In general the maximum economical transformation ratio is 5:1 or 2:5. Calculation of the frame size is also dependent on the transformation ratio, and for this reason no fixed guide can be given. The effect of the ratio can be seen in the following examples:

FRAME VA = AUTO VA × ((HIGH VOLTAGE - LOW VOLTAGE)/HIGH VOLTAGE))

Consider two 500 VA transformers:

(a) 250/200V 50 Hz

Frame VA = 500 × ((250 - 200)/250))

Frame VA = 100 VA

(b) 500/220V 50 Hz

Frame VA = 500 × ((500 - 220)/500))

Frame VA = 280 VA

 

AUTO WOUND FRAME SIZE CALCULATOR

System highest voltage               

System lowest voltage                  

Auto wound KVA                         

Frame size KVA                          

In a tapped auto-transformer the maximum VA must be calculated from the voltage and current of each tapping and the frame size calculated accordingly. If the tappings are required for simultaneous operation, the design becomes involved and each transformer requires individual consideration. The auto-transformer may be step up or step down.