Buck and boost for load increased portion only

[dannyrzk]

Good afternoon, designing power for a 240v equipment with the following rating (240v/60amp/60hz - 2 wires, 1 grd) The manufacturer say to use a 2KVA buck and boost, which was strange considering the rated 60amp of the equipment. Here is their response:

The buck boost transformer is responsible for handling only the load increase from 208v to 236V, not the whole 60amps that the machine requires. 2Kva leaves a comfortable margin for boosting the power.

Can you please help me understand what he means by that? I understood that a buck and boost shall be rated for the load, not just an increased portion. Please let me know what you think about it.

Thanks

 

[LarryFine]

They are correct, the transformer is sized according to the load, but does not need to be able to accommodate it in its entirety.

Here's why, in simple terms: The transformer secondary is placed in series with the incoming power, and thus must be able to supply (236 - 208) 28 volts at 60 amps, which is (28 x 60) 1680 va.

1680 va at 208v is (1680 / 208) 8 amps, which is where the power to produce the voltage boost comes from.

Remember, though, that this power must be supplied to the transformer primary, so your circuit must be able to supply (240 x 60 = 14,400) (14,400 / 208) more like 70 amps at 208v.

 

[masterinbama]

The buck boost transformer is responsible for handling only the load increase from 208v to 236V, not the whole 60amps that the machine requires. 2Kva leaves a comfortable margin for boosting the power.



Thanks

The transformer only has to handle the difference in load between 208 volts and 240 volts. A 2 kva is plenty.

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[GoldDigger]

The quote was misleading in it's statement that the transformer did not need to supply the entire 60A the load requires. More understandable, perhaps, would be to state that it does not need to supply the entire 240V the load requires.
A completely correct statement is that it does not need to supply the entire 240V times 60A of power (or VA).

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[jim dungar]

Actually, the resultant autotransformer (i.e. buck - boost) needs to be able to carry the entire load. However, as has been pointed out, the isolation transformer which is connected into the proper winding configuration only needs to be sized for the difference.

Be aware that an isolation transformer, such as a 240v-24V is a 10:1 ratio. So, when connected to a 208V source you will only have 208+20.8 = 228.8V for a boosted output.
This is why I always make use of the sizing charts provided by all transformer manufacturers.

 

[LarryFine]

Be aware that an isolation transformer, such as a 240v-24V is a 10:1 ratio. So, when connected to a 208V source you will only have 208+20.8 = 228.8V for a boosted output.
This is why I always make use of the sizing charts provided by all transformer manufacturers.

They're also available with 16/32v secondaries; you can obtain 236v.

 

[topgone]

From your data:
Series winding rating = 240-208 = 32 volts
Common winding rating = 208 volts

Therefore, a 2 kVA autotransformer (208/240) will have the capacity:

= (32+208)/32 X 2 kVA = 15 kVA!​

Your 2 X 2 kVA transformer will then be able to deliver 30 kVA @ 240V. Bulk of the power delivered is conducted while a little portion is "transformed".

Please read some more on this LINK.

 

[dannyrzk]

thanks

thanks

I would like to thank everyone for your contribution, this has been my best forum dialogue by far!

Again Thank you all!