A $6$ pole $3$ phase wound-rotor induction machine is driven by another machine at $180 \; \mathrm{rpm}$. The rotor of the induction machine is connected to a $50 \mathrm{~Hz}$ system. If the mechanical rotation of the rotor is in the same direction as the rotor winding fluc rotation, then the frequency of the stator voltage will be
- $50 \mathrm{~Hz}$
- $140 \mathrm{~Hz}$
- $150 \mathrm{~Hz}$
- $200 \mathrm{~Hz}$.