PROTECTION OF SQUIRREL-CAGE INDUCTION-MOTOR UTILIZING INSTANTANEOUS POWER AND PHASE INFORMATION

A new method to protect a squirrel-cage induction motor, which checks its condition before starting and while running, is presented. The check before running is made by successively connecting the windings of the motor to an impulse generator and analyzing the resulting current and voltage. The running motor is checked by monitoring the ac-component of the power signals in all three phases, the sum of which gives information about the motor condition. The presented method is very sensitive to short circuits in the motor wiring, interturn short circuits, ground faults, and phase failure but is not sensitive to load changes and small deviations in the supply voltage.