Transformer voltage increase

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jborkowski

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I need to convert a 200 Amp panel 120/208 single phase panel to 200 Amp 120/240 single phase..If this was 480 volt, I would use a 50 KVA transformer...Wouldn't I still need a 50 KVA transformer to convert the voltage that I need? 200 amps in/ 200 amps out....Buck boost transformer KVA ratings are not offered at such large KVA ratings....Any thoughts?
 
Re: Transformer voltage increase

Originally posted by jborkowski:
I need to convert a 200 Amp panel 120/208 single phase panel to 200 Amp 120/240 single phase..If this was 480 volt, I would use a 50 KVA transformer...Wouldn't I still need a 50 KVA transformer to convert the voltage that I need? 200 amps in/ 200 amps out....Buck boost transformer KVA ratings are not offered at such large KVA ratings....Any thoughts?
Click to expand...
Maybe you could use a couple of smaller units and tie them together as an auto transformer.
 
Re: Transformer voltage increase

208 volts times 200 amps equals 41.6 KVA. 240 volts times 200 amps equals 48 KVA. Yes, I think you would still need a 50 KVA. How do you know that you can't get one that size? Have you asked any vendors for suggestions?
 
Re: Transformer voltage increase

The rating of the boost transformer would only have to be 6.4kVA. 32 volts times 200 amps.
Don
 
Re: Transformer voltage increase

Originally posted by don_resqcapt19:
The rating of the boost transformer would only have to be 6.4kVA. 32 volts times 200 amps.
Don
Click to expand...
Don,
I don't see how 6.4kva would work. I agree with charlie's 41.6kva scenario or you could use two 25kva transformers tied as an auto tranformer. I'm looking at 200amp 208v steped up for 200amps, 240v out.
 
Re: Transformer voltage increase

You lost me on that one. I must not know how a boost transformer is wired. I had thought it would just be one primary winding and one secondary winding on the same core, with a turns ratio of 208:240. If that were the case, all of the power being transferred to the load would pass through the secondary winding, and the transformer would have to be rated for the 48 KVA that I calculated above.

If that is not the way it works, then how does it work? :confused:
 
Re: Transformer voltage increase

A buck-boost transformer has one winding with a tap. The current into the transformer, and out to the load basically cancel each other out in the common winding. The result is only the difference in voltage needs to be used to compute KVA.

Steve
 
Re: Transformer voltage increase

Don and Steve are correct. A buck/boost transformer is like an autotransformer with the secondary winding in series with the primary winding and based on polarity it either adds or subtracts the voltage.

It will look a bit funny, because you need wiring capable of carrying 200 amps although the voltage only increases by 32 volts.

Jim T
 
Re: Transformer voltage increase

Could there be a problem with the neutral connection on a 208V - 240/120 V buck-boost autotransformer? I can't figure out how to get the 120/240 with a grounded neutral for the 120V unless a "normal" two-winding 208V - 120/240V transformer is used.

IMHO - Any autotransformer connection that boosts the 208V to 240V will have 120V to ground/neutral on one leg and 148 volts on the other, assuming the 208V comes from a 120/208V three phase system. The boost transformer will add the 32 volts in phase with the 208V at one end of the winding, creating 145V to the original 120/208 neutral/ground.

(I was wrong last week, I am I wrong again?)
 
Re: Transformer voltage increase

It will look a bit funny, because you need wiring capable of carrying 200 amps although the voltage only increases by 32 volts.

Jim T [/QB][/QUOTE]

It did seem a little weird to me at first. But now I'm feeling all better.Don's calcs and transformer are correct.
 
Re: Transformer voltage increase

I think I agree with Rcwilson. You can get a single phase output, or a 3 phase output, but I don't see a 240/120V output with a balanced neutral. It seems like you could do one if the small winding was divided in two, and placed on each end of the main winding.

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Steve
 
Re: Transformer voltage increase

Your neutral would be 139 volts instead of 120 volts three phase or 153 volts single phase.

[ September 29, 2005, 08:56 PM: Message edited by: ronaldrc ]
 
Re: Transformer voltage increase

You guys are correct, you can't use the buck/boost transformer on a 3 phase 120/208 wye to get single phase 120/240.
Don
 
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