# Auto transformers



## GEORGE D (Apr 2, 2009)

I have zero experience with auto transformers and was wondering if it will suit my application. Attached is the diagram for a Chinese 3 phase 380-volt machine that requires a neutral. I couldn't source a delta/wye transformer, all my supplier could come up with is this. Please help


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## Mshea (Jan 17, 2011)

What is your service configuration? is it grounded or not? If the service is ungrounded then you need an isolation transformer to get a neutral or grounded service conductor.
If the service is grounded and the voltage correction is for a machine then a 3 phase autotransformer might be the thing you need take 3 hots and the neutral. Neutral does not have a connection to the auto transformer The hots are connected to the correct input output taps and the load is connected to teh auto transformer at the correct voltage tab, the neutral carries through to the load.


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## brian john (Mar 11, 2007)

The machine uses 3-phase 3-wire or 3-phase 4-wire?


What is your available voltage?

I have bought 208 felt to 380/220 VAC transformers


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## GEORGE D (Apr 2, 2009)

Mshea said:


> What is your service configuration? is it grounded or not? If the service is ungrounded then you need an isolation transformer to get a neutral or grounded service conductor.
> If the service is grounded and the voltage correction is for a machine then a 3 phase autotransformer might be the thing you need take 3 hots and the neutral. Neutral does not have a connection to the auto transformer The hots are connected to the correct input output taps and the load is connected to teh auto transformer at the correct voltage tab, the neutral carries through to the load.


Ok, yes the service does have a neutral. I see now on the diagram, just wanted some input before purchasing transformer. Thanks


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## GEORGE D (Apr 2, 2009)

brian john said:


> The machine uses 3-phase 3-wire or 3-phase 4-wire?
> 
> What is your available voltage?


480/277, 3-phase.


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## GEORGE D (Apr 2, 2009)

GEORGE D said:


> I have zero experience with auto transformers and was wondering if it will suit my application. Attached is the diagram for a Chinese 3 phase 380-volt machine that requires a neutral. I couldn't source a delta/wye transformer, all my supplier could come up with is this. Please help


I'm sorry, I meant attached is the diagram to the transformer I looking at, not the machine diagram. .. In case this confused anyone


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## brian john (Mar 11, 2007)

And the machine uses 3-phase 3-wire or 3-phase, 4-wire?

I cannot see the PDF you posted can you provide a model number?


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## Mshea (Jan 17, 2011)

So your service is 480/277 and you want to power a 380/220 volt machine. I added the derived neutral voltage for fun.
Neutral goes straight to the load Your auto transformer needs a 480/380 volt ratio or about 1,26 buck connection on a 3 phase auto transformer. Rating of a buck boost transforemr is about %10 of the full load current of the machine.

There may be a neutral connection but it is not necessary. That said if there is one, use it. you should see multiple taps on each phase with instructions to what taps for the voltage included. Output wire is bigger than input wire as current at 380 volts will be higher than at 480 volts.
remember that the input and putput are electrically connected and to an Ohmeter the resistance reading is very low. You are relying on magnetism to control the current.
Sorry no easy reference to a diagram.


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## JRaef (Mar 23, 2009)

Depending on the machine, you might be worse off getting that transformer. If it is just an electric motor, the machine is likely designed for 380V *50*HZ, you will be giving it 380V *60*Hz if you use a transformer, that will over heat the motor AND make it run fast.

AC motor design is a factor of the ratio of volts and hertz, usually within a +-10% range. A motor designed for 380V 50Hz is therefore looking for 380/50 = *7.6 V/Hz*. If you use that transformer, you will be giving it 380V 60Hz, so the ratio will be *6.33 V/Hz*, which is beyond the 10% range. The motor will have about 80% of it's design torque, 70% of it's peak torque (used for re-accelerating loads) and it will spin 20% faster. The slip will be higher, it will draw more current while doing less work. In short, it will burn up.

If you just go ahead and apply 480V 60Hz power to it, it will still run 20% faster, but the ratio will be *8.0 V/Hz*, well within the tolerance range for the motor (+5%).

If there are other things on the machine that need 220V single phase power, open it up and separate those from the 380V motors and get a separate single phase off-the-shelf 480-240V transformer. Then hope the frequency difference is OK for those things.

Better yet, don't buy cheap crap machinery from China that was not designed to be used here...


