Help understanding auto transformer shorting

[kenneth edmiston]

Will someone please help me understand the significance of shorting an auto transformer? This feeds a motor. At reduced voltage, the motor starts at 65% of line voltage, a run contactor pulls in bypassing the transformer and the shorting contactor pulls into take the transformer out of the circuit.

How does this happen? What is the significance of shorting them out? Why is it necessary? And what if they were left open? If anyone can explain in detail I would greatly appreciate it!

Thanks!

 

[Tom.G]

I assume this is related to your other thread. https://www.physicsforums.com/threa...ge-sag-at-large-chiller-motor-startup.942228/

For the circuit to operate as intended, that is start at reduced voltage then switch to full line voltage, would require M and S contacts to close for start-up. Then when speed is reached, S contacts open and R contacts close. Make sure ther is no overlap of the S and R contacts, that could lead to spectacular results. Probably the safest way is have the S and R contacts on the same contactor that the manufacturer guarantees not to have contact overlap under any failure conditions.

 

[Baluncore]

How does this happen? What is the significance of shorting them out? Why is it necessary? And what if they were left open? If anyone can explain in detail I would greatly appreciate it!

During the start period, the voltage is reduced by the Y or star connected 3PH autotransformer.

When running, the common terminal at the centre of the Y is disconnected from all autotransformer phase windings.
The remaining windings are shorted by the run contacts, so the transformer is effectively without voltage or current.
The autotransformer is NOT energised while it is shorted.

If the run contacts were left open, the autotransformer would appear to be inductors in series with the motor.
Series Inductors would reduce the power factor, which is really bad.

 

[kenneth edmiston]

During the start period, the voltage is reduced by the Y or star connected 3PH autotransformer.

When running, the common terminal at the centre of the Y is disconnected from all autotransformer phase windings.
The remaining windings are shorted by the run contacts, so the transformer is effectively without voltage or current.
The autotransformer is NOT energised while it is shorted.

If the run contacts were left open, the autotransformer would appear to be inductors in series with the motor.
Series Inductors would reduce the power factor, which is really bad.

Exactly what I was looking for. Thank you sir

 

[kenneth edmiston]

During the start period, the voltage is reduced by the Y or star connected 3PH autotransformer.

When running, the common terminal at the centre of the Y is disconnected from all autotransformer phase windings.
The remaining windings are shorted by the run contacts, so the transformer is effectively without voltage or current.
The autotransformer is NOT energised while it is shorted.

If the run contacts were left open, the autotransformer would appear to be inductors in series with the motor.
Series Inductors would reduce the power factor, which is really bad.

I assume this is related to your other thread. https://www.physicsforums.com/threa...ge-sag-at-large-chiller-motor-startup.942228/

For the circuit to operate as intended, that is start at reduced voltage then switch to full line voltage, would require M and S contacts to close for start-up. Then when speed is reached, S contacts open and R contacts close. Make sure ther is no overlap of the S and R contacts, that could lead to spectacular results. Probably the safest way is have the S and R contacts on the same contactor that the manufacturer guarantees not to have contact overlap under any failure conditions.

It is related, and it isn’t. The diagram I uploaded isn’t the motor I’m working on. I just want to know all I can about what I’m working on. How an auto transformer even actually works on a physical level is still a mystery to me haha. Thanks for the article and the reply

 

[kenneth edmiston]

During the start period, the voltage is reduced by the Y or star connected 3PH autotransformer.

When running, the common terminal at the centre of the Y is disconnected from all autotransformer phase windings.
The remaining windings are shorted by the run contacts, so the transformer is effectively without voltage or current.
The autotransformer is NOT energised while it is shorted.

A couple of questions:
Are all 3 windings (primary/secondary x3) connect at the common terminal? So as to be consider a single autotransformer and not 3 singles.
And does the shorting process change the configuration to bypass the y connection so to speak?
Right now I have an auto transformer in a primary reactor RV starter. Wired like a PR. Power company installed PFCC and it fixed the problem slightly, but not completely.
I think it’s the set-up but I want to be certain.

One more question— A primary reactor is still left in the circuit, just bypassed by a run contactor. why is that inductance not an issue like it would be with a transformer? (The circuit does have 20uf of capacitance to offset some of the L.

Thanks a lot!

 

[kenneth edmiston]

Here’s the circuit. Wired the same only reactor has been replaced with xfmr tapped to 80%.
It seems to be causing some major issues. I just surge tested the “c” phase winding and it appears to be almost dead shorted. I wonder if leaving a medium duty xfmr in the circuit has been causing heat as well.

Thanks again chaps

 

[kenneth edmiston]

Or do I have this completely backwards and the 0% is the star point and is needed to provide the reduced voltage, then is disconnected after motor reaches desired speed? This would make sense too

 

[Asymptotic]

My experience is limited to 200 to 400 HP, 460V, delta-wye starters, and not 4160V machines, but from what I can make out in the schematic there is no delta-wye transitioning. Rather, the motor is started by closing contactor 1K with contactor 2K de-energized (allowing current to flow through the auto-transformer to achieve the reduced voltage start), then (presumably, once a timer relay has timed out) closing contactor 2K to bypass the auto-transformer windings.

Have you tried using the lower taps?

