Digital Implementation Issues on High Speed Permanent Magnet Machine FOC Drive Under Insufficient Sample Frequency

Abstract

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 over rotor fsample operating 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 digital controllers is compared specifically at low fsample /fe. It is shown that digital controllers using forward difference and bilinear approximation are able to maintain the drive stability if the voltage delay and inductance cross-coupling are compensated. However, the drive performance strongly depends on the accuracy of the inductance parameter. By contrast, the controller using direct digital design achieves the better FOC performance independent to machine parameters. Different types of controllers have experimentally compared to find a suited high-speed FOC drive. This paper includes the design guidance of high-speed FOC drive for PM machines with different time constants.

Keywords

• Machine drives
• digital controller
• discrete-time approximation
• field oriented control