SSC JE Electrical Engineering 2014 Set 2 Paper

∵ ${\mathit{E}}_{\mathit{b}}=\frac{\mathit{\varphi }\mathit{Z}\mathit{N}}{60.}\frac{\mathit{P}}{\mathit{A}}$ and $\mathit{V}={\mathit{E}}_{\mathit{b}}+{\mathit{I}}_{\mathit{a}}{\mathit{R}}_{\mathit{a}}$
∴ $\mathit{N}=\left(\frac{60\mathit{A}}{\mathit{P}\mathit{Z}}\right).\frac{{\mathit{E}}_{\mathit{b}}}{\mathit{\varphi }}$
= $\left(\frac{60\mathit{A}}{\mathit{P}\mathit{Z}}\right)\left(\frac{\mathit{V}-{\mathit{I}}_{\mathit{a}}{\mathit{R}}_{\mathit{a}}}{\mathit{\varphi }}\right)$
or $\mathit{N}\propto \frac{\mathit{V}-{\mathit{I}}_{\mathit{a}}{\mathit{R}}_{\mathit{a}}}{\mathit{\varphi }}$
Hence in order to increase the speed of dc shunt motor from rated speed 1050 r.p.m to 1200 r.p.m we have to reduce ϕ. Hence field resistance control scheme will be used.
∴ Option (c) is correct.