Question

A dc shunt motor develops a torque of 20 N-m when the armature current is 40 A. If it is changed as a cumulative compound motor by adding series field, which increases 20% of flux, and the armature current is 60 A, then the developed torque is

• A. None of the above
• B. 18 N-m
• C. 36 N-m
• D. 24 N-m

Answer 1

The correct option is C 36 N-m

$\text{We know that,}$
$\text{Torque,}T\propto \varphi I_a$

$\text{So,}\frac{T_1}{T_2}=\frac{{\varphi }_1{I}_{a1}}{{\varphi }_2{I}_{a2}}$

$T_2=\frac{{\varphi }_2{I}_{a2}}{{\varphi }_1{I}_{a1}}\times T_1$

${\varphi }_2=1.2{\varphi }_1,I_{a1}=40A,I_{a2}=60A$

$T_2=1.2\times \frac{60}{40}\times 20=36N-m$