Question

For a D.C shunt machine given stray loss = 150 W, full load Copper loss = 400 W, Brush loss = 50 W, field loss = 200 W. To get the maximum efficiency at full load the iron loss is

• A. 60 W
• B. 50 W
• C. 40 W
• D. 70 W

Answer 1

The correct option is B 50 W

$\text{For maximum efficiency,}$
$\text{Constant loss = losses proportional to square of variable Cu loss =}{I}^2\text{R}$
$\text{Brush loss}\propto \text{I (so it is not included in constant losses)}$
$\text{So, Constant loss = 150 + 200 +}{P}_i$
$150+200+P_i=400$
$P_i=50W$