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## Buck and Boost

I need to boost 208 vac to 240vac what KVA Transformer do I need? The fla is 17,

2. The manufacturers have online calculators that will size the units. I'm sure you can find it with a quick google. I know square d has one.

3. gar
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181911-1414 EDT

Boggsc71:

Quite obviously the quick answer is (240-208)*17/1000 = 0.544 . Other factors might modify this.

.

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Originally Posted by gar
181911-1414 EDT

Boggsc71:

Quite obviously the quick answer is (240-208)*17/1000 = 0.544 . Other factors might modify this.

.
Thanks for the quick response.

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Originally Posted by gar
181911-1414 EDT

Boggsc71:

Quite obviously the quick answer is (240-208)*17/1000 = 0.544 . Other factors might modify this.

.
I'm confused. Why are the two voltages subtracted from one another? I would think it would be 240*17/1000 = 4.08 kVA, assuming the 17 Amperes is carried on the 240V side.

6. Originally Posted by Carultch
I'm confused. Why are the two voltages subtracted from one another? I would think it would be 240*17/1000 = 4.08 kVA, assuming the 17 Amperes is carried on the 240V side.
With an autotransformer, you essentially only need the kva for the voltage change.

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Originally Posted by electrofelon
With an autotransformer, you essentially only need the kva for the voltage change.
That's not entirely accurate. The current through the common part of the winding is the difference between the input and output current but the current in the part of the winding that is used by only one side of the autotransformer is the full current for that side and needs to be sized for that current. In this case, the shared winding would carry 2.6A and the secondary only part of the winding would carry 17A. If the autotransformer was wound with a single size conductor it would have to be rated for 17A. Sometimes autotransformers are wound with two conductor sizes to take advantage of the reduced current in the shared winding.

8. Originally Posted by pv_n00b
That's not entirely accurate. The current through the common part of the winding is the difference between the input and output current but the current in the part of the winding that is used by only one side of the autotransformer is the full current for that side and needs to be sized for that current. In this case, the shared winding would carry 2.6A and the secondary only part of the winding would carry 17A. If the autotransformer was wound with a single size conductor it would have to be rated for 17A. Sometimes autotransformers are wound with two conductor sizes to take advantage of the reduced current in the shared winding.
So what do you think the kVA should be?

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Originally Posted by Carultch
I'm confused. Why are the two voltages subtracted from one another? I would think it would be 240*17/1000 = 4.08 kVA, assuming the 17 Amperes is carried on the 240V side.
Multiply the delta V by the current.

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Originally Posted by pv_n00b
That's not entirely accurate. The current through the common part of the winding is the difference between the input and output current but the current in the part of the winding that is used by only one side of the autotransformer is the full current for that side and needs to be sized for that current. In this case, the shared winding would carry 2.6A and the secondary only part of the winding would carry 17A. If the autotransformer was wound with a single size conductor it would have to be rated for 17A. Sometimes autotransformers are wound with two conductor sizes to take advantage of the reduced current in the shared winding.
Most commonly for buck and boost applications under 600V, the transformer used has separate primary and secondary coils. These coils are then connected in an autotransformer configuration.

The secondary coil(s) are 12 or 16 or 24V, and often there are 2 coils which can be in series or parallel. These transformers have 600V insulation on both the primary and secondary side. In this case the transformer secondary will be rated to carry the full current needed to give the rated kVA at the secondary voltage. The secondary coil(s) will be wound with different size wire than the primary.

The other common autotransformer is the variable autotransformer. In this case there is only a single coil, generally made with a single size wire. The kVA rating will be at full output voltage, and at low voltage output the available kVA is less than the rating.

-Jon

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