Question

A 100 VA, 120/12 V transformer is to be connected so as to form a step up auto transformer. If a primary voltage of 120 V is applied to the transformer, then its maximum volt ampere rating in this mode of operation is

• A. 200 VA
• B. 1200 VA
• C. 100 VA
• D. 1100 VA

Answer 1

The correct option is D 1100 VA



$V_H=132V$
The maximum volt-ampere rating in either winding of this transformer is 100 VA.
$\text{The voltage on series winding is 12 V.}$
$I_{series,max}=\frac{S_{max}}{V_{series}}=\frac{100}{12}=8.33$ A

$\text{Since}{I}_{series}\text{is equal to}{I}_H\text{, so secondary apparent power is}$

$S_{out}=V_s{I}_s=V_H{I}_H=132\times 8.33$
$=1100VA$