High Speed Permanent Magnet Synchronous Machine Drive Under Insufficient Sample Frequency

This paper investigates digital implementation issues on high speed permanent magnet (PM) machine drives. The machine operating speed is designed beyond 30krpm where the ratio of controller sample frequency fsample over rotor electrical frequency fe is less than 5. Because microcontrollers are used in variable frequency drives to realize the field oriented control (FOC), the discretized effects must be considered to maintain the controller stability under low ratios of fsample/fe. In this paper, the FOC based on different current controllers are compared specifically at low fsample/fe. It is shown that current controllers using forward difference and bilinear approximation are able to maintain the drive stability if the voltage delay and indcutance cross-coupling are compensated. However, the drive performance strongly depends on the accuracy of inductance parameter. By contrast, the controller using direct digital design achieves the better FOC performance independent to machine specifications. In this paper, different types of controllers are experimentally compared to find a suited high speed FOC drive.