On-line copper loss minimization control method of induction and PM motors with periodic fluctuation load

This paper discusses the efficiency of permanent magnet motors (PM motors) and induction motors when periodic fluctuation occurs in the load. PM motors generate magnet torque by permanent magnet in the rotor. The magnet torque of PM motor is independent to d-axis current. However, if d-axis current of PM motor change rapidly in order to achieve high efficiency control, output torque also change rapidly without transient current because the reluctance torque is proportional to d-axis current. Thus, the torque response is quick according to electric time constant of the PM motor. On the other hand, induction motors generate output torque by the secondary flux. There is a relationship of first order lag between the secondary flux and the excitation current in the induction motors. Consequently, the secondary flux cannot be rapidly changed by the load torque. When the load torque varies in faster speed, the high efficiency control method increases the copper loss because the transient current flows in the excitation current. This paper clarifies the minimum copper loss control method for induction motors with torque fluctuation. When the load fluctuation is faster than the secondary flux of the induction motor, the high efficiency control method using the effective value of the load torque is used. The copper loss in the load fluctuation condition is analyzed in order to derive the boundary condition that copper loss is minimized. The validity of the boundary condition is confirmed by the experimental results using 3.7-kW induction motor.