Autotransformer starting scheme question 1

[rockman7892]

This is strictly a theoretical question that I have been curious to know the answer for some time. In a motor Autotransformer starting scheme, what will happen if for some reason the Autotransformer contacts and the By-Pass contacts are for some reason closed at the same time? I would venture to guess that there would be very little current flow through the Autotransformer sinced the bypass contact would in esence be shorting it out. Therefore the By-Pass contact would cause the motor to start across the line with the Auto-Transformer having little effect?

However if there was current flowing through the Autotransformer the it may be phase shifted by some amount. Would this present a problem when joining with in phase current across bypass contactor?

 

[rhatcher]

Rockman7892, in most cases there are mechanical interlocks that physycally prevent the start contactor and run contactor from being closed at the same time.

However, for the sake of your theoretical question, I believe that you are right. The run contactor would short circuit (bypass) the portion of the AT above the taps and the portion below the taps would have full voltage applied. There would be no circulating current flow through the bypassed portion of the AT to the line but the AT may be damaged by having full voltage across the portion below the taps.

I hope this makes sense. A picture would be worth a thousand words right now.

 

[waross]

It depends on the switching scheme. With some designs all that will happen is that you will be wasting the heat generated by the magnetizing current of the autotransformer.
With some switching schemes and interlock failures you may have part of the autotransformer energized and part shorted resulting in heavy circulating currents and rapid destruction of the autotransformer. A combination of an interlock failure and a contactor switching misoperation during the transition of a closed transition switch may be the worst case.

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

 

[rockman7892]

Attached is a schematic that I have for this arrangement. I understand what you are talking about above and below the taps.

It would appear that the (3) Autotransformer contacts are arranged in a "wye" configuration. Does this mean that when in operation the voltage across the 100% tap on these is 2300V? If that is the case then as you mentioned this 2300V would be seen across the bottom portion of the taps in the event that both sets of contacts were closed at the same time.

 

[rockman7892]

Sorry I keep forgetting attachments.

 

[rhatcher]

I said "A picture would be worth a thousand words right now" and Rockman7892 provided one. Nice job!

Assuming that your line voltage is 2300V (L-L) then, yes, during starting the voltage across each leg of the transformer is 2300V/1.73=1330V between the 100% tap and the 0% tap. The voltage between the 30% tap and the 0% tap would obviously be 2300/1.73*0.3=398V. The voltage seen by the motor during starting is 2300*0.3=690V.

If the start and run contactors are closed at the same time then the voltage between the 30% tap and the 0% tap would be 1330V instead of 398V. I am certain this would smoke the transformer.

I believe that waross must have been talking about a different configuration because it is not clear to me how this would generate circulating currents. Perhaps he will enlighten us.

 

[rockman7892]

I should have mentioned that our L-L voltage is 4.16kV so across each leg of the transfomrer between 0% and 100% tap would be 2300V, and this full 2300V would be seen between 30% and 0% if both contacts closed at the same time.

 

[rhatcher]

Yes...although for 4160V the L-N voltage would be 2400V.

Otherwise, if you substitute 4160V for 2300V in my post and do the math, my answer remains the same.

 

[rockman7892]

rhatcher

You are right, 2400V is what I meant to say.

 

[rhatcher]

So, is your question answered?

Also, having seen the label of the schematic decribing the motor as a quarry crusher, I now understand why you call yourself Rockman. Cool name.

I've been called a rock before, but it wasn't for a good reason...intelligence=rock...haha

 

[rockman7892]

rhatcher

Yes my question has been answered and it makes sense now. Thanks a lot for the help.

I guess the one last thing that I think I already know the answer for is that although 2400V is seen across the transformer winding during startup and the tap voltages are a percentage of this voltage the L-L voltage seen across the winding of the motor will still be a percentage of the system L-L voltage. For instance at the 50% tap there will be 1200V between the 0% tap and 50% tap on each transformer however there will be 2080V across winding in a delta configured motor?

 

[rhatcher]

Yes, you are right. Just remember, the motor is connected L-L to the AT. When you change the L-N voltage of the AT, by definition, you also change the L-L voltage that the motor sees.