Non-uniform notch structure for sinusoidal air gap flux density of brushless wound field synchronous motor

Abstract

In general, permanent magnet synchronous motors (PMSM) are used as motors for vehicles because of high torque density and power density. Among the PMSM, Interior permanent magnet synchronous motor IPMSM) has higher torque density per unit motor volume than induction motor and reluctance motor as this employs both magnetic and reluctance torques. In addition, wide operating speed range can be obtained with the help of field weakening control. For these reasons, IPMSM is applied as an application of a traction motor for hybrid electric vehicle (HEV) or electric vehicle (EV) [1]-[3]. However, The reserves of rare earth permanent magnets are limited, and most of the rare earth permanent magnets used are supplied from China. For this reason, the magnet price cannot be easily predicted and controlled. In addition, environmental problems in the process of mining permanent magnets of rare earth become an issue. In this regard, the motor that does not incorporate rare earth permanent magnets have been studied worldwide that can further increase the torque density and improve the efficiency. Induction motor, switched-reluctance motor, the ferrite permanent-magnet synchronous motor, and the wound field synchronous motor (WFSM) are some of the designs that do not incorporate rare earth permanent magnets. Among them, WFSM has the advantage of being able to satisfy both high torque at low speed and a wide operating range at the same time by increasing the control parameter field current. However WFSM requires the supply brush and slip-lings or a rotary transformer and rectifier. Such additional devices result in problems, such as increased maintenance costs, greater size, and heat issues. This paper deals with brushless wound field synchronous machine (BL-WFSM) without these additional devices for HEV. BL-WFSM with reduced additional equipment incurs additional air-gap, which reduces the torque density. In the case of a PMSM, rotor eddy current losses can be reduced by stacking the core. Since BL-WFSM apply both axial and radial magnetic flux paths, the solid core is applied. Therefore, there is a disadvantage that large eddy current loss occurs in the rotor. This was improved by applying a non-uniform notch structure. A non-uniform notch structure reflecting the influence of the notch was proposed. And the comparison studies are discussed between non-uniform notch structure and conventional model in terms of their air gap flux density, toque, and eddy current loss. BL-WFSM is designed to have compatible performance of 2kW Brushed WFSM for HEV application, and their size, characteristics, and efficiency over the main operation region are compared. Non-uniform notch (single step) is finally built and tested to assure validity of the proposed methods in the paper.