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05-16-2005, 06:01 AM #1
I was wondering if there is any way to tell if the capcitors in a VFD are failing. I have an old(1994) Omron vfd on my surface grinder, and as soon as you shut off the power the drive shuts down. But on my mill, the drive stays on for at least a minute(without the motor running). The vfd on the mill is an Allen Bradley, no more than 2 years old, less than 20 hours run time.
I let both of the drives sit powered up, but not running the motor, for at least 8 hours.
Is there any way to check the condition of the capacitors if that isn't a indication of there condition?
05-16-2005, 09:33 AM #2
Most VFDs have some sort of DC bus discharge circuit that bleeds off the DC bus when power is shut off. Different drives use different circuits, and some bleed down quicker than others. This alone would not indicate a problem with the caps.
One way to check caps is to look at the tops (or bottoms depending how you look at it). If they are bulging, it is not a good sign. Most newer drives have smaller circuit board mount caps, while a lot of older drives have larger caps with screws for mounting to bus bars. On those older caps there is usually a little rubber "button" on the same end as the screw holes. Normally this button should be flush with the top of the cap. If the button is popped up it can indicate a problem.
05-16-2005, 11:27 AM #3
All electrolytic caps must "breathe". The polymer (plastic, rubber, etcetera) seals allows this to occur.
Should the cap overheat, the gasses produced by the boiling electrolyte may "blow" the seals out. This is usually catastrophic.
Non-catastrophic heating can cause the seals to "bulge". This is still serious.
Blown or bulging caps should be replaced.
On PCB-mount caps the seals are facing the PCB itself, so the only clue as to the impending or actual failure may be bulging in the metal case opposite the mount.
05-16-2005, 01:15 PM #4
Thank you. As far as I know, the caps look fine, I didn't notice any bulging.
Under normal conditions how long do the caps generaly last in vfd's? And what usually cause vfd failure?
05-16-2005, 10:23 PM #5
"Under normal conditions how long do the caps generaly last in vfd's?"
For 105 degree caps, a long time. Perhaps even the economic lifetime of the unit.
"And what usually cause vfd failure?"
IGBT failures (in the inverter section) or diode failures (in the converter section).
05-17-2005, 02:25 PM #6
I believe the "expected" life of most DC bus caps, under perfect conditions, is about 10 years. Unfortunately most VFD installations are nowhere near perfect conditions, so 5-7 years is more realistic, assuming continuous use.
06-30-2005, 04:24 PM #7
your caps only have to store their energy for 17 Milli Seconds, your Allan Bradley device does not seem to have a working capacitor discharge resistor in it and one day may kill a service engineer, its definately suspect.
07-01-2005, 05:26 PM #8
I agree with Chucki. If the caps are storing energy longer than needed, it's not their fault! A cap failure means they cannot store charge at all, or very little.
07-03-2005, 01:28 PM #9
I was just wondering if the capacitors degrade over time. And how to tell if capacitor failure is imminent. All of my drives work fine, I was only asking because I was curious.
As far as the Allen Bradley drive killing someone, that would probably be his fault, if he didn't check to make sure it was discharged. The display goes out one the charge is gone. Also the only person touching it will be me
07-03-2005, 09:14 PM #10
RE - Chucki's comment about storage time of capacitors. I have a Teco-Westinghouse 15 hp VFD and the manual clearly says to wait at least ten minutes after the unit shuts down due to risk of electrical shock. Appears it is designed to slowly discharge the capacitors?