If an attempt is made to reduce the supply frequency at the
rated supply voltage, the air gap flux ψm will tend to saturate,
causing excessive stator current. Therefore, the region below
the rated frequency should be accompanied by the
proportional reduction of stator voltage so as to maintain the
air gap flux constant. Fig. 1 shows the plot of torque-speed
curves at Volt / Hz = constant. It is clear from Fig. 1 that the
starting torque at the minimum frequency is much less than
the breakdown torque at higher frequencies, and this could be
a problem for loads which require a high starting torque.