Transformer to Run 440V Woodworking Machine Motors

I have some small woodworking machines with 440V motors and 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 only need to run 440V motors up to 3 hp for brief times. Thoughts?

Sure, you can use three single phase transformers. A 3 phase transformer is just three single phase transformers in a single package.

That's what I'd do.

Mike

I could easily be wrong but I don't think it's possible. On 220 three phase with a rotary converter you have two legs of 110v and the third one is bumped up with capacitors. For 440 you would have to have 220v on both L1 & L2 before you could get to L3 The 440 is determined by putting a volt meter across any two legs. I don't think you could bump up 110v to 220v and have the amperage to make the motor work. It's kind of a stretch to make 220 three phase. 440 may be too far.

I have a single phase 240 volt panel, a rotary phase converter, to give me 240 three phase. then i have a transformer to 460 volt three phase and another one to get 600 volts three phase. Been running this setup for the past 12 years.

I have 6 KVA 460 - 240 transformer that i can sell you if you are interested pm me.

How large of a motor can you run off of a transformer? I've always been told that you loose amperage with a step up transformer.

I have a couple of 15KVA transformers. My phase converter will start up to 7.5 hp and run up to 22hp I have a couple of 7.5 hp motors on machines, and have no problem running them.

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Originally Posted by Edward Dyas

How large of a motor can you run off of a transformer? I've always been told that you loose amperage with a step up transformer.

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Quote:

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Originally Posted by Edward Dyas

How large of a motor can you run off of a transformer? I've always been told that you loose amperage with a step up transformer.

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Motors don't run on amps, they run on watts. Voltage x current x power factor = watts. Raise the voltage and lower the current proportionally and the watts stay the same (- the transformer losses of course)

You can also use two single phase transformers and wire them open delta rather than needing three. For small units, an autotransformer is probably the way to go and most single phase units will be delta to delta- open tranformer units. A 15 kva three phase unit will generally be an isolation transformer- delta LV to wye HV. Using them backwards ( step down used as a step up ) can cause grounding issues so it is best to read up on transformer use. A 15 kva isn't large enough to worry about losses or inefficiencies. The isolation tranformer may deliver the most balanced power but an autotransformer will be cheaper and lighter in this application. For larger needs I would go with an isolation step up tranformer. Dave

As David said you have to make sure you can ground the high voltage side if you are going to hook it up like that. your typical 480 to 240 transformer does not have a neutral point on the primary side so it should not be hooked up backwards.

I backfeed a couple big transformers. A 112.5kva and a 225kva.

Technically amps are halved when 240 to 480, you still have the same capacity.

It does suck when you have to dedicate a 400amp disconnect to get 200amps of 480v to basically run two, maybe 3 machines.

Can someone elaborate on the need for neutral with the 3 phase transformer? If you have a 120V circuit, the neutral and ground are separate and current flows through the neutral wire. They are only tied together at the breaker box. With 220V single phase there are 2 hot wires and a ground. With 3 phase, there are 3 hot wires and a ground. So where does the neutral come in when using a step up 3 phase transformer? Seems like if I go with 3 separate single phase transformers I will have the option to wire them in any way that is best. If I need to have a neutral wire on the high side of the transformer, what would I connect it too? Many of the 3 phase transformers I see have delta-wired primaries (high voltage) and wye-wired secondaries (low voltage).

Another thing I will mention is that the rotary phase converter I will have uses a delta-wound idler motor. It's controller uses SCR's to switch in various capacitors on the fly during operation to keep the voltages balanced. The manufacturer of the controller (Eurotech) says to use a delta-wound motor for the idler, so that is what I bought. All of the motors I will be running are wye-wound.

I know of a 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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Originally Posted by Bill Webster

I know of a 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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No. Just feed it with the correct sized breaker, or fused disconnect. You can feed it less amps to have less on the high side.

If you don't bond the neutral to a ground rod or building steel, no you cannot just tie it to the ground wire in your supply cable. You will have an isolated system with no voltage to ground. Everything will probably work just fine until you have a problem.
A transformer is considered a separately derived system according to the NEC and must be bonded on the secondary side somehow back to the ground at the main service.