SSC JE Electrical Engineering 2012 Paper

Forward field slip speed
${\mathit{S}}_{\mathit{F}}=\frac{\left({\mathit{N}}_{\mathit{S}}-{\mathit{N}}_{\mathit{r}}\right)}{{\mathit{N}}_{\mathit{S}}}$
Syncronous speed ${\mathit{N}}_{\mathit{S}}=\frac{120\mathit{f}}{\mathit{p}}$
$120\times \frac{50}{4}=1500$
$\frac{\left(1500-1300\right)}{1500}=0.133\mathit{H}\mathit{z}$
Rotor frequency due to forward field
$\mathit{f}=0.133\times 50=6.67\mathit{H}\mathit{z}$
Backward slip of an induction motor
${\mathit{S}}_{\mathit{p}}=\left(2-5\mathit{f}\right)=2-0.133$ $=1.87\mathit{H}\mathit{z}$
Rotor frequency due to backwork field
${\mathit{S}}_{\mathit{b}}\times \mathit{f}$ = 1.87 × 50 = 93.35 Hz