# DC Bus Overvoltage Problem



## Ratt1976

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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## oliquir

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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## RIVETER

Ratt1976 said:


> 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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## Ratt1976

oliquir said:


> 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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## Ratt1976

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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## Big John

Ratt1976 said:


> ...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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## Ratt1976

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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## John Valdes

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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## Big John

John Valdes said:


> 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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## oliquir

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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## varmit

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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## varmit

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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## RIVETER

Big John said:


> 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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## DriveGuru

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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## DriveGuru

RIVETER said:


> 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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## DriveGuru

varmit said:


> 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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## Vicdog

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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## DriveGuru

Vicdog said:


> 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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## Ratt1976

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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## DriveGuru

Were the drives enabled when the fault occurred? Or were they just powered up?


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## Ratt1976

the brand of drive: Lenze 8200
Diode bridge front end


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## Ratt1976

DriveGuru said:


> Were the drives enabled when the fault occurred? Or were they just powered up?


 
Just powered up. The fault would not allow me to enable!


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## DriveGuru

Do you have more info on the line reactor? What you are describing doesn't make sense to me.


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## DriveGuru

A line reactor is just a choke coil, mounted in a core. If it meggers good with respect to ground, and the resistance checks about the same on each coil they are probably ok, also look for signs of heat(blackened insulation). Was polarity observed when wiring the reactor? I'd still like to see a scope put on that input.


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## Big John

Ratt1976 said:


> 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....


 There are other full time drive guys who know more about this than I do, but like I said above that sounds for all the world like resonant condition. 

The inductance put in the circuit by the line reactor happens to coincide with the cable/front-end capacitance well enough to set up a tank circuit: It dumps energy back into the front end at the peak of every AC cycle, and you end up with severe over-voltages.

I've seen this happen in distribution circuits, I know it can happen on the load side of drives. I was not aware it could happen on the line side of a drive, but I see no immediate reason it's not possible.


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## DriveGuru

Big John said:


> There are other full time drive guys who know more about this than I do, but like I said above that sounds for all the world like resonant condition.
> 
> The inductance put in the circuit by the line reactor happens to coincide with the cable/front-end capacitance well enough to set up a tank circuit: It dumps energy back into the front end at the peak of every AC cycle, and you end up with severe over-voltages.
> 
> I've seen this happen in distribution circuits, I know it can happen on the load side of drives. I was not aware it could happen on the line side of a drive, but I see no immediate reason it's not possible.



With a diode bridge nothing is switching, nothing is spiking, some harmonic current, in 30 years I've not run across what he's describing. Part of why I asked about the polarity of the wiring through the reactors. I was kind of assuming that all these drives are in the same cabinet, in close proximity to the drives they are feeding? I would say a phone call to a lenze engineer may be in order, they have great customer support.


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## JRaef

I saw this thread on my phone on Saturday and tried to respond then, but I was sitting in the sun on a deck at a winery in Sonoma, listening to some live blues, sipping some Zinfandel and when the iPhone messed up my post, I was not motivated enough to try it again.

Are you SURE it is just a line reactor and not a filter? A Passive Harmonic Filter (PHF) will look similar to the untrained eye but they are a combination of a Line Reactor and another tuned reactor feeding capacitors. If that's what you have, they typically also come with a contactor that is supposed to be open until the VFD is above a set speed, because otherwise the motor inductance is too low so the caps become leading and interact with the VFD caps to over charge the DC bus. Here's a diagram of what a Passive Harmonic Filter is supposed to look like:










But there are two issues: 1) not all are created equal, sometimes the filter supplier does not include the contactor, leaving that to the installer, who doesn't read the instructions and just hooks up wires, or 2) the contactor is welded shut, often because the snubber circuit that protects the contacts burns out from being cycled too often when someone doesn't understand the complexities of applying a PHF. 

Also, the tuned reactor is designed based on the available fault current in a system, and if the PHF is removed and used somewhere else, it may not work correctly.

Any of the above conditions can cause DC bus over charging, and I have seen many times where people mistake a PHF for a line reactor, because it's just a heavy thing in a box with terminals, that LOOKS like a line reactor.