 

[kenneth edmiston]

This is a typical auto-xfmr start

 

[kenneth edmiston]

We can only run this chiller 1ce per year. Makes troubleshooting a pain

 

[kenneth edmiston]

The 1st diagram was a primary reactor start setup, which we have, only the reactor has been replaced by an autoxfmr. I can only assume the auto xfmr is now being used as a series inductor and not a xfmr, so line current is not being reduced at start-up and power factor is being affected.

 

[kenneth edmiston]

Lower taps should increase inductance, lower stress on motor, but leave line current unchanged. If I understand this right.

Sorry for blowing up my own thread. Thanks to all for the help

 

[Asymptotic]

This is a typical auto-xfmr start

This diagram is different than the first. Is this how it is wired?

If so, it looks like the 'M' contactor and 'S' contactor are closed during start. They drop out and the 'R' contactor closes when the start timer completes.

50% tap: 50% current & 25% torque
65% tap: 65% current & 42% torque
80% tap: 80% current & 64% torque

 

[kenneth edmiston]

The “autoxfmr diagram” was just to show you how a typical auto xfmr start is wired. How it’s shorted etc.

What I have is an auto xfmr in place of a reactor in the 1st diagram. So it’s not being connected during start up, and is only reducing motor voltage by way of dropping voltage through inductance of xfmr winding.

So it looks like the original reactor start was not a good choice for that motor and caused the voltage dip during start. so someone replaced it with an auto xfmr to try and reduce start up LINE current but did not install a means to connect the windings during start to actually reduce voltage. Instead it ended up operating the exact same way the reactor did.

 

[Baluncore]

Sorry for blowing up my own thread. Thanks to all for the help

I think you just learned something. A forum is not like a fast two way conversation.

You need to get a reasoning.text file. Rather than thinking out loud in posts, do your reasoning and construct your questions in that file. You will find it advantageously disciplines your reasoning. When you have noted your observations and refined your question(s), you will probably realize what the answer must be.

If you cannot find the answer, then you will at least have a succinct question, with relevant observations, to post on the forum.

 

[kenneth edmiston]

I debated on even responding because it contradicts your post . Yeah but noted, I’ll take it as constructive criticism. I’m a newb to the forums. I learned more than one thing today though. Many thanks

 

[Baluncore]

I debated on even responding because it contradicts your post .

Most answers need to be questioned.
To answer a question we must first work out what the question should have been.

“The only interesting answers are those that destroy the questions”. Susan Sontag.

 

[kenneth edmiston]

I completely understand what you’re saying.
But I’m really good at questioning myself to the point of confusion. I’ve found that my biggest revelations come from thinking out loud to one or many so to speak and piecing together the conversations. I’ve always thought there were no such thing as dumb questions, only dumb answers. I still do, but I’ve never thought of it the other way around. But the truth is, I didn’t know what the question was, I was in the middle of troubleshooting and flying by the seat of my pants. But I believe I was able to understand what I was dealing with by being on this thread. Maybe the way this thread was formatted was exactly how I needed it. Who’s to say. But I’ll quit rambling on, unfortunately this isn’t a physics/philosophy forum

 

[Tom.G]

But the truth is, I didn’t know what the question was, I was in the middle of troubleshooting and flying by the seat of my pants. But I believe I was able to understand what I was dealing with by being on this thread.

Glad we could help. And Bravo for continually looking for answers when you realized they were needed. Too many people just throw up there hands and say 'That's the way it is.' I think all of us have learned a bit on this thread.

Cheers,
Tom

 

[Tom.G]

Just a thought. For trouble shooting can you find a lower supply voltage to drive the circuit? I'm thinking 208V or 440V. Maybe you can run a temporary line from somewhere else in the plant to keep things under control while tracking down the problem.

 

[kenneth edmiston]

Just a thought. For trouble shooting can you find a lower supply voltage to drive the circuit? I'm thinking 208V or 440V. Maybe you can run a temporary line from somewhere else in the plant to keep things under control while tracking down the problem.

There’s probably a way to do it, although I don’t think the low voltage would provide the torque needed to turn the motor. Even if it would, they wouldn’t spend the money for it. After figuring out the problem, it turns out they are content with putting a reactor back in and only running that particular machine once a year. (One start, and continuously running through summer). I work for a small company full of “that’s the way it is”. If it’s out of the norm, it’s scary. I don’t think my superior fully understood what I was trying to explain and reverted to the original design when making the executive decisions. The auto-xfmr would have dropped the inrush current on the source if it had been connected properly. But the boss was able to sell the easy way out.

...That’s the way it is

 

[Tom.G]

Bummer. I've worked for a few of those "Bosses", usually quite briefly.

Too often it's a case of There Are Two Rules:
1) The boss is always right.
2) If the boss is wrong, see rule #1.

Ah well. A roof over the head and food in the belly is good.

 

[kenneth edmiston]

Bummer. I've worked for a few of those "Bosses", usually quite briefly.

Too often it's a case of There Are Two Rules:
1) The boss is always right.
2) If the boss is wrong, see rule #1.

Ah well. A roof over the head and food in the belly is good.

It’s alright though, I’ll chalk it up as a good learning experience and sleep like a baby.