Motor autotransformer starting

[jes1983]

I have complete installing a diesel generator that will start a 1500 hp 4160v motor useing a autotransfomer (two coil) . The Auto transfomere taps will be switched with 3 bypass contactors (close transition) using a a PLC.I will set the tap at 80% My question is the timeing on the contactors. For long do I leave the taps at 80% before opening the transition contactors and then the contactors to give me the 100%? thanks again

 

[roydm]

jes1983,
It depends on how long it takes the load to accelerate, my guess is you go to 100% once it reaches maximum revs at 80%. Another method might be to switch when the amperage falls back, I'm sure you have that available in a motor that size.
Regards
Roy

 

[Marke]

Yes, Roy is correct. You change to full voltage once the motor reaches full speed or as close to it as it will go.
If you switch to full voltage at less than 90% full speed, you might as well use a full voltage starter.
You can detect when the slip is getting small by a significant drop in current.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jes1983]

Thanks for the comments, how much time should i program for the opening of the transition contactors and the final bypass contactors for 100%. Are both done instantanous for is their a small time I should allow?

JMarquez

 

[roydm]

Hopefully you are not proposing to use Open transition method.
The recommended method is to put one contactor at star point of transformer and hardwire the motor to the taps.
Sequence of Events
Close Star point Contactor
Close main contactor applying reduced voltage to motor.
Open Star point contactor (motor is now suppied thru transformer as a reactor.
Close 3rd contactor between Line and Taps thus shorting reactance.
I mmagine that you can close the shorting contactor as soon as the Star point opens (verify with N/C auxillary contact.
It's a while since I worked on one, perhaps someone else can add to this.
Roy

 

[jraef]

That is called the "Korndorfer" starting method and if you do a search on that specific term you will get additional information.

 

[7anoter4]

hi jraef
The "Kondorfer" starting method is indeed the correct
definition.It seems that the link from 2003[jacphi] is not available now.Never the less there are a lot of references on web.
On the attachment are 2 schemes -one of AEG one of ALSTOM.
But I did not find theoretical development yet.
Best Regards

 

[jes1983]

Gentlemen the set up I am using is the autotransformer with close transition. the time in question is the opening of the transition contactors 6 and 7 and closing bypass contactors 1 and 5 for 100% I am attaching a file on my setup. Pls see thanks.

Jmarquez

 

[itsmoked]

You won't burn up the motor by having it say in reduced speed mode a little long. So you can set the times beyond your expectation and then stopwatch it all to see how long it really needs to get to the first transfer point then the second. You can hear the motor level out at each step.

Just remember that while doing all this you DO NOT exceed the allowed motor start frequency.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[LionelHutz]

You can switch quickly. Just use a N/C aux off the start contacor (6 & 7) to allow the run contactor (1 & 5) to close. You could program a small time such as 200mS but it's really not necessary. There are some Y-Delta timers that have a small delay built in that could be used.

As for the transition speed. Set a long time and start the motor. Monitor the current and/or the speed. When the motor quits accelerating or the current falls, dial back the timer until the transition occurs. If you can't adjust the timer, then time the start and set the timer to a little longer time than you found, say around 3-5 seconds.

 

[rockman7892]

I have always been told and understood that when playing with these transfer times with an Autotransformer you have to be aware of the Autotransformer Duty Rating. If the transition time is too long and you run through the Autotransformer longer than its Duty Rating, then you can damage the transformer. Starting frequency (starts/hr) applies to the transformer the same as it does the motor.

 

[jraef]

Absolutely true, and most autotransformers by the way have about a 15 second duty rating by the way, as well as a starts-per-hour rating that is almost always less than the motor's will be. In most cases you will have a thermal cutout switch (Klixon) embedded in or attached to the transformer which will drop out the start circuit if it overheats. That's not to say you can't buy one with a higher duty rating, but if you don't ask, that is what you will get.

 

[jes1983]

Thanks for all your comments and suggestions they have been a big help. I should be testing soon and will let you know how it went.

Regard
JMarquez

 

[nightfox1925]

In relation to autotransformer starting. How is the autotransformer size determined? Is it based from 100% FLA or 100% LRA?