A three-phase slip-ring induction motor, provided with a commutator winding, is shown in the figure. The motor rotates in clockwise direction when the rotor windings are closed.
If the rotor winding is open circuited and the system is made to run at rotational speed fr with the help of prime-mover in anti-clockwise direction, then the frequency of voltage across slip rings is $f_1$ and frequency of voltage across commutator brushes is $f_2$. The values of $f_1$ and $f_2$ respectively are
- $f$ + $f_r$ and $f$
- $f$- $f_r$ and $f$
- $f$ - $f_r$ and $f$+$f_r$
- $f$ + $f_r$ and $f$-$f_r$