Contacts Weldinmg and Sporadic MCB Tripping

[mnickels]

Good afternoon Gents. I wonder if anyone can help with a problem we are having with our AC supply.

We have a 400Vac generator supplying our AC, this passes through a contactor which has a rating of 115A but we are seeing contacts damaged through arcing.

I believe that the operator is knocking off the DC supply to the coil and the contacts are breaking with load but there are other possibilities.

We run a compressor off the supply which has an operating current of 14.2A, a starting current of 124A and an inrush current of 228A.

1. It has an MCB rated at 13/18A which sporadically trips. Should this be a higher rating?

2. Does the 115A seem sufficient for a starting current of 124A and an inrush current of 228A?

3. What is the best way to record the current/ back EMF in this circuit? What apparatus do you reccommend?

I have attached a circuit diagram but the scanner isn't great, sorry.

 

[bobmcbride]

Obviously if "the operator is knocking off the DC supply to the coil and the contacts are breaking with load" arcing can occur.

Have you checked the voltage going to the coils of the contactors?
With low coil voltage they will be more prone to contact bounce, which won't help. For example I've seen some 24v contactors that will close with only 9v but chatter at first.

I'm going to assume the contactor is rated 115A RMS. So for a 14.2A RMS compressor I'd say this is fine. But with out the specifications of the contactor I can't say for sure.

I really can't read the schematic so I'm a little limited on saying exactly how to record back EMF/current in your circuit.
However I like the TEK THS720P scopes for this sort of thing. They have 2 fully floating channels with allows you to connect the ground clips of the scope to up to 600VAC?.. if I remember right. With this 2kv probes, and a current probe I'd say you'd be set.

Hope this helps.

 

[jraef]

Wow it is really difficult to make out those devices, but it appears as though you have an MCB that feeds a "box" that appears to say Over Excitement(?) which in turn feeds what looks to be a motor starter ahead of the motor. So is that the contactor that you say is welding, the one right ahead of the motor?

If so, and that is a motor starter, meaning the contactor is DESIGNED as a motor controller, then the rating should be fine. When a contactor has an inductive rating of N amps, it is intended to be able to handle the short duration starting current, typically defined as N times 6. So from a sizing standpoint I don't see an immediate problem, but I am not there to see the actual contactor. A mfr and part number would be helpful.

A common cause of contactor failure in comprtessor systems is what is called "short cycling", where the pressure control device has no hysteresis, it cycles on and off in rapid succession. Standard contactor selection for a motor would allow maybe 1,000,000 operations, but there is a limit as to how many operations per hour it can take. So if for example it is short cycling, it may only be capable of 100,000 operations, and if it is cycling 10 times per minute, that is not a long time. So watch the operations for a while and see if it chatters on and off, then find out why if it does.

Another possibility on a compressor is if you have an unload valve that is supposed to allow the compressor to start unloaded, but it is failing in some way. That will cause the motor to draw locked rotor current for too long and fry the contacts.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[mnickels]

The contacts that are getting damaged can be found in the device at position E7 on the diagram. It isn't great so again apologies. This contactor is rated at 115A and allows the AC supply to travel from the generator, through the excitation unit and to the devices requiring the 3 phase supply.

I need to measure the maximum current at this device on breaking as well as the back EMF from the motors it supplies to pin point what is causing the arcing.

In any case I believe it is a consequence of the operator cutting the DC supply to the coil and breaking the contacts on load. I just need to prove it...

Regarding the compressor MCB tripping, it is a seperate issue and to avoid confusion I will leave it for now.

Thanks for your help guys