Star Delta starter on 550 hp motor - 480 volt

[JVShermco]

We have a star delta or Wye Delta starter on a 550 hp air compressor motor. We are tripping the 800 amp feeder breaker feeding the starter. It does not trip on every start. We have done inrush measurements (se attached pdf). The motor gets up to speed during the reduced voltage portion of the start. There is an approximate 1/4 second delay between low voltage and high voltage start. At the initiation of the delta contactor, the cuerrent spikes. This is when the breaker trips. Is there a timing issue with the transitional period between low and high voltage contactor closing? Is there a timng issue with the two Delta contactors? Is there a test to see if there is a contactor timing issue?

 

[waross]

It looks as if your instantaneous trips are set to low on the breaker. Try 500% on the magnetic trips.
For timing, look at the residual motor voltage. Us potential transformers for isolation and record the voltage across each motor phase winding. You will see the voltage decay when the wye contactor opens. If the residual voltage is too high you may have current spike problems, but first, review your breaker instantaneous settings.

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

 

[itsmoked]

Classic wye-delta starter issue.

The tight-rope walk between waiting long enough to prevent The Big Jolt or the RPM dropping so far that the delta is a jolt all by itself.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jraef]

When I worked for a soft starter manufacturer, this was our best sales weapon against the cheapness of Y-Delta starters. In some cases, there is nothing you can do that is going to consistently solve the problem. Waross' suggestion is about the best hope, but any slight difference in loading or line voltage will still affect the timing issues.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[Skokian]

Is this an "open transition" (three step) or a closed transition (four step) starter?

With open transition, if the motor does not get up to speed, or looses speed during the open transition period, there will be a current surge of LRA (LRC) plus offset on one or two phase currents for a few line cycles.

If the motor does get up to speed, there will be a transition spike due to the motor back EMF being out of phase with the line voltage. This spike can be up to 18x the FLA (FLC) current.

Most industrial NEMA Design "B" squirrel cage induction motors have an open circuit time constant of 3/4 to 1/5 seconds. The back EMF is around 85% of 58% of the line voltage to being in the Wye (Star) configuration.

More info. can be found here:

http://www.slideshare.net/JamesSNasby/fire-pump-motor-starting-presentation

Hope this helps.

Regards . . .

Jim S. Nasby

 

[Scramode]

If you can, ($) go to a closed transition star delta open transition just causes to much greif
scrammode