Application of capacitors for power-factor improvement of induction motors

POWER-FACTOR improvement of induction-motor loads by means of shunt capacitors has been a common practice for many years. Recently, the practice of connecting the capacitors directly at the motor terminals has developed in order to permit switching of the capacitors and motor as a unit. This will not result in adverse motor operation if the number of capacitors is selected properly. There is very little written material regarding operating experience, although capacitor and motor manufacturers have accumulated some experience.^1,2,3,4 This experience has shown that frequently too large a capacitor rating was used, resulting in overvoltage due to self-excitation when the motor starter was opened; in some cases the voltage rose to 200 per cent causing failure of motor insulation, indicating lamps, and other devices. In several instances, the motor coupling and shaft were damaged when too large a capacitor bank was used and the motor switched by a reduced-voltage autotransformer-type starter. In some cases, motors have been damaged from overload because allowance was not made in the overload setting for the reduction in line current due to the capacitors. In other cases, instruments and meters connected in the current-transformer secondary circuit have been damaged due to overvoltage.⁶