Low voltage can lead to overheating
, shortened life, reduced starting ability, and reduced pull-up and pullout torque. The starting torque, pull-up torque, and pullout torque of induction motors all change, based on the applied voltage squared. Thus, a 10% reduction from nameplate voltage (100% to 90%, 230V to 207V) would reduce the starting torque, pull-up torque, and pullout torque by a factor of .92.9. The resulting values would be 81% of the full voltage values.
In this case, it is easy to see why it would be difficult to start “hard-to-start” loads, like you’re a/C compressor, when the voltage happens to be low.
When you subject a motor
to voltages below the nameplate rating, some of the motor's characteristics will change slightly and others will change dramatically. To drive a fixed mechanical load connected to the shaft, a motor
must draw a fixed amount of power from the line. The amount of power the motor draws has a rough correlation to the voltage 2current (amps). Thus, when voltage gets low, the current
must increase (proportional with the decrease in voltage) to provide the same amount of power. An increase in current
is a danger
to the motor only if that current exceeds the motor's nameplate current rating. When amps go above the nameplate rating, heat begins to build up in the motor. Without a timely correction, this heat will damage the motor. The more heat and the longer the exposure to it, the more damage to the motor.