Optimization of the current operating state of induction machines based on electrical quantities

Abstract:
This paper describes a method for optimizing the current operating state of induction machines based on directly measured electrical quantities. The objective is to maximize the efficiency of the motor at a given speed and torque, or to minimize the total losses in the machine, without directly measuring the mechanical quantities. The total losses are composed of winding, iron, ventilation and friction losses and can be divided into two main parts: the load-dependent and load-independent losses. From the motor data sheet, the information about the efficiency in part-load operations can be obtained. This allows to separate the total losses approximately into the load-dependent and the load-independent portions. Based on this information, the current operating state can be optimized for lowest power losses and best efficiency. In the rated operation of an induction machine, its efficiency corresponds to the rated value. However, if the torque is much lower than the rated torque, the efficiency of the machine also decreases. By lowering the stator voltage, the drop in efficiency can be counteracted. The model, proposed in this paper, is used to calculate the optimum stator voltage and the optimum stator frequency. These must be set in order to achieve the optimum operating point and thus the best efficiency at a given torque and speed.