3 Phase Transformer
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  1. #1
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    Default 3 Phase Transformer

    I have been looking for a used 3KVA 3 phase transformer to step up the output of my rotary phase converter to 440V, no luck so far. Used single phase 220V/440V machine transformers are easier to find, so I was thinking about getting 3 identical 1KVA machine transformers and wiring them up as a 3 phase transformer instead. I did find some 15KVA 3 phase transformers for sale, but they are 5 times bigger than I need. I don't know how bad the losses would be from using a transformer that is so oversized. Maybe the loss wouldn't even matter. I want to run 440V motors up to 3 hp for brief times. Thoughts?

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    For $568 is it worth the bother? But yes, you can use 3 single phase transformers. Did it many years ago when I was a field service engineer.

    DAYTON Three Phase Transformer,3kVA,480V, 44YV05, ASA 61 Gray 190735891483 | eBay

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    A three phase transformer has a certain balancing action from interaction between the phases, but three single transformers will work. I was just looking at the service to a shop with about ten each lathes and mills. For a reason that escapes me, the power company has three single phase transformers on a pole instead of one three phase.

    Bill

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    Quote Originally Posted by 9100 View Post
    A three phase transformer has a certain balancing action from interaction between the phases, but three single transformers will work. I was just looking at the service to a shop with about ten each lathes and mills. For a reason that escapes me, the power company has three single phase transformers on a pole instead of one three phase.

    Bill
    For two reasons probably. One, if there is significant unbalanced load, they can size the xfmrs accordingly, 2, cheaper to replace one trans than all three.

    Tom

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    Quote Originally Posted by markz528 View Post
    For $568 is it worth the bother? But yes, you can use 3 single phase transformers. Did it many years ago when I was a field service engineer.

    DAYTON Three Phase Transformer,3kVA,480V, 44YV05, ASA 61 Gray 190735891483 | eBay

    Over the years I have seen many 220/440 machine transformers dirt cheap, just wish I could find 3 identical ones right now. This is for my home hobby shop, so for me it is definitely worth the bother to avoid spending $500+.

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    You might want to check HGR Industrial Surplus, you can search for them on the web. They are in Euclid, Ohio not far from Cleveland. I've picked up quite a few transformers from them really cheap! They keep lowering the price as time goes by until they sell or they scrap them.

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    I know of a 3-phase 220/440 step up transformer for sale cheap, but it is 12KVA and I only need 2KVA, maybe 3KVA at the most. Would it still work ok or would there be huge losses or other problems?

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    Quote Originally Posted by bill1608 View Post
    I have been looking for a used 3KVA 3 phase transformer to step up the output of my rotary phase converter to 440V, no luck so far. Used single phase 220V/440V machine transformers are easier to find, so I was thinking about getting 3 identical 1KVA machine transformers and wiring them up as a 3 phase transformer instead. - I want to run 440V motors up to 3 hp for brief times. Thoughts?
    It can be done with two single phase units, 480/240 x 240/120, in a open delta configuration. The 480V output windings, output ungrounded open delta, as shown in the drawing. To ground the output winding's, to make it safe to use and have a fault return path, you would ground the center of one of the 480V output windings, at the center tap, at the junction of H2 &H3.

    Have a look at this post. Clausing drill press 480 to 240?

    This will not yield a WYE 277/480, but a high leg open delta, 240V to ground from two legs, and 415V on the wild leg.

    It's not useful for things like VFD's that want a symmetrical Wye 277/480V voltage to ground.

    But is perfectly, suitable for basic 3Φ motor loads.
    Small 1Φ transformers, less than 3KVA, usually have some compensation windings added to one side, to adjust to the proper output voltage. So you need to use units 3KVA or above to get the correct ratio, for the correct output voltage. Smaller units will throw your output voltage off, due to the compensation windings.

    Two 1Φ 3KVA units would yield you 5.16KVA output. And you still need to add the correct overcurrent protection on both sides of the transformer bank.

    If you needed a 277/480V output from Three 1Φ units, they would need a 277V HV coil, which is a lot rarer in the used market.

    Three 480V Primary coil units, can be wired in closed delta, which would then need to be corner grounded, or center tapped grounded, yielding the same high leg 415V. Both of these output connections are not suitable for most VFD units. Operating any transformer bank without grounding the 480V output winding, would be a violation of the NEC, and an unwise decision.

    SAF Ω

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    Quote Originally Posted by TDegenhart View Post
    For two reasons probably. One, if there is significant unbalanced load, they can size the xfmrs accordingly, 2, cheaper to replace one trans than all three.

    Tom
    All three look alike. The load is almost all three phase motors including six CNC machines with electronic drives. About the only unbalanced loads are the air conditioners which are 240 single phase. This is corner grounded, so to get a reading on the grounded leg I have to clip an ammeter around both of the other wires and read the sum. As near as I can tell with incomplete documentation, six air conditioners are divided between all three legs and they are only 15 amps, not enough to bother anything. This shop is the only load on the transformers. They have lost three CNC controllers recently, which really eats into profits. The power co has a monitor on it that shows some fairly large voltage swings. I have spent several hours staring at an oscilloscope and can tell when the larger machines turn on but I haven't seen anything that can damage a controller.

    Bill


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