DC bus correction for inverter VFD via PWM modulation
Speed variation due to changes in the mechanical load and limited torque capability at low speed are the principle disadvantages of open-loop V/f control. Speed and torque capability variations due to mains voltage fluctuation will be less noticeable. If you want to improve the performance without abandoning the open loop control scheme, you might consider providing a manual adjustment for V/f to increase V/f slightly at low speed. Also consider a scheme for estimating operating torque and increasing the frequency slightly to compensate for motor slip increase with load torque increase. A scheme to maintain constant V/f with fluctuating mains voltage will improve performance, but will probably provide less improvement that the two schemes that I suggest.