Double Wound Transformer Sizing
To identify a transformers’ size the first thing you need to determine is it’s output power rating, expressed as VA.
This can be done by applying the following formula :
VA = (output volts) x (output amps)
n.b. for three phase transformers this figure needs to be multiplied by 1.732
These transformers are used to save space and cost. They can be used where the voltage change is 50% or less. When the voltage difference between the input and output is small then the larger the reduction in cost and size.
Below is an example showing the difference between a double-wound transformer and the equivalent auto-transformer
Input 415V 3 phase / Output 380V at 18.5amps x 1.732 = 12176VA (nearest standard 3ph double-wound frame size is 12500VA)
Frame size for an auto-wound is ((415V-380V)/415V) x 12176VA = 1027VA frame size. (nearest standard frame size is a 1250VA
Therefore the size of a 12176VA double-wound transformer is 420 x 240 x 500mm (90kgs). The equivalent auto-wound transformer would be designed on a 1250VA frame size and would measure 180 x 175 x 240mm (12kgs).
This shows a considerable saving can be made but this is dependent on the input and output ratios.