Motor Starting Current graph analysis

[noel0589]

Hi all,

I saw the subject on contactors come up again.

http://rapidshare.de/files/1496465/hb.pdf.html

Please download the above.
Page 3 of attachment (hb.pdf) shows wye delta starting of a 40HP/200V motor using 208V/60Hz/3Phase power supply.
Scale is 250A / div. (A) and 50ms/div (horizontal) Page 3 shows no problem during start.
Now please look at page 2
In this case, heavy arcing occurred. On delta connection the graph on page 2 shows how the current was interrupted during 2 cycles. I am not sure to attribute this to bouncing of the contactor or to a voltage drop in the power supply affecting the contactor magnetic coils capacity to hold.
Page 1 shows line to line voltage (V is 300V peak) and current under heavy arcing condition.
My thinking is that the surge of current(transient current) results in a large voltage drop in the voltage supply line. This drop in voltage results in contactor coil magnetic field weakening and causes delta contacts to break at high current causing the large arc. When delta is disconnected, supply voltage recovers and contactor closes once again.
Can you provide me with advice on my reasoning?
My main question is the time scale. Can all of the above happen within such a short time span of 2 cycles?

Thank you!!

 

[electricpete]

I can't find the file. Can you give better instructions how to get it.

Reminds me of something I saw once.. probably not related to what you saw but here it is...

We instrumented switchgear for a 13.8kv motor prior to starting. When we reviewed the traces afterwards we saw several very steep-sloped blips of current over about a 2 cycle period before we saw the normal pattern we expected. We thought we were looking at some weird contact bounce. But it didn't make sense that the actual current could change that quickly in an inductive motor circuit, even if the contacts did bounce. Further investigation showed that the instrumentation was picking up false readings due to switching of relay coils within the breaker prior to closure.

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[jraef]

I saw the file and it does not look that unusual to me for a Y-Delta starter profile. What you probably experienced was a severe transition spike because when you went from Y to Delta. Under the right conditions, the current can drop to zero like that and spike again right after the delta contactor closes.

Is this still the same problem you posted back in September thread237-104589 ? You also have posted several other issues with 40HP Y-D elevator motor starters. I found them when searching for some of the many threads to direct you to in this forum that have dealt with this issue. You seem to have a recurring problem with this. If this is a different application with similar problems, you may have a bigger issue to contend with, i.e. supply problems or transformer sizing issues. That would be especially suspicious if it is happening on the same phase each time. If it is the same elevator and you upgraded to 50HP but the problem didn't go away, I would suggest that you check into a solid state starter and be done with it.

"Venditori de oleum-vipera non vigere excordis populi"

 

[jraef]

E-pete, as someone else informed me on one of his previous posts you just need to click on the "Free" button at the bottom of the linked page and wait a few seconds before it redirects you to their FTP site. Then you need to download his .zip file of images. You will also need to be capable of rotating them because one is scanned upside down.

"Venditori de oleum-vipera non vigere excordis populi"

 

[itsmoked]

I would agree with jraef. Cut to the chase, and go with a soft starter so you can avoid that nasty transition.

 

[electricpete]

Thanks jraef.

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