Mathematical Modeling of Combined Thermal Protection of a Deep-Groove Asynchronous Motor

Abstract

A mathematical model is developed for a deep-grove asynchronous motor with combined thermal protection. In the model measurement data on the instantaneous currents and voltages of the stator are used to determine the heating temperatures of the windings of the stator and rotor via two channels: in the first of these, by comparing the initial and instantaneous resistances of the direct sequence resistance of the stator and rotor and in the second, in terms of a thermal model based on the calculated instantaneous power losses in the stator and rotor. A contactless method of measuring the slip, based on including the frequency dependences of the resistances of the rotor owing to the skin effect is developed and used in the protection algorithm. To prevent overheating of the windings of an asynchronous motor when nonsynchronous regimes develop, in the protection algorithm the instantaneous values of the phase currents and voltages are used to determine the moduli of the voltages, currents, and resistances of the reverse sequence, which are compared with the permissible values. The workability of the proposed algorithm for combined thermal protection is confirmed by the results of a mathematical model of the direct startup regime and operation with different loads and power supply asymmetries for a 630 kW deep-groove asynchronous motor with a 6 kVstator voltage.