3000 HP Motor (4160 V) Starter

Huh? Have you combined both autotransformer and wye-delta into one starter? And if so, why?

Yes, the auto transformer is tapped at 80% in a wye configuration, then switched to delta (normal feed) by bypassing the auto transformer when the motor is at 90% FLA. The main contactor provides power for the operation (centrifugal chiller). This configuration allows straight delta wound motors. I've been tasked to correct a momentary flash over problem with the auto transformer. Thanks.

Huh? (New one on me; where does the center of the Y go, and what does it do?)

The main contactor feeds the 0 % end of the auto xfmr which is connected to the motor windings via the 80% tap. The 100 % end of the the auto xfmr is connected (bolted) together by the start contactor. At transition the start contactor opens and the run contactor closes effectively shorting out the auto transformer and providing line to line 4160 v in delta (normal) config. The 4160 v is presented to the motor at 80 % of line to line Y volts (3 phase). This is old equipment (1970's) and very slow (mechanical linkage, etc.).

In an auto-transformer starter, the main advantage is a closed transition, when switching from partial to full voltage. When starting the Υ of the autotransformer is made first, and then the motor is started. After adequate speed is reached, the Υ is opened, and the three part-windings of the autotransformer act as series inductors while the transition to full voltage is going on. So, closed transition. When running at full voltage/speed, the autotransformer is completely out of the circuit. So which Δ are you talking about here?

Yes, that's right. What I meant was usually a typical wye/delta starter involves a 12 lead motor (not pratical over 500 hp). This auto transformer mode allows a standard motor to be started wye (with tapped lower voltage) and run delta. I disagree that it is closed transition because the run and start contactors cannot be closed simultaneously. My original question was: before applying power to the auto transformer should the wye be made up or vice-versa? Or does it make any difference? Momentary flash-over (to ground) occurs at the transformer during start-up. Thanks.

Please check the circuit i have given. The start and run contactors cannot be on at the same time. In the transition period, the star point of the autotfr is broken and the motor continues to get supply through the part windings of the autotfr, which will now act like series reactors. The run contactor then switches on, eliminating the autitfr from the circuit. This avoids open transition, eliminating the dangerous "frozen rotor" situation which has been known to shear motor shafts. It is called a Korndorfer closed-transition autotfr starter.Please google and check.

If this is the circuit you are using, then yes, the star-point should be made first, then power should be applies to the 100% point of the autotfr. If your scheme is different, then ....

Logic still says that the starpoint should be made first, as otherwise, it is not really an autotfr.

Thanks for your explanation. Our starter is a little different than the one you illustrated. The SEL 710 has a form "C" contact to transition the control voltage (not the 4160 v) without losing power. As you said the run and start contactors cannot be closed at the same time. It looks like your circuit is powering the xfmr prior to making the star. I am designing a circuit to make the star first (as you suggested).

I appreciate your input. Thanks

I believe you are mixing your terminologies.

The "Wye" connection you refer to has nothing to do with the motor connections, it is a shorting connection for the autotransformer itself. Referring to the motor changing from Wye to Delta is related to a Wye-Delta motor starter, which does NOT have an autotransformer in the circuit. The issues are mutually exclusive.

If you are getting a flash situation during transotion fro Start to Run on an autotransformer starter, it is because your autotransformer starter is very old fashioned and is NOT using the closed-transition Korndorfer method as mentioned above. Do some research on that term, you will see that is specifically what it was intended to address.

hughes838

i sincerely hope that you are not offended by anything i have said. If so, i apologise...certainly not intended.

That done, back to your query....

1. 80% tap is rather high, almost DOL. Well, one assumes that the original designer knew his business. So, we will leave it at that.

2. The motor is very expensive obviously, there is a SEL710 relay protection relay.

3. So, in light of the above, it is logical that the original designer would use a Korndorfer starter

If you ponder these points, it seems most likely that your circuit is most likely to be a closed transition AT starter...

http://en.wikipedia.org/wiki/Kornd%C3%B6rfer_autotransformer_starter

If you read the Wiki page, it gives the switching sequence....

Good hunting...

In no way was I offended! I most certainly appreciate your valued input. I will evaluate your recommendations and let you know how this pans out. Thanks