V. I was expecting about 470*1.414 = 665V. I measure the voltage with my fluke 87 on the UG+ and UG- terminals and it shows 980V. I disconnect the motor and check it again and the DC bus is still 980V. I say 980V for simplicity but it may fluctuate +/-10V. How is it even possible to get the bus voltage this high with out even running? I installed a breaking resistor (10W, 100kOhms) to try and bring down the bus voltage and it dropped maybe 70V.

I am stumped!
 

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High harmonics can create that error, you may have very high voltage spikes on your ac input and the vfd rectify them. You could try installing a line reactor before the vfd
find an oscilloscope to see what is happening there
 

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I have a 5.5kW motor & VFD combo. The VFD and motor can take voltages from 400 to 500V, 3-phase. My voltage is 470, 471, 472 phase to phase. Right after I apply power to the combo unit I get the DC bus overvoltage fault. I'm not able to enable the drive because it faults right away. I check the DC bus voltage on the display an it is showing 980V. I was expecting about 470*1.414 = 665V. I measure the voltage with my fluke 87 on the UG+ and UG- terminals and it shows 980V. I disconnect the motor and check it again and the DC bus is still 980V. I say 980V for simplicity but it may fluctuate +/-10V. How is it even possible to get the bus voltage this high with out even running? I installed a breaking resistor (10W, 100kOhms) to try and bring down the bus voltage and it dropped maybe 70V.

I am stumped!
It would seem that the resistors you installed should have worked if they were properly sized dynamic resistors. The over voltage should...or would only have been there for a short period of time. If you measured the extra high voltage some time after the fault I have no answer. Try starting the unit without a load , or at least a very light load.
 

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Discussion Starter · #4 ·
High harmonics can create that error, you may have very high voltage spikes on your ac input and the vfd rectify them. You could try installing a line reactor before the vfd
find an oscilloscope to see what is happening there

There are 6 of these units and each one has it's own line reactor for the mains coming to the vfd. Each one has the same fault. I was able to go up after 5pm (when everybody else has gone home) meaning nothing else in the area was running and the voltage was up to 494 +/- 1 on all three legs and the units would enable and run.
 

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Discussion Starter · #5 ·
This morning I tried to run them again and same problem they were all faulted. The mains voltage was 460 +/- 2 on all three legs.
 

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...I was able to go up after 5pm (when everybody else has gone home) meaning nothing else in the area was running and the voltage was up to 494 +/- 1 on all three legs and the units would enable and run.
Did you take a DC bus voltage at that time?

Is there a line-side reactor or isolation transformer on these drives? As much as I hate it when folks say "It's probably harmonics" that really is what I'm leaning towards.
 

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Discussion Starter · #7 ·
I did not take the DC bus voltage at that time. It is a line-side reactor. I am going to bypass the reactor today just to see if the problem gets better or worse. I'll make sure to measure the DC bus voltage when I do.

I appreciate all your help!
 

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You got a drive problem. If it faults right away after power up, the drive is probably bad.
I don't think bus voltage is adjustable and since you are not regening anything, it sounds like its time to try another drive. Or have this one repaired.

If you are able do a diode test. Its should be explained in the manual.
The dc bus voltage is way to high for your nominal input voltage.

The line reactor has nothing to do with this problem.
 

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You got a drive problem. If it faults right away after power up, the drive is probably bad....
I won't dispute that might be the case, just very strange to me that he is able to independently verify the presence of that voltage with another meter. It suggests it's very real voltage passing through the front end.

It's also strange that the problem apparently goes away and the drive still operates, so this isn't a permanent fault.

I don't know what except for resonant ringing would actually produce almost double the peak line voltage. With no load present we know it can't be regen voltage, and the rectifier is simply passing what voltage is present on the AC side; it can't manufacture anything. I don't see any means that would create those DC bus values if it weren't a legitimate system voltage.

OP, can you get a meter that reads harmonics, or THD and put it on your line-side during operation? See what that says.
 

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you found your problem, when nothing runs harmonics are very low and you dont have high voltage problem. Get an oscilloscope or harmonics analyser you will be surprised to see whats happening
 

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Are all of these drives newly installed? If so, I am inclined to think that this is a bad batch of drives. It happens sometimes with this fine "made in china" stuff.

OR

If there is nothing regenerating back to these drives, it does not seem likely that a 460 input would give an almost 1000 volt DC buss voltage without some component problem. Line reactors are not a cure all, but usually will filter enough to allow VFDs to work.