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## DriveGuru

JRaef said:


> I saw this thread on my phone on Saturday and tried to respond then, but I was sitting in the sun on a deck at a winery in Sonoma, listening to some live blues, sipping some Zinfandel and when the iPhone messed up my post, I was not motivated enough to try it again.
> 
> Are you SURE it is just a line reactor and not a filter? A Passive Harmonic Filter (PHF) will look similar to the untrained eye but they are a combination of a Line Reactor and another tuned reactor feeding capacitors. If that's what you have, they typically also come with a contactor that is supposed to be open until the VFD is above a set speed, because otherwise the motor inductance is too low so the caps become leading and interact with the VFD caps to over charge the DC bus. Here's a diagram of what a Passive Harmonic Filter is supposed to look like:
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> But there are two issues: 1) not all are created equal, sometimes the filter supplier does not include the contactor, leaving that to the installer, who doesn't read the instructions and just hooks up wires, or 2) the contactor is welded shut, often because the snubber circuit that protects the contacts burns out from being cycled too often when someone doesn't understand the complexities of applying a PHF.
> 
> Also, the tuned reactor is designed based on the available fault current in a system, and if the PHF is removed and used somewhere else, it may not work correctly.
> 
> Any of the above conditions can cause DC bus over charging, and I have seen many times where people mistake a PHF for a line reactor, because it's just a heavy thing in a box with terminals, that LOOKS like a line reactor.



Hmmm...I've not seen that configuration in the Midwest, unless it was a detuned PF bank. Is this an alternative for using a 12 pulse front end? Is this seen more to comply with European standards? Thankfully not many people around here care about harmonics unless there's a problem, it's poco's problem,lol


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## Ratt1976

I'm attempting to attach a pic of the line reactor!


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## MDShunk

DC Bus Overvoltage was one I'd see a lot on new installs where the motor was some distance from the MCC. Took a great deal to "charge the capacitor" (the wire between the VFD and the motor). In each instance, I successfully solved the matter with the installation of a load reactor. I already had line reactors as standard fare in the buckets.


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## Ratt1976

MDShunk said:


> DC Bus Overvoltage was one I'd see a lot on new installs where the motor was some distance from the MCC. Took a great deal to "charge the capacitor" (the wire between the VFD and the motor). In each instance, I successfully solved the matter with the installation of a load reactor. I already had line reactors as standard fare in the buckets.


 
Why would a load reactor help in this particular issue, when the drive won't enable because it faults as soon as it is powered up. I take the line reactor away and the fault goes away. How could the line reactor cause this?


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## MDShunk

Ratt1976 said:


> Why would a load reactor help in this particular issue, when the drive won't enable because it faults as soon as it is powered up. I take the line reactor away and the fault goes away. How could the line reactor cause this?


Sorry. A load reactor won't help you. I didn't read your issue clearly. I read "when the drive is powered up" and heard "when the motor is powered up".


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## DriveGuru

Ratt1976 said:


> I'm attempting to attach a pic of the line reactor!



That looks like a plain line reactor to me. Does it megger good? Winding resistance seem about the same. There looks to be a bolt missing on the center choke bolting that pole to the core, is that missing on all of the reactors?


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## Ratt1976

DriveGuru said:


> That looks like a plain line reactor to me. Does it megger good? Winding resistance seem about the same. There looks to be a bolt missing on the center choke bolting that pole to the core, is that missing on all of the reactors?


 
Good eye! I am not sure, I will have to check on the bolt. I don't have access to a megger but I have used a Fluke 87 and have checked phase to phase and phase to ground, they all show open. The manufacturer sent me the following specs:

inductivity: 3x 2,5mH (50-60Hz)
current: 3x 13A

I don't think I have a meter that can measure inductance. I'll have to check!


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## DriveGuru

Unfortunately your 87, even though your reading sounds promising, doesn't put out near enough voltage to properly test the insulation resistance to ground. I would recommend a 1000V megger as a minimum for testing, personally I like my 1587 by fluke. If you have a large number of motors, and are using drives, you should really own a megger. Beat up the boss


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## DriveGuru

What is the current rating of the inverter it was feeding?


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## DriveGuru

You can check the resistance on each coil individually, should be pretty low, and should all be about the same.


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## Ratt1976

The motor rating for the 5.5kW is 11.9A @ 360V
The inverter rating for the input: 16.8A / 13.4 A @ 400V / 500V
The inverter rating for the output: 13.0A / 10.4A @ 0 - 400V / 500V


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## Ratt1976

I checked resistance and each coil shows about 0.3 ohms


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## Ratt1976

Is the only difference between a line-reactor and load-reactor is where it is placed? By this I mean can I take the line-reactor and put it in a load-reactor spot?


