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

Auto-Transformer Sizing

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.