secondary fuses blow when shutting down 2

[deetz]

We have a 3 phase 480v washer with heaters, a conveyor and exhaust and such. For some reason it blows the secondary fuse of the control transformer when the master stop is pushed.. It has run for years and has never done this, to my knowledge anyway. I've checked all heaters, pumps and motors and they all are working properly. There are no diodes or snubbers on any of the contactor coils, but there never has been from the beginning. I've also noticed that when they shut it down by turning all the switches off instead of the master stop that it won't blow the fuse. Yeah, I know, have them do that from now on. Also if you turn the switches in the on position and then turn the master start on, the main control transformer fuse blows. Just one of them. If the transformer was going bad, wouldn't it just be bad?

 

[FacEngrPE]

This is likely just a troubleshooting problem. The equipment is giving you some hints. Assuming you have already checked the wiring for obvious problems, look at your master stop / start buttons. It is uncommon, but internal problems can develop inside buttons and switches.

 

[cranky108]

I usually ask the first question: what has changed? Have you changed any of the control relays, lights, etc. lately?
Then go look for ageing problems.

 

[deetz]

Nothing has changed, again, to my knowledge. Hard to troubleshoot something that is in use alot. When I am able I'll start to disconnect stuff, then step by step reconnect them and try it until I get it to trip. I'm kind of leaning towards a solenoid valve. Just a premonition.

 

[thebard3]

I think I would just try adding an MOV or something to the MCR and see if that fixes it. I know it doesn't answer why it just started doing that. Sometimes you can avoid a protracted troubleshooting effort by just throwing a couple of parts in there and just accept that it's a little bit of a mystery.

Brad Waybright

The more you know, the more you know you don't know.

 

[cranky108]

There are components that can fail, and give no indication they have failed. MOV's and spike suppression capacitors come to mind.

 

[deetz]

Thanks for the replies. I haven't really gotten back to it because they ahve been using it pretty steady. It's running fine as long as they shut the different functions switches off before pushing the master stop/e-stop.

 

[sunnysky]

The aging master switch may be arcing during shutoff and should be inspected for voltage drop when running or signs of wear. Flashover with stored kinetic energy ought to be snubbed with an RC film filter.

 

[jraef]

I’m going to go with the arcing issue. Does the “master switch” work via a Master Control Relay (MCR)? If so, I would suspect that the contacts are arcing and possibly flashing to ground inside of the MCR. It will only happen if being de-energized or energized under load, which seems to be the case from your descriptions.


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