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## JRaef

Ratt1976 said:


> Is the only difference between a line-reactor and load-reactor is where it is placed? By this I mean can I take the line-reactor and put it in a load-reactor spot?


Where it is placed, and *how much current it is rated for*.You can always remove a LOAD reactor to the Line side, but not vice-verse in most cases. Line side current it less than Load side current, because on the Load side, the current from the drive to the motor is INCLUSIVE of the reactive current that must flow to the motor. On the line side, the DC bus capacitors are storing reactive current, thus correcting the power factor as seen by the line side.

Shorter version: do the current calculations for a given motor at .95 PF (Line side) and .75 PF (Load side). 

That photo does appear to be only a reactor, but again, _*I cannot see how the removal of the reactor would make the problem go away*_. Reactors store a small amount of energy by virtue of the Inductive Time Constant, but they cannot charge up a circuit and increase a DC bus voltage as capacitors can, at least as far as I am aware. Reactors in fact present a voltage DROP across themselves. So this is a new one on me. I can only imagine that maybe there are other components in that over-stuffed box than you can see with that camera shot, maybe even underneath the reactor.


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## JRaef

Duplicated post, please remove.


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## DriveGuru

JRaef said:


> Where it is placed, and *how much current it is rated for*.You can always remove a LOAD reactor to the Line side, but not vice-verse in most cases. Line side current it less than Load side current, because on the Load side, the current from the drive to the motor is INCLUSIVE of the reactive current that must flow to the motor. On the line side, the DC bus capacitors are storing reactive current, thus correcting the power factor as seen by the line side.
> 
> Shorter version: do the current calculations for a given motor at .95 PF (Line side) and .75 PF (Load side).
> 
> That photo does appear to be only a reactor, but again, _*I cannot see how the removal of the reactor would make the problem go away*_. Reactors store a small amount of energy by virtue of the Inductive Time Constant, but they cannot charge up a circuit and increase a DC bus voltage as capacitors can, at least as far as I am aware. Reactors in fact present a voltage DROP across themselves. So this is a new one on me. I can only imagine that maybe there are other components in that over-stuffed box than you can see with that camera shot, maybe even underneath the reactor.



With the bolt missing from the center coil attaching the pole piece, could the reactor somehow be acting like a transformer??? I realize I may be reaching here But I can't say I've ever encountered one that was missing what I would consider essential hardware, to keep the pole piece from vibrating if nothing else It would sure be nice to see some scope readings with the reactor connected and not.


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## Ratt1976

Thank all of you for responding and helping troubleshoot this problem. I will be working this problem for a while and will continue to update my progress. 

Thanks!


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## TimGoon

Had a similar problem with high dc link voltage on 6 identical drives. They would sit at roughly 730vdc and would jump to 900+ when machines would run that had no connection to them. First thought incoming voltage, then network, then braking resistors, then spikes and nothing seemed to work. Come to find out the 6.9kvac supply was cycled and set these German made drives to the default parameters. All I had to do was set the incoming mains voltage parameter on the drives to be looking for 480vac instead of 400vac and *duh* it works. Felt pretty dumb after that one.


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## kcarter

*Vfd DC over voltage*

Depending on brand, Weg and dura pulse have an extensive troubleshooting fault code in the user manual. You might need more than one resistor or try a in line reactor on input voltage and output voltage. Good luck


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## STEM

I know for you that as long as it is running then then problem is solved. For me though, it's too strange a problem not to follow up. Can you give us the brand-model-nameplate data off the inductors and the drives?


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## Ratt1976

STEM said:


> I know for you that as long as it is running then then problem is solved. For me though, it's too strange a problem not to follow up. Can you give us the brand-model-nameplate data off the inductors and the drives?


See attachments

Thanks!


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## STEM

Thanks for posting the photos. Rather than blather on about what I do or don't know about the subject, check out this link and see if there's something in here that rings a bell.
http://ecmweb.com/design/line-reactors-and-vfds

Could the reactor be limiting your current (too much)?

Any chance you have bad caps? Why don't you measure and see if they measure up to their rated value? This has happened to me recently on our induction heater caps.

When you measured p-t-p and p-t-g voltages were they before or after the reactor? Any difference between the two?

If nothing can be found at the component level, you might have to look at your whole system instead of just individual components to solve this one.

Just a few ideas and questions I would have if it were my problem to solve.


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