Effective frequency control for induction electric drives under overloading

Abstract

This paper considers features of mechanical characteristics of induction motor drives, as well as methods that increase their efficiency in a static regime under overloading. The induction motor vector diagrams are constructed by an L-shaped equivalent circuit taking into account and neglecting the stator resistance in order to describe the physical process of reducing the electromagnetic torque with increasing frequency of the supply voltage. Using vectors of rotor current codirected with the main magnetic flux of magnetizing current and the angle between them, it is shown how the shape of a torque triangle changes with increasing frequency of the supply voltage. Mathematical analysis of the elaborated formula of Kloss showed that the derivative of the electromagnetic torque of the induction motor has a negative sign in terms of the frequency of the supply voltage. Due to this, the conventional methods of correction under overloading, which increase the supply voltage frequency with the increasing stator currents, lead to even higher currents and reduction of the actual engine speed. Algorithms for improving the efficiency of frequency control of induction electric drives are proposed. Under a heavy load, it is required to increase the flux in the motor voltage by increasing the voltage, and if this is not possible, by reducing the frequency. The proposed algorithms are confirmed by experimental studies of the traction drive of a shaft self-propelled car.