# Autotransformer



## Jlarson (Jun 28, 2009)

Just get a regular 600-480Y/277 step down xformer.


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## GEORGE D (Apr 2, 2009)

He's already set on an auto transformer. Any clue how to size those?


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## JRaef (Mar 23, 2009)

As to the current, it's just going to vary by the voltage change ratio. 600/480V = 1.25, so your 60A current capacity on 600V side will translate to 60 * 1.25 = 75A max. on the 480V side (minus a small amount of losses in the transformers).

Normally, I'd tell you "Don't look for "autotransformers", look for "buck-boost transformers". In your case, you are going to be "bucking" from 600V down to 480V and unfortunately B-B Xfmr sites aren't going to list those ratios for you.

So because that is actually a nicely convenient number, there is another trick you can do, buy a motor starting autotransformer. They typically come with 3 sets of taps, 50%, 65% and 80%. a 1.25 ratio is the inverse of 80%. So you would feed your 600V into it, and tap your 480V off of the 80% taps and you are done. There will be two problems in doing this though; 

1) The motor starting autotransformer will be sized for a motor HP, not a current load per se, so you would have to translate it. However at 600V, HP and Amps are pretty close to 1:1, so a 60HP would be 60A. But see below.

2) Typical motor starting autotransformers are NOT designed for continuous duty, they are only expected to be on-line for 15-30 seconds or so. You would have to over size it, but only the transformer mfr would be able to do that for you.

Also, they come in 2 coil or 3 coil versions, the 2 coil versions give you an unbalanced output, which is fine for starting a motor once in a while, but that's different than what you are doing, so get the 3 coil version. Get it with the thermal switches in each coil also.

Look at page 155 of this catalog, but call them and talk about sizing for continuous duty. By the time you are done, it may just be simpler and cheaper to get a standard 75kVA 600-480Y277V distribution transformer. Sure, it's not necessary, but commercial off-the-shelf transformers tend to be less expensive than custom stuff.


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## GEORGE D (Apr 2, 2009)

Thanks J, as always, VERY helpful! Let me throw this at you, surely you'll have a great answer, the purpose of this transformer is for him to bench test motors, using a variac. I'm not familiar at all in variacs, does this seem like the right direction?


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## JRaef (Mar 23, 2009)

Different animal...
A "Variac" is a VARIABLE autotransformer, basically a giant rheostat. He wants that so that he can "dial in" a specific voltage going to a motor, whatever the motor needs.

Variac is a brand name of variable autotransformer, made by Staco Energy. There are other mfrs as well, that's just the "Kleenex" name everyone uses.

For doing test stands however, he probably really wants what they call a PPS, Provider Power System. It's a complete packaged variac based system with safety controls and a motor operator on the variac so you don't have to expose yourself to change it. You could theoretically buy the variac itself and rig up the test stand using it on your own, this just does all that for you and you just install it.


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## GEORGE D (Apr 2, 2009)

Awesome, I'll forward all mentioned and let him decide. Thanks again big guy!


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## Handasee (Dec 18, 2009)

I have used quite a few autotransformers to hook up 480 volt US machines. Autotransformers are used for motor loads, industrial machinery and inductive loads such as electric heating. I will never use an autotransformer for any machinery that has electronic controls (CNC etc).
All transformer manufacturers in Canada have a section devoted to autotransformers and sizing calculators.
Here is one web site with a calculator on the right side toolbar.
http://www.canadatransformers.com/75-kva-autotransformer-rc75j-h.html

Deep inside all the specifications you will also find that the manufacturers suggest you increase the size by 20% if the motor is started more than once per hour.


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