Autotransformer up-rating

[hirsch56]

I need a 120/240V -> 12/24V Xformer to serve as a boost autotransformer giving 230V, 20A output. My question is: Can I get by with the .75kVA Xformer in hand? In a normal primary/secondary application one would need ~5kVA, but when wired as an autotransformer, don't kVAs get up-rated by a factor of 10? Is that correct?

 

[jim dungar]

I need a 120/240V -> 12/24V Xformer to serve as a boost autotransformer giving 230V, 20A output. My question is: Can I get by with the .75kVA Xformer in hand? In a normal primary/secondary application one would need ~5kVA, but when wired as an autotransformer, don't kVAs get up-rated by a factor of 10? Is that correct?

Actually you only need a 250VA unit.
The final ratio of the autotransformer determines the "factor".

I use the on-line buck-boost calculator from Square D.
http://www.squared.com/us/squared/corporate_info.nsf/unid/C16C1C0106A0374985256A5400653380/$file/softwareFrameset.htm

 

[hirsch56]

I tried the Square-D on-line calculator, but it gives me a 4.7kVA answer. How do you come up with 250VA? Input will be 208V and output 230V.

 

[hirsch56]

OK, I see that the calculator is saying there's a 4.7kVA load, but the Xformer it is recommending is a 0.5kVA model. That's what I thought, that it can be uprated by a factor of 10. You're saying that the factor of 10 comes from the fact that it is a divide by 10 Xformer (240 -> 24). Likewise, if the Xformer was 240 -> 36V, that's a factor of 6.7, so a 4.7kVA load could run from a 0.705 kVA unit. I get it!! Thank you Jim!

 

[steve66]

I think the math comes from the difference in voltage times the current.

Since you are stepping up 10 volts, its 10V * 20A = 200VA. If you were going from 208V to 240V, it would be 32V* 20A = 640VA. (At least this works for one configuration of a autotransformer, since the current cancels out in the part of the coil with the common voltages.)

Why do you need to change from 240V to 230V?? Motors and equipment with a 230V rating on them are normally connected to a 240V supply.

Steve

 

[hirsch56]

OK, Steve. You're saying to multiply the step voltage by the output current. That makes more sense.

I am changing 208V, 1ph. to 230V/20A to run a helium cold-head. In the ideal case, I can turn 208 into 229 with a 240/24V Xformer. A 21V step times 20A needs 420VA. The Xformer I have on hand is a 240/32V, 750VA. Instead of 229V it will give me 236V. I'll ask the OEM if 236V is close enough, so I might avoid buying a different Xformer.

Thanks to all. This forum is very helpful. -Hirsch

 

[LarryFine]

I'd say the 236v is ideal, since 230v motors are actually designed to run on 240v systems.