METHOD FOR PROTECTION OF AN ASYNCHRONOUS ELECTRIC MOTOR WITH A SQUIRREL-CAGE ROTOR AGAINST THERMAL OVERLOAD

摘要

A method for protection of an asynchronous electric motor with a squirrel cage rotor against thermal overload comprises preliminary definition at given initial temperature of machine cold condition, before putting asynchronous electric motor into operation: resistances of stator windings; a magnetizing branch in non-working stroke test and functional relation of a rotor resistance from sliding in a test supplying three-phase voltage with different frequencies to a stator when machine is fixed; in operation condition - continuous measurement of instantaneous magnitudes of phase currents and voltages, sliding, stator heating temperature; calculation of rotor active resistances for current sliding at current temperature, and for current sliding at temperature of cold condition, definition of rotor heating temperature based on comparison the above mentioned resistances. If stator or rotor heating temperature exceeds allowable values, the motor is disconnected from supply main. In dynamic conditions, it is additionally measured voltage which is proportional to derivatives of stator phase currents, with help of which from differential-integral equations of stator and rotor contours, derivatives of currents of magnetizing branch and rotor contour are determined, thereafter temperature of rotor heating is measured.