How to compensate for grid quality changes in three phase power
The following diagram shows the effect of induction motor operation of small to moderate changes in voltages. The load torque is assumed to be constant. Any change in starting and maximum torque can be interpreted as a torque variation over the entire torque vs. speed (T-S) curve. If the load torque is constant, the change in the T-S curve will result in a small change in operating speed. If the load torque increases with speed, the speed change will be smaller.
It will be very difficult if not impossible to determine if a change in motor current or power is due to a motor problem, a voltage change or a load change.

Image by Ed Cowern