If you over correct an induction motor when using staic correction, problems can certainly occur. Generally with static correction, the capacitor is controlled by the same contactor as the motor. i.e. the capacitor is effectively permanently connected accross the motor terminals. When the motor is over corrected, the effective resonant frequency of the motor inductance and the correction capacitance is less than the line frequency. During start and run, there is no problem, however when the motor is disconnected from the supply, it acts as a generator, generating voltage at the speed of it's rotation. Initially, (assuming that the motor is at full speed) the output voltage will be at the line frequency and as the motor slows, the frequency will drop. The actual voltage generated is dependant on the residual magnetism in the rotor. While slowing with ovewr correction, the voltage will pass through the resonant frequency of the motor / capacitor combination. At resonance, very high voltages will be generated and damage to the motor and capacitors can result. There will also commonly be very high torque transients that can cause major mechanical damage as well. Correction should only be applied up to 80% of the magnetising current of the motor.
Mark Empson
I always thought saturation would limit voltage in such a situation. How is it possible to operate an compensated asynchronous machine as generator if resonance between machine and capacitor can lead to destructive overvoltage?
