Wye-Delta Open Transition Problems

[noel0589]

First off, I'd like to thank all responders, some more than others... you know who you are!!!

I have a problem with my delta contacts (open transition) which seem to be burning out on our hydraulic elevator systems. The only reason I heard that makes sense is because of the surge which can occur during the instant of switching from wye to delta.
Now... the actual physical problem is that roughly 1 in every 4 or 5 starts is when an abnormal amount of arcing/sparking occurs. Does this mean that 1 in every 4 or 5 starts the contactor is closing upon a critical point on the applied voltage wave resulting in a huge surge?
Also, the abnormal amount of sparking always seems to appear on ONLY ONE of the lines, the arcing is not distributed among the three lines. So one contact in the housing has much more deterioration than the others. Is this also a symptom of wye to delta transition current surges?!?! THANK YOU!

 

[freddycor]

noel:

some questions:

1-how old are your contactors? ( no mechanical problems? )
2-ratings of equipment? ( motor,contactor,etc.)
3-always is the same equipment failure or are differents equipments with the same failure ?
4-have you capacitors in this equipments ?.

I think you need add a little more information like this an some other.

Good luck

 

[Marke]

Hello noel0589

Certainly, there will be a variation in closing voltage in the transition from star to delta due to the random nature of the phase angle at reclose. I am surprised that the arcing as at close though. The highest current through the contacts is at reclose, but the contacts should be closing fast enough to avoid significant arcing. The arcing usually occurs during opening.

I would expect that there is a mechanical issue here that is preventing that contact from closing at full speed, or that contact is bouncing on closure.

Is this a new contactor?
Is the contactor mounted according to the manufacturers recommendations? I have seen similar problems where the contactor is mounted side ways rather than vertical.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[noel0589]

Mark, Freddy

The contactors are new and the silver tips are being replaced every three months. It is a 208V 50 HP motor.
Starter is rated for 40HP. It is not properly sized, but in other hydraulic elevator applications with the same type system there is no problem. Mounting is proper. 4 out of 5 times nothing happens. But 1 out of 5 on the wye to delta switch there is a huge arc created on one contact during the delta connection. A loud sound is made due to the contacts hitting so hard and it does sound as if bouncing occurs. But this is only one in 5.
I am trying to investigate problems with open transition wye-delta starting and if these symptoms match what would occur due to angular positioning and magnitude of flux during wye to delta switch.
Arcing is definitely occuring during closing. I think that it should occur during closing, but I am puzzled at why it only occurs across one of the lines.

Regards,
Noel

 

[noel0589]

Actually, also, I have heard that arcs are only created during disconnect?!? I don't know how true this is. If this is the case, then I think that what is occuring here is that the contacts may be bouncing off and this may lead to the excessive spark.
So would that mean that sometimes during the wye-delta switch(depending on phase angle at reclose) that there may be more magnetic force pushing the contacts together, and that it may be strong enough at times to cause the contacts to bounce?
Would someone please explain what would cause contacts to bounce or "chatter"?
Thanks!!!

 

[aolalde]

My opinion is that the contactors do not close at the same time nor the voltages are the same at each starting event.
If we think in milliseconds or microseconds, after the first contactor has closed the other two must have some time delay to close, at least a few microseconds but could be up to 1 millisecond.
The second factor is the relative voltage between phases or lines if the second and third closings are near a zero or a maximum. That will be random depending on the time required to close each contactor and sometimes it is worst than others.

 

[Sanditech]

The problem you appear to have has been well documented in the past. Basically at transition the phase voltage has not fully decayed before the line voltage is applied and depending on the instance of transition the contactors can have an increased voltage across one pole.
All modern Star(wye)/Delta timers now have a dwell time incorporated to give a 20-30ms gap from opening the star (wye) contactor to closing the delta. This allows the phase voltage to decay prior to applying the line voltage.
There is also a way of wiring star(wye)/delta starters that iliminates the problem altogether I believe Schneider use this as there standard scheme so a visit to there site may assist.
As an aside the problem was first identified by a former lecturer of mine in the UK (Mr Tomlinson)in the early 70s (that 1970s).
A local machine shop had installed 20 lathes, all with star/delta starters. For some reason 10 of these lathes starters were burning there contacts out in 3-4 months the rest showed signs of normal wear. On investigation he found that the starters were wired in a slightley diffrent way. He then proved by vectoral diagrams the reason for the excess voltage during transition and a paper was published in anational journal, which I came across last year.

If you would like a copy I will have to dig it out and scan it.
training@switchgear.co.uk

Hope this helps.

To resolve a problem you must take ownership of it!

SANDITECH

 

[jraef]

There are a number of problems that can occurr during Y-D transition, most of which will cause flashing. It could very well be that the flashing is always happening, but only occasionally does the ionized air surrounding the contactor allow it to be a big enough flash to be seen from the outside.

It could also be related to the transition time itself, in that it must be sufficiently long to allow the field to collapse in the motor, but not so long that the motor slows down. That is why special Y-Delta transition timers are made, because it is so critical. Often, some OEMs try to get away cheap by using only the mechanical interlocks on the contactors as the transition delay. That works when they are brand new in the shop, but in the field the mechanical components quickly wear and start to hang up. You keep changing the contacts, but if the other parts of the system are worn out you would keep recreating the problem.

You could try replacing the entire assembly, but if you want the problem to go away completely, replace it with a solid state starter. There are several marketed now that have been designed as drop-in replacements specifically for the elevator industry.

"Venditori de oleum-vipera non vigere excordis populi"