VFD failure.
VFD failure.
(OP)
So I'm ditzing with a VFD that has an issue. We'd just replace it if they were available in the USA and a measly 1kW unit wasn't way over $1k.
This VFD is for the Z-axis of a machine tool.
As soon as the drive is commanded to output a drive signal it faults on over current? I went thru the usual "is the brake stuck on?" stuff. Nope.
I finally swapped the drive with the Y-axis and, sure enough, the problem jumped to the Y-axis -> it's the VFD.
I tore it apart looking for the current sensing and on the third of 4 boards there were two large current coils. Hunting down the part nums showed they were actually Hall effect sensors. That explained the four terminals!
While working out what everything was I discovered (6) PC928 ICs. I assumed they were opto-isolators but on pulling up the data sheet it turns out they're opto gate drivers. They also monitor the actual current thru their associated IGBTs via the change in VCE. They put out an over-current signal at some threshold. Unfortunately, complicating this is the need to isolate the overcurrent signal from the PC928 back to the in-charge CPU. Opto couplers have crummy life expectancies since the IREDs age quickly.
SO! the question:
Whada you think has failed, a PC928, or one of their over-current signal optos, or maybe one of the Hall effect current sensors? Anyone heard of "the typical" over current problem when it's in the VFD?
This VFD is for the Z-axis of a machine tool.
As soon as the drive is commanded to output a drive signal it faults on over current? I went thru the usual "is the brake stuck on?" stuff. Nope.
I finally swapped the drive with the Y-axis and, sure enough, the problem jumped to the Y-axis -> it's the VFD.
I tore it apart looking for the current sensing and on the third of 4 boards there were two large current coils. Hunting down the part nums showed they were actually Hall effect sensors. That explained the four terminals!
While working out what everything was I discovered (6) PC928 ICs. I assumed they were opto-isolators but on pulling up the data sheet it turns out they're opto gate drivers. They also monitor the actual current thru their associated IGBTs via the change in VCE. They put out an over-current signal at some threshold. Unfortunately, complicating this is the need to isolate the overcurrent signal from the PC928 back to the in-charge CPU. Opto couplers have crummy life expectancies since the IREDs age quickly.
SO! the question:
Whada you think has failed, a PC928, or one of their over-current signal optos, or maybe one of the Hall effect current sensors? Anyone heard of "the typical" over current problem when it's in the VFD?
Keith Cress
kcress - http://www.flaminsystems.com





RE: VFD failure.
There was never anything wrong with the FET itself, but the solder joints holding it in place would fatigue and crack through after a few months of heavy service, resulting in erratic operation and then no operation.
I didn't have reflow equipment. If you do, consider fluxing and reflowing any surface mount devices that carry more than a few tens of milliamps, just for fun.
Mike Halloran
Pembroke Pines, FL, USA
RE: VFD failure.
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD failure.
RE: VFD failure.
I'm curious if you figured out what the cause of over current was on the drive, any developments or did you just scrap the unit?
Chuck
RE: VFD failure.
So, no not scrapped but repaired!
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD failure.
Ok, Thank you. Food for thought, we have a DC drive tripping out on over current fault it's sporadic and may need to be addressed if it gets worse so I'll keep that in mind.
Chuck
RE: VFD failure.
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD failure.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: VFD failure.
And Mike, about your cooked solder joints, I've seen that a lot. Essentially any part that you cannot leave your finger on will suffer premature joint failure given enough time. I think the solder literally evaporates over time. To fix it I either find a larger part like using a 5W resistor in place of the cooking 1W resistor or I stand a replacement on its full length leads by wrapping the leads around something to put little pig-tails in them so the resistor's heat has to travel much farther to reach the joint and the loops provide more heatsinking to the air.
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD failure.
Military is not forced to use lead-free. They need stuff that works.
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: VFD failure.
RE: VFD failure.
I decided to try repairing one. I couldn't fix it. Tried a second and DID fix it. Tried a third and fixed it, a fourth and fixed it, a fifth and fixed it. I stopped there, figuring the machine I got back on-line and three spares would be enough. The last two someone else had tried repairing and failed, I couldn't bear to give them effort seeing what they'd done already. The company that quoted the $2,400 was very disappointed. :)
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD failure.
bad electrolytic caps?
RE: VFD failure.
Each board had about 30 electrolytics that were absolute misery to replace. They were glued on and the 10 layer boards tried to defy all attempts to de-solder the caps. Couldn't get the holes to 'suck' clean either no matter what tactic was employed. Finally resorted to re-drilling or chasing every hole with board fab drills.
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD failure.
Since you enjoy doing this, keep in mind that as dc bus caps age they allow more and more ripple to pass, and this can also be cause of worsening cases of seemingly untrue overcurrent detection faults.
www.KilroyWasHere<dot>com
RE: VFD failure.
I did not figure out which one was at fault because they were all ganged and to run the drive the four boards all had to be stacked to operate. The power traveled between boards via screws which rigidly positioned them. This GREATLY restricted access to the pins of interest in the assembled hot stack. I'd have had to solder wirewrap wires to all the SMD pins of interest which would've run the time past all reason instead of just past reason as was the case..
Keith Cress
kcress - http://www.flaminsystems.com