how to convert 480VAC 3 phase to 120VAC single phase?

Since you only need single phase, and you mention using only two legs of the 480V supply of a (presumably single) machine you can, using those two legs, use a single phase 480 to 120V control transformer and save a fair bit money compared to a three phase transformer. This is also common practice, if fact it is more common than using a three phase transformer to drop to 120 for control circuits.

If you want general purpose 120V , as compared to control power fro a single machine, a 480V to 208V three phase "Y" secondary transformer would be common, phase to neutral on a 208 Y service is 120V. A 240V three phase secondary is much less common as phase to neutral would not be 120V, one would need a center tap in one of the secondary winding, then you have a "wild" leg and it tends to be more hassle than it's worth.

3 Phase Step Down Transformer

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RE:

how to convert 480VAC 3 phase to 120VAC single phase?

I have to build a control circuit using two legs of a 480VAC 3 phase machine to get 120VAC single phase

any ideas?

In standard practice, 480 v 3 phase power is reduced to 240 v 3 phase using a 3 phase step down transformer, and one leg of the 3 phase transformer supplies the 120 v power.

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I am sorry, and mean no offense, as you have stated that you don't want to hear this, BUT... It would be foolish, and possibly hazardous, for anyone to try to explain this to you here. You "say" you "are one," but you also say you don't know about this, and that's the same as not being one where this project is concerned. You need to sacrifice a little pride and money, and hire a LICENSED MASTER Electrician to go to your job-site and literally POINT to, identify each of the phase legs, and explain which are to be used for your intended project. You don't have to hire him to do the work, only to advise and educate you. How about one of the guys you work with who does this work, he probably would do it for a few beers and laughs. There is a universal convention for which phase to use for what you want to do, and someone who knows what he/she is doing needs to precisely identify which phase line is which. The reason for this is that whoever originally wired the 3-phase may NOT have followed the convention, and misuse could result in problems. Better safe than sorry.

Use a single-phase control power transformer (CPT) rated 480V / 120V. Obtain it with both primary fusing (2 fuses) and secondary fusing (1 fuse) (or supply separate fuse blocks). Ground the unfused leg of the 120V. This is a very-typical setup used in combination motor starters. Many transformer manufacturers offer CPTs. Here's one example:

http://cmsapps.sea.siemens.com/controls/icc/06%20I...

Here are several, mostly without fusing:

http://www.alliedelec.com/search/searchresults.asp...

Google control power transformer.

i am trying to build a system that can product 4 outputs with each giving 48VDC 36 to 46amps to drive 4 motors. i have a 25HP engine with a PMG or PMA generator.

any one can help out here with ideas???

I have a SQD 9070T50D1 TRANSFORMER and have 480v 3ph supply would like to know haw to hook it up for 120v secondary

480v Transformer

I would do just that then, because tapping the correct two legs of the four legs required for 480VAC 3ph would be the common or neutral leg and one other leg: These two legs will provide a 120VAC 1ph supply current needed for your control circuit.