Drive implementation of a permanent magnet synchronous motor

Abstract

The North-West University has been focussing on the development and implementation of active magnetic bearings (AMBs) as well as the modelling and control of the AMBs. Recently the need for a high speed flywheel system arose and research has shown that permanent magnet synchronous machines (PMSMs) are an effective solution for high speed applications. The university decided to combine the AMB and PMSM technology to develop a high speed flywheel system with a magnetically suspended rotor. The aim of this project is to develop a 3-phase ac drive that will control the speed of the PMSM. The power amplifier consists of power electronics for the voltage generation as well as protection systems to ensure safe operation of the power amplifier. A control algorithm was developed to control the speed of the PMSM. A flyback converter will provide power for the small signal electronics. The design process includes the derivation of a mathematical model that describes the behaviour of the PMSM. From the model a control algorithm is designed that will ensure synchronization between the stator magnetic field and the magnetic field of the permanent magnets. The control algorithm is a constant V/f algorithm that controls the flux in the motor. From start-up to half speed constant torque control is implemented and from half speed to rated speed constant power control is implemented. The control algorithm is realised with a dSPACE® real-time development tool. The power amplifier is designed to operate from a 310 V dc supply. The amplifier delivers adequate power to the PMSM to enable the motor to achieve a speed of 30 000 rpm and deliver 2 kW of power with a maximum torque of 0.6 Nm. The switching devices of the power amplifier operate at a switching frequency of 50 kHz and can withstand 25 A current. The drive comprises various protection systems. The thermal protection ensures that the temperature of the heat sink does not increase above safe operating levels for the power electronic devices. The short circuit protection protects the switching devices from a short in the phases of the PMSM. An external enable let the user decide when the switching devices should be turned on and protects the devices from switching on together during system initialization. An ac filter is implemented between the output of the power amplifier and the input of the motor. The filter greatly reduces the current ripple and minimizes the effect of electromagnetic interference (EMI).

The simulated results showed good correlation to the experimental results. The power amplifier performed according to the design specifications and the control algorithm proved to be sufficient for the application. The project is concluded and any unforeseen phenomena are discussed. Recommendations are made based on the experimental results to improve the performance of the drive in the future. The knowledge acquired on PMSM drives will be useful for future development and will ensure technological advancement in the research group.