A 3-phase, 400 V, 4 pole, 50 Hz star-connected induction motor has the following parameters referred to stator:
\[
R'_1 = 1 \, \text{ohm}, \, X_s = X'_1 = 2 \, \text{ohm}
\]
Stator resistance, magnetizing reactance, and core loss of the motor are neglected. The motor is run with constant \( v/f \) control. The output frequency from the drive is closest to: