How many buck transformers for 3 ph?

D. Rey · Sep 15, 2008

I want to drop from 240-208V (3 ph), called temco today and talked to their tech says I need two buck transformers one leg goes straight through to the machine.

I have read threads here in the history that say you need three transformers one for each leg.

Know a guy that runs a shop his lathes have two buck transformers.

Which is it two or three ?

Dave

peterh5322 · Sep 15, 2008

You can get 208 line-to-line by using three autotransformers in buck mode, but 208 is a Y system, whereas 240 is a ∆ system, and by using autotransformers you will get a 208 ∆ system, not a 208 Y system.

If you need a 208 Y system, then you need a conventional 240 ∆ to 120/208 Y transformer.

In modern premises, it is more common to be served by 120/208 Y or 277/480 Y, and then to derive any 120/240 single-phase or 240 ∆ from the Y services.

Motors and their controls are usually OK with 208-240, and really don't care if the source is ∆ or Y.

Multi-family premises are often served by 120/208 Y, and the cooking, heating and washing equipment are special 208 versions.

Jraef · Sep 16, 2008

What that means is, if your machine is using 208V for the motors and 120V for controls, the 120V is usually derived by tapping off of one leg of 208V and Neutral. So if you use 2 transformers to get to 208V, you will not have that ability to get 120V. You could likely source your 120V from somewhere else, or use a control power transformer, but it will mean some investigation and modification.

If, on the other hand, all you need to do is supply the motor only (or some other 208V 3 phase only load), then the two transformer method will be fine. But as Peter said, before making the expenditure, look into the voltage tolerance of the machine because most of them are OK with 208V even if then say 220V or 230V.

D. Rey · Sep 16, 2008

Power is coming from a 20 hp phase perfect phase converter.

That is delta right?

So two buck transformers are needed or three?

The machine is a CNC lathe, has it's own 120 source in the control cabinet right?

Forgive me I do not know the difference between delta and Y, the machine has inputs for L1,2,3 and ground.

Does that help?

Dave

peterh5322 · Sep 16, 2008

PP accepts L1 and L2, which correspond to A and C, and produces B.

I would install two autotransformers, A to B and C to B, and get A' and C', respectively.

A' to B would be 208. C' to B would also be 208. A' to C' would be a tad different, but probably not too different to matter.

But, before I went to the expense of doing so, I would run your specific case by an application engineer at PP.

D. Rey · Sep 16, 2008

Hey that helps a lot thanks!

Dave

jim rozen · Sep 16, 2008

Read the manufacturer's instructions. In my case the answer was an unequivocal
"two."

Jim

peterh5322 · Sep 16, 2008

The reason I suggested contacting an application engineer at PP is it may be possible to employ only one transformer if the B phase closely tracks L1 and L2, as is very strongly suggested in the PP patent document.

The PP takes L1 and L2, which are passed through as A and C, and then converts it to a higher voltage dc source which is subsequently inverted back to an ac voltage source, but at such a voltage and phase relationship that a fully functional B phase is produced.

(A PP does not utilize the neutral).

If B tracks as closely as is described, then only one transformer is required.

The PP application engineer should be able to help you identify the specific transformer which will achieve your specific goal.

With transformation before the PP, using only one transformer, A, B and C would be identical, and balanced.

With transformation after the PP, using two transformers, A and C would be identical, but B would not.

D. Rey · Sep 16, 2008

Called PP today they say a common answer is to use two BB transformers should not be an issue at all.

On to another question.

I have two machines I run from this PP, Haas VF-2 and this nakamura lathe. The haas with it's 20 hp motor actually has a lower rated amp draw than my 10 hp nakamura lathe go figure.

So as you have written I could run one BB before my PP and change the taps on the haas to match the incoming 208. Any disadvantage to running the Haas at a lower voltage? It's been perfectly happy (with appropriate taps) at 240+V.

Matt100 · Sep 17, 2008

I have a simular situation. I wired one BB ahead of my 30hp Phase perfect and have balanced 208 going to the whole shop which has a mixture of 208/240 equipment both manual and cnc. This is how PP sugested I do it. I bought the BB thru them. If using 2 or 3 is prefered I'm sure they would have loved to sell me more.
Matt

D. Rey · Sep 17, 2008

Problem here after talking with PP is that I was told I can not get a big enough "buck" using one transformer before the PP.

I'm going from 240 to 208. Discussed with a tech today at the mfg and all was easily explained over the phone as far as what to connect to what.

How did you get away with one, do different mfgs have different abilities as far as how big a step they can take?

Mine are federal pacific brand.

I'll let you know how it goes.

Dave

Matt100 · Sep 17, 2008

I went with the BB recomended by Phase Perfect, which as I said I purchased direct from them. The brand Is a dongan model 85-Y055.

D. Rey · Sep 18, 2008

Alright up and running.

Wall, to phase perfect, to two Federal pac BBs, I get 208 dead nuts on all legs.

Woo Hoo.

I appreciate all of your help no doubt. Thank you.

Dave

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