SSC JE Electrical Engineering 2017 Set 4 Paper

In DC series motor, field winding is in series with armature then armature current ${\mathit{I}}_{\mathit{a}}$ is proportional to field flux.
${\mathit{I}}_{\mathit{a}}\propto \mathit{\varphi }.$
Torque produce by motor.
$\mathit{T}\propto {\mathit{I}}_{\mathit{a}}\mathit{\varphi }.$
for DC series motor
$\mathit{T}\propto {{\mathit{I}}^2}_{\mathit{a}}\mathit{\varphi }.$
If ${\mathit{I}}_{\mathit{a}}$ reduce to by5%
${\mathit{T}}_1\propto {\left(05{\mathit{I}}_{\mathit{a}}\right)}^2$.
${\mathit{T}}_1=\mathit{T}\times .25$
Torque is reduce to 25%.