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wye delta contactor burning

wye delta contactor burning

(OP)
thread237-380026: Contactor getting welded when shifting from Star to Delta
Hello Sirs,

I am using a 10HP, 380/420V, 50Hz wye delta starter in an textile industry, two motors are used in m/c's, top for forward direction, bottom for reverse direction, both totally separated, different supply, no linkage b/w the two.

Fault :-
The starter at bottom one, the wye & star contactors are getting welded during transition, but top starter is working fine in forward direction.
We changed the welded contacts of bottom & interchanged the motor starter from top to bottom & bottom to top.
The motor starter in bottom (after interchange) gave the same problem of contact welding in wye delta.

The fraction of time taken to shift from star to delta is negligibly small causing a short circuit
I checked the motor specification of motor's inrush / current drawing in both revere & forward direction & found no difference.

The fraction of time taken to shift from star to delta is negligibly small causing a short circuit.
Applied all methods as used in thread237-380026: Contactor getting welded when shifting from Star to Delta.

I would like to get more comments and somebody please help because I am getting no ideas.





RE: wye delta contactor burning

I am investigating a similar problem right now. But it is not about contactor welding in the first place. It is more about motor overheating and overly cautious setting of the Wye time. Timer is now set to 4 - 5 seconds while no more than 1 second is needed. There is a recording, showing current in one phase, attached. To the extreme right, you can see the transfer from Wye to Delta and the associated high current peak.

We are now testing with a shorter time, around 1 second, and are going to test with an even shorter time so that the induced voltage from rotor flux is reduced. The motor and load (high inertia) are then rotating near their design speed and the reduced rotor flux avoids the high current peak.

There are also other things to consider, like the rotor position fall-back and the phase shift during switch over.

I try to avoid Wye/Delta. It is expensive, causes problems and most grids are tough enough to handle a DOL start nowadays.

Jeff will certainly add a few view-points. He is more versed in US practice. I handle the EU problems bigsmile


Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: wye delta contactor burning

Is there a difference in the starting of each motor, for example the bottom one starts last or the bottom one has more load.

RE: wye delta contactor burning

Has anyone checked the motor?
A ground near one end of a winding will do this. If there is a ground near the winding end that is connected to the wye contactor, there will be little ground fault current when in the star connection but a ground fault when connected in delta.
Interchanging starters is a good troubleshooting technique.
You say that whichever contactor is connected to the bottom motor fails.
Check the motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: wye delta contactor burning

Interchanging suspect components with working components to isolate a problem is a tried and true troubleshooting method.

Quote (OP)

We changed the welded contacts of bottom & interchanged the motor starter from top to bottom & bottom to top.
The motor starter in bottom (after interchange) gave the same problem of contact welding in wye delta.
This indicates that the problem is external to the starter.

Quote (OP)

MR. Waross, Motor has been checked by its vendor & did not find anything at their end
This leaves the wiring between the starter and the motor to be checked.
I understand that this is not a new machine and that this is a problem that has developed on a machine that has been working well for some time. Is this correct?
If the wiring checks out, then review the steps already taken.
Were the starters physically interchanged or did they remain in place and only the wiring connections were changed?
I understand that both starters have suffered welded contacts now. Is this correct?
Check the wiring with a megger using the 1000 Volt range.
Recheck the motor with a 1000 Volt megger.
Did this problem occur spontaneously or did the problem start after someone worked on the motor?
If any work has been done on the machine, check for a reversed winding in the motor, or a wiring error that reverses the connections to one winding.
To repeat:
If the problem developed spontaneously, check the wiring and the motor with a 1000 Volt megger.
If anyone has worked on the machine, checked for a wiring error that reverses one winding.
Last, inspect the motor for mechanical damage.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: wye delta contactor burning

I'm with Bill on this, the problem follows the motor, so it is most likely in the motor or the leads going to it.

You say you had the motor vendor check it? To me that's a little bit like asking the fox to investigate the henhouse security. "Yes Mr. Farmer, everything is fine here. No gaping holes in the floor covered up by straw."

Do the megger tests he suggested. If you or your technicians don't have a megger (megohmmeter) and are responsible for maintenance of 3 phase machinery, go buy one, you need it. The cost of the down time you have already suffered would have paid for it many times over by now.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

RE: wye delta contactor burning

Kapil ,
I did not a long time ago see motor with insulation class E. Check that motor maybe at 400 V should works in star connection .
Good luck !

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