SSC JE Electrical Engineering 2017 Set 3 Paper

Field flux of a DC motor can be controlled to achieve the speed is higher then rated speed.
In field flux controlled method, resistance ‘R’ is connected in series with field winding in DC shunt motor due to increase in resistance across field current ${\mathit{I}}_{\mathrm{f}}$ reduce.
If µ f due to ${\mathit{I}}_{\mathrm{f}}$ reduce then flux produce in control reduce
$\mathit{N}\propto \frac{1}{\mathit{\varphi }}$
At constant supply voltage at flux reduce then speed of motor increase.