snubber board

[bigheadted]

we recenlty experienced a dc drive occasionally tripping on regen stops,
I scoped the current output from the drive during motoring and regenerting conditions and all pulses were present every time i checked. the drive would trip on speed feedback error. initially i thought the encoder coupling but because of the way this system works that cant be the case as once it had slipped it would never run again.
looking at the way it derrives a speed feedback error it is a calculation based on armature volts, current and speed which narrows it down to just about anything really.
Because this doesnt happen all the time i decided to change the snubber board thinking a failure may be causing erratic armature volts on occasion, the fault hasnt occured since but i really expected to see a blown component on the board which has resistors varistors and capacitors on it.
from your experience do snubbers go bang or do they fail silently?
was it a good guess to change this board first or do you think i have just been lucky and desturbed a loose connection
cheers

 

[itsmoked]

Caps can fail quietly open as can resistors though not reeeeeal often. Yes your disturbed connection could be valid too. Glad you got it working and kudos to the idea of changing the snubber board. I don't think I'd have gone there very quickly.

Varistors - MOVs absolutely wear out. Usually dramatically.. Hence the shrink wrap on them all the time.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[Marke]

Hello bigheadted

Power film capacitors are commonly made of metalized polypropylene film and any insulation breakdown in the capacitor will cause the local metalization to be vapourised. These capacitors are often described as "self healing" and will slowly lose their capacitance. It is possible that the value of the capacitors on the snubber are much lower than they used to be.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[OperaHouse]

Where the lead is "welded" to metalized film of the capacitor is the weakest part of the construction. Not like the days when they actually used metal. This is like nailing jelly to a tree. The highest resistance is located there which causes localized heating and eventual destruction. It could be a random failure or an indication that the capacitor is under rated for a motor with above average spikes.