Developing New Thermal Protection Method for AC Electric Motors

Monitoring the thermal state of windings of electrical machines is a backbone for protection from unacceptable overheating. A large number of different methods and systems aim to solve this problem. This article discusses the main known methods of thermal protection of electric motors and provides their comparative analysis. This paper shows that the most promising methods are those based on control of the current active resistance of the stator winding, as its value uniquely depends on temperature. It is demonstrated that the known methods have a number of disadvantages. A new phase method for thermal protection of AC motors is proposed. The method is based on the fact that a temperature-induced change in the active and reactive components of the winding impedance causes a corresponding change in the angle between the vectors of phase voltages and currents. This allows for thermal protection by controlling the change in this angle. This article provides tabular analytical substantiation of the proposed method, which is based on the direct measurements of voltage and current and the subsequent algorithmic calculation of physical values functionally related to the sought angle. The authors develop a structural block diagram of a device that implements the proposed thermal protection method. All relevant experimental studies were carried out. In this case, a small-sized electronic thermometer with a remote digital temperature sensor connected to the USB port of a personal computer was used as a temperature meter. The results obtained confirm the functional capability and efficiency of the proposed technical solution.