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How do I size and wire a Buck Boost transformer

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How do I size and wire a Buck Boost transformer
Buck Boost transformers can help convert power available to the needed voltage for machinery. Multiple versions of buck/boost transformers are available to convert voltages available to differing machine requirements.
Using an open delta boost or buck is a cost effective solution to changing 3 Phase power to the needed value.

"Boost" means adding to the supply voltage and "Buck" means to reduce the supply voltage. 

Transformers are commonly available for small boost and buck needs.  An isolation transformer can be used for bigger transformations.  

Example 1:
To boost 208 to 240 you need a 32 volt boost.  This is a common need so transformers are available for 208V primary and a 32 volt secondary.  Wired in open delta boost as in the diagram, you can get 240V out of 208V cost effectively.   
User-added image
L1 L2 L3 are the inbound 208v power
H1 H2 H3 are the outbound 32v power

The rating of the transormer is calculated by taking the "Boost" voltage (I.E. 32v) and multiplying by the current of the load and the safety factor.

EXAMPLE: 

The largest motor on the machine = 7.5 HP with a Full Load Amps of 21 amps at 240v.
This would size the transformer at 32 volts(Boost Voltage) x 21 amps(FLA of Motor) x 1.5(Safety Factor) = 1008 V.A.
Thus, we will use a 1 KVA Buck/Boost transformer.                    



Example 2:
To run a 480V machine on a 600V feed, we can use a 480 to 120 V isolation transformer.  The 120V winding is wired in series with the 480V winding and we create a buck transformation.  

Sizing the buck\boost transformer is generally done using the current across the buck or boost winding at the current draw of the system, times a safety factor of 150% 

This auto transformer wiring solution is very cost effective and only requires 2 transformers.  The alternative is an isolation transformer that would cost up to 8 times more.  
 
 
 

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