ECN Forum Forums Technical Discussion Electrical Theory and Applications Buck Boost KVA rating

Can somebody explain how to calculate the required KVA rating of a buck boost transformer?

There is a good section in http://www.federalpacific.com/literature/drytrans/FPTS-RTB-0701.pdf

Depends on series/parallel primary/secondary connections, 1ø or 3ø, 2- 3- 4-wire, delta or wye source/load.




[This message has been edited by Bjarney (edited 01-19-2003).]

Re: Figuring KVA rating for Buck / Boost Transformers:

Basically, the capacity - in Amperes - of the Secondary side is what determines the overall KVA rating for these setups.

These Transformers are simply small / low KVA rated ISOLATED TRANSFORMERS with low voltage Secondary Winding(s),
being connected in a fashion which creates an AUTOTRANSFORMER type setup.

The Primary Winding(s) do not carry the full KVA that is achieved through the new connection scheme - only a small amount of Apparent Power (Volt-Amps, or VA) is drawn in the Primary as compared to the total Apparent Power going to a load.
The Secondary side is the crucial player in these designs!

Along with increasing (or decreasing) the output Voltage, the Secondary Winding(s) will be carrying the load current - so the Winding(s) need to have capacity to do this.

A simple example for figuring the Maximum Line KVA of a Boost (or Buck) Transformer arrangement is shown below, using a normal Dry-Type Isolated Transformer (Primary Windings not Electrically connected to Secondary Windings):

Transformer Rating:
120/240 VAC (Pri) × 12/24 VAC (Sec) - 1 KVA @ 150° C maximum

Figuring the Line (Input) Voltage is 120 VAC, the Primary Windings are connected in Parallel (for 120 VAC).

Figure the Current Rating ( I ) of the Secondary side:

VA = KVA × 1000 - convert the Kilo Volt-Amps figure to Volt-Amps,

I = VA ÷ E - Find Current ( I ) by dividing Apparent Power ( VA) by the Voltage ( E ).
KVA = Isec × Ein ÷ 1000

For a connection with 132 VAC output ( + 12 Volts ), the Secondaries are connected in Parallel.
The KVA Rating will be:

Isec = 83.34 Amps
120 VAC × 83.34 Amps = 10,000.8 VA - or simply 10 KVA
Output will be 10 KVA @ 132 VAC

For a connection with 144 VAC output ( + 24 Volts ), the Secondaries are connected in Series.
The KVA Rating will be:

Isec = 41.67 Amps
120 VAC × 41.67 Amps = 5,000.4 VA - or simply 5 KVA
Output will be 5 KVA @ 144 VAC

The same calculations work for Voltage Reducing ("Buck") connections also.

If you have further questions, feel free to ask away!!!
Also, feel free to contact me directly via E-mail if needed.

Hoping this data is helpful enough to answer your question!

Scott35 s.e.t.

beautiful

Thank you.

No problem!!!

I hope this stuff makes sense and is as helpful as possible to you and everyone else!

Scott35 S.E.T.

Wouldn't the 83.3A go through the 132V load for(11kVA)?

and the 41.7 A go through the 144V load for (6kVA)?

[This message has been edited by engy (edited 04-02-2003).]