Rotor protection apparatus and method

Abstract

Apparatus and method for protecting a squirrel cage induction motor against excessive temperatures which can occur in the rotor, provides a thermal model of the rotor. The thermal model has three regions A, B and C representing the starting portion of the rotor conductor, and the rotor core. The rotor is frequency sensitive and currents of higher frequency induced in the conductors tend to be in region A, and currents of lower frequency tend to be in region B. Equipment connected to the stator winding determines values for the positive and negative sequence components of the supply for the fundamental frequency and preferably also for the second and third harmonics present, and provides signals representing these values. Heat calculation circuitry receives the values for all the positive and negative sequence components and a signal representing speed, determines the currents that would be induced in regions A, B and C, and derives equivalent currents which would produce the same heat. A summing circuit sums these equivalent currents for each of the regions A, B and C and applies the summed values to a first, second and third storage devices which stores the values representing temperatures. Interconnections between the three storage devices simulate the thermal resistances between the regions A, B and C and the thermal resistance to ambient. A temperature detector is connected to each storage device and when the value stored in the storage device reaches a predetermined level representing a critical temperature, the detector provides a trip signal which interrupts the supply of power to the motor.