VFD line side imbalance
- Thread starter fireryan
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- Placerville, CA, USA
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- Retired PV System Designer
Is there a corresponding voltage imbalance on the input side?
Since the line-to-line voltage peaks directly drive the DC bus through a bridge rectifier, the current drawn will be higher for higher voltages.
Since the line-to-line voltage peaks directly drive the DC bus through a bridge rectifier, the current drawn will be higher for higher voltages.
Voltage is fine on line side
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
It is possible that there is an issue with the VFD input rectifier, but more likely a small input voltage imbalance is causing a large current imbalance
Had this problem maybe 10 years ago and it was from a defective IGBT. We had over 500 VFD'S that we PM'ed from once to four times a year. Never had a rectifier or rectifier bridge go bad. When I had such a large difference in output ampere I would place the drive in hand and start at 10% and take both ampere & voltage readings using the LCB ( What Danfoss calls Local Control Panel ) = touch screen then raise it 10% more and keep taking readings until It reached full speed ( 100 % ). ( First few times gave tech support guys this info )Danfoss & Allen Bradley had great tech support and ABB the worst. Would call them a few times a year and they are great at locating problems. I kept troubleshooting problems & fixes in a copy book to help in troubleshooting. Appears with drives probably 85% of the problems where from same maybe 5 reoccurring problems: Bad IGBT, if tripping out at full speed but not at 90 to 94% full speed problem with load such as large air handlers lovers only partially open, runs on hand but not automatic often a control issue, if you have bypass with two or most times three contactors the M2 ( OUTPUT contactor ) goes bad at least twenty times more then the M1 input contactor, Had 12 & 15 year old drives in a secured area that parameters would change on their own and some times people change parameters without telling or recording it. Had people change maximum speed output lower. As far as the DC BUSS issue if the line voltage is 480 volts then the DC Buss should be from 650 to 675 volts DC. Old drives had terminals where you could take measurements but in at least last 20 years you can read DC Buss from the touch screen. If the DC is low and all three input lines at or close to 480 volts and DC Buss low then it's a rectifier problem. Some drives use three separate rectifiers but majority use a one piece three phase input rectifier. 3 line wires in & two DC wires out. From one to 6 or more capacitors are wired across the DC Buss.They are rated for 75,000 hours but had several old drives with over 125,000 hours run time with original capacitors. Did you try bypassing the drive & running motor and taking amp readings ? If it runs across the line this would eliminate one of the six motor windings ( if dual voltage ) being source of problem.
kwired
Electron manager
- Location
- NE Nebraska
His current discrepancy is on one of the input leads. About has to be a rectifier problem in that particular input, or any other circuit component up to where that current enters the DC bus.
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
garbo,
Are you saying that in your experience an _input_ current imbalance was caused by an _output_ IGBT problem?
Could you explain how this happens further? I could see if perhaps the output frequency was 60Hz, and there was and output failure, then an output imbalance might be reflected back to the input. But I can't see how an _output_ error could cause a consistent and steady input current imbalance.
If you've experienced this sort of issue, I'd like to understand how the failure works.
Thanks
Jon
Are you saying that in your experience an _input_ current imbalance was caused by an _output_ IGBT problem?
Could you explain how this happens further? I could see if perhaps the output frequency was 60Hz, and there was and output failure, then an output imbalance might be reflected back to the input. But I can't see how an _output_ error could cause a consistent and steady input current imbalance.
If you've experienced this sort of issue, I'd like to understand how the failure works.
Thanks
Jon
kwired
Electron manager
- Location
- NE Nebraska
OP indicated output balance was fine though. Which is very possible if not too heavy of a load being supplied per drive rating, can completely have loss of one input lead and still normal output if not too heavily loaded.
- Location
- San Francisco Bay Area, CA, USA
- Occupation
- Electrical Engineer
Input current imbalance is not going to be caused by anything on the output side, it can’t. It can’t even be caused by anything on the DC bus. The input is just a bridge rectifier. If there is no input voltage imbalance, that only leaves a couple of possibilities but keep in mind that a seemingly minor voltage imbalance has an effect of an amplified current imbalance; it can be deceptive. But just in case, here are the other possibilities I have come across.
On some drives, they use SCRs instead of diodes for the rectifier in order to not need a pre-charge resistor setup. Sometimes a gate circuit for an SCR gets old and fires a little bit late, which reduces the current through that pulse on the bridge. If this is the case, you would have to replace the gate drive board for it. But an SCR bridge on a 10 HP drive would be unusual unless this is a relic of the late 80s or early 90s. If so, it might not be worth fixing.
If it’s a newer drive with an IPM (Intelligent Power Module) where the diode bridge, firing circuits and transistors are all in an integrated and potted module, it might indicate some bad internal connections to the diodes, a sign of imminent failure. IPMs cannot be repaired and are all but impossible to replace but would also likely cost as much as a new drive. I would consider a prophylactic drive replacement when you can do so with an orderly shutdown.
On some drives, they use SCRs instead of diodes for the rectifier in order to not need a pre-charge resistor setup. Sometimes a gate circuit for an SCR gets old and fires a little bit late, which reduces the current through that pulse on the bridge. If this is the case, you would have to replace the gate drive board for it. But an SCR bridge on a 10 HP drive would be unusual unless this is a relic of the late 80s or early 90s. If so, it might not be worth fixing.
If it’s a newer drive with an IPM (Intelligent Power Module) where the diode bridge, firing circuits and transistors are all in an integrated and potted module, it might indicate some bad internal connections to the diodes, a sign of imminent failure. IPMs cannot be repaired and are all but impossible to replace but would also likely cost as much as a new drive. I would consider a prophylactic drive replacement when you can do so with an orderly shutdown.
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
A suggestion is to rotate the three phases feeding the drive. Then see if the lower current stays on the same phase (which would imply a voltage balance issue) or if it stays on the same drive input terminal (which would indicate a drive rectifier problem).
synchro
Senior Member
- Location
- Chicago, IL
- Occupation
- EE
Good imerhod.
As far as my post above, the OP could also just exchange the low current phase with one of the other phases, because rotation wouldn't matter for a rectifier.
When I had a defective IGBT I was keying in on mainly output current imbalance. When I ran a VFD at lower speeds say 20 & 30% the output currents were close to being balance but as I increased speed the current became more & more unbalanced. I replaced a bad IGBT on a 125 HP air handler and it started ok. First I rang out the motor leads & meggered the motor then ran it on bypass for a few minutes. Started drive up at 10 or 15% then increased 5% at a time . When I got up to around 35% had a loud what sounded like an explosion. Secured power and had smoke coming out of drive. Called our great drive tech in. First thing he asked me before replacing IGBT did I replace the little maybe $150 board that controlled the expensive IGBT. He told me in some models they have a board just to control firing ( believe up to 5,000 times a second )IGBT'S. I would call the tech support and ask if control board could cause such a problem. On a normal drive set up I found that the output frequency increases at the same rate of output frequency. Good way of checking is in a 480 volt drive when the output voltage reaches say 240 volts output frequency should be at 30 Hertz. Years ago I went to an Allen Bradley & a Fincorr drive and never found any company that supplies a detailed troubleshooting Manuel. We had a small hand held A&B troubleshooting device that some times throw a curve ball. If power supply fuse blew it would display bad power supply. Please let me know what the problem was. Even though I'm retired 4 years now still trying to learn, keep up with the NEC and giving any assistance to others.
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