OR

In the drive parameters, see what the input HZ are set for. If the input circuit is set to fire for the wrong frequency it could cause the buss voltage to be high or low.
 

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Did you check the line side (input) frequency? If there are multiple drives on the same power distribution system, I have seen defective VFDs impose higher frequencies (130 HZ or more) on the input of other VFDs causing them to trip.
 

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I won't dispute that might be the case, just very strange to me that he is able to independently verify the presence of that voltage with another meter. It suggests it's very real voltage passing through the front end.

It's also strange that the problem apparently goes away and the drive still operates, so this isn't a permanent fault.

I don't know what except for resonant ringing would actually produce almost double the peak line voltage. With no load present we know it can't be regen voltage, and the rectifier is simply passing what voltage is present on the AC side; it can't manufacture anything. I don't see any means that would create those DC bus values if it weren't a legitimate system voltage.

OP, can you get a meter that reads harmonics, or THD and put it on your line-side during operation? See what that says.
I can't know it all...just ask my wife. So, I will ask you how you think that a harmonic, or multiples of such could pass through the utility to that location without causing problems elsewhere. Even if there were PF correctors installed at or near the service how would they pass it on to one piece of equipment?
 

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Hmmm... Interesting problem. What brand of drives are they? Buss capacitors are going to charge to 1.414 x input voltage, period.Capacitors cannot charge to a higher voltage than the peak of what is being applied(well actually just a little bit less than peak). I would want to see a scope on the input to look for extreme line notching and over voltage. Does the drive have an active front end(line Regen)? Or just a diode bridge. Are there other line regenerative drives running on this power system that could be causing a higher line voltage? I would also check to see if there are any EMC filter screws that need to be removed for certain input voltage configurations(ie grounded delta).
The output transistors have fly back diodes in them, if you look at the schematic its a 3phase bridge in reverse, that's why it's possible to regenerate to the buss if a motor is being overhauled. Are the motors being spun by an external source when not being powered(say like in a dual fan plenum, motors can be spun when not powered by airflow). If a motor has enough residual magnetism it is still possible for it to generate when not powered, if something else is physically spinning it. Being able to read that voltage on the buss constantly is bothersome, sounds like that voltage is actually present, what brand meter is being used?
 

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I can't know it all...just ask my wife. So, I will ask you how you think that a harmonic, or multiples of such could pass through the utility to that location without causing problems elsewhere. Even if there were PF correctors installed at or near the service how would they pass it on to one piece of equipment?

An automatic power factor bank can cause spikes when the capacitors switch in and out. Also I've seen people over excite large synchronous motors to correct for power factor.
 

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Are all of these drives newly installed? If so, I am inclined to think that this is a bad batch of drives. It happens sometimes with this fine "made in china" stuff.



OR



If there is nothing regenerating back to these drives, it does not seem likely that a 460 input would give an almost 1000 volt DC buss voltage without some component problem. Line reactors are not a cure all, but usually will filter enough to allow VFDs to work.



OR



In the drive parameters, see what the input HZ are set for. If the input circuit is set to fire for the wrong frequency it could cause the buss voltage to be high or low.

Although line reactors on the input can offer some protection by somewhat limiting the current caused by voltage spikes, their primary purpose is to block high frequency noise from the drives they are feeding, to keep them from effecting other equipment on the same line.
 

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You need to look at the waveform. I had a site with the same problem. Only visible issue was the drive would fault on over voltage. Fluke read 480. Scope showed a distorted waveform with a peak over 750V. DC bus will charge almost to peak voltage as stated before. Does your meter read "crest factor"? See what that is.
 

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You need to look at the waveform. I had a site with the same problem. Only visible issue was the drive would fault on over voltage. Fluke read 480. Scope showed a distorted waveform with a peak over 750V. DC bus will charge almost to peak voltage as stated before. Does your meter read "crest factor"? See what that is.

If you don't have a scope you might try the peak hold on a fluke87;)
 

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Discussion Starter · #19 ·
UPDATE:

I bypassed the line reactor and everything worked. The bus voltage was about 640V. After this I hooked the line reactor back up to make sure and the drive faulted and the bus voltage was 928V. I bypassed all six units and they all work. Could I be getting reflections or noise from the line reactor, especially if it isn't sized appropriately? How to determine if the line reactor is bad? I know it is an inductor, it's not shorted to case or the other inductors (other phases).
 
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