Discussion of Field Weakening Operation of a High-speed PMSM for Flywheel Application

This paper discusses the benefits of field weakening (FW) operation of a permanent magnet synchronous machine (PMSM) for the application in a flywheel system, which has 24 000 min(exp -1) maximum speed and a power rating of 28 kW. The discussion focuses on the loss and efficiency performance compared to non-field weakening (NFW) operation. According to the calculation, FW-strategy can reduce the iron losses at high speed by applying a demagnetizing current. Despite the increased copper losses, the total losses are slightly lower than for NFW-operation. However, the benefit (less than 200 W) is small compared to the rated power. The second part of this paper presents measurements for a prototype flywheel system, applying a FW-strategy. Different loss groups are analyzed. High iron losses and PWM-based additional losses are observed at high speed. The measured efficiency with full power operation is above 94 % for the speed range 12 000...24 000 min(exp -1).