SSC JE Electrical Engineering 2015 Paper

Torque in DC series motor $\left(\mathit{T}\right)\propto \mathit{\varphi }{\mathit{I}}_{\mathit{a}}$
field is series with armature winding then ${\mathit{I}}_{\mathit{a}}\propto \mathit{\varphi }$
So Torque ∝ ${\mathit{I}}_{\mathit{a}}^2$ , if field saturate and flux become constant $\mathit{T}\propto {\mathit{I}}_{\mathit{a}}$
So, Torque ${\mathit{V}}_{\mathit{a}}{\mathit{I}}_{\mathit{a}}$ graph in DC series motor is pardoloa up to full load and straight line after saturation.