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## GEORGE D (Apr 2, 2009)

JRaef said:


> Depending on the machine, you might be worse off getting that transformer. If it is just an electric motor, the machine is likely designed for 380V 50HZ, you will be giving it 380V 60Hz if you use a transformer, that will over heat the motor AND make it run fast.
> 
> AC motor design is a factor of the ratio of volts and hertz, usually within a +-10% range. A motor designed for 380V 50Hz is therefore looking for 380/50 = 7.6 V/Hz. If you use that transformer, you will be giving it 380V 60Hz, so the ratio will be 6.33 V/Hz, which is beyond the 10% range. The motor will have about 80% of it's design torque, 70% of it's peak torque (used for re-accelerating loads) and it will spin 20% faster. The slip will be higher, it will draw more current while doing less work. In short, it will burn up.
> 
> ...


Haha! No doubt, I'm just just the poor schmuck stuck with the task of getting it powered up. Here's the nameplate, they just sent it to me. This is a print inspection machine, it supposedly first of its class in the printing world. Tomorrow I will be face to face with the machine and report back before I proceed with ordering any transformer. I do know or has motors and an Arsenal of high powered cameras.


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## GEORGE D (Apr 2, 2009)

Sorry for double picture, trying to get used to Android phone


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## newbi (Dec 17, 2011)

Learned something new. I have never heard of the 10% V/HZ tolerance.


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## GEORGE D (Apr 2, 2009)

Ok actually is not the inspection machine but a printing machine. (Confused the two). Here is a pic of the control cabinet and a pic of the largest motor I can see. It appears the motor is controlled via vfd as indicated in cabinet,as well as another which I have no idea where the motor is. My guess would be that the large motor in picture is the main drive motor for machine, followed by the additional motor. The only other visible motors are for auxiliary things, like blowers, small pumps(pictured).The last 3-4 pics are of a transformer that shipped with another similar machine. I can't make much sense of nameplate but it appears to be a Y output. 

























































































The contactors in cabinet might be for heaters or something since they have no overload protection. Please help me whether or not the auto transformer I'm inquiring about will work. I am on site and can answer most questions about machine.


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## BlackHowling (Feb 27, 2013)

Looks like that VFD will give you the voltage that you want. But that equipment wouldn't fly up here. No CSA or CUL certification.


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## micromind (Aug 11, 2007)

If the transformer (8th pic from the top; the one with the gold nameplate) is part of the machine, it can be supplied with 480 3ø no neutral. 

You'll need to make sure the taps are set for 480, and since the transformer is 3 wire input and 4 wire output, it generates its own neutral.


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## GEORGE D (Apr 2, 2009)

micromind said:


> If the transformer (8th pic from the top; the one with the gold nameplate) is part of the machine, it can be supplied with 480 3ø no neutral. You'll need to make sure the taps are set for 480, and since the transformer is 3 wire input and 4 wire output, it generates its own neutral.


Right, my problem is I need a similar transformer for a different machine that's very similar to that one. That transformer is exactly what I need but can't find without resorting to China, which is why I'm trying to source locally.


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## oliquir (Jan 13, 2011)

BlackHowling said:


> Looks like that VFD will give you the voltage that you want. But that equipment wouldn't fly up here. No CSA or CUL certification.


he could make it inspected and get special inspection from CSA tag (cost over 2000$ though), i see that very often in cnc shop here. China machine cost half the price of other machine :blink:


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## oliquir (Jan 13, 2011)

GEORGE D said:


> Right, my problem is I need a similar transformer for a different machine that's very similar to that one. That transformer is exactly what I need but can't find without resorting to China, which is why I'm trying to source locally.


any transformer shop should be able to make this, they make custom transformer exactly to your voltage needed.
a 480 to 380v xfmr is not rare, alot of machines needs 380-400v input


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## BlackHowling (Feb 27, 2013)

oliquir said:


> he could make it inspected and get special inspection from CSA tag (cost over 2000$ though), i see that very often in cnc shop here. China machine cost half the price of other machine :blink:


 Yea we had to get quite a few motors/vfds/mccs done that way once.

"Our house... In the middle of the street"


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## 8V71 (Dec 23, 2011)

I hear land is tight over there....but daammnn! :blink: :laughing:


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## JRaef (Mar 23, 2009)

Looks like there are two VFDs in that cabinet. The VFDs get around all of the V/Hz issues with the motors, so you are good there. Lots of people make those transformers now, but maybe the problem is that you are looking for an *auto*transformer? Most will be isolation transformers. Just remove any reference to the TYPE of transformer you search for, you should be able to find plenty.

Look at Group J on the last page of this for example.
http://www.solahevidutysales.com/pdf/transformers/GenPurpose.pdf


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