Novel Control Strategies for High Speed Permanent Magnet Brushless Motor Drives

Permanent magnet (PM) brushless motors are eminently suitable for high speed drive systems due to high power density and high efficiency. This thesis presents a thorough investigation into control strategies for high speed PM brushless motor drives, considering both brushless AC (BLAC) operation and brushless DC (BLDC) operation.
In order to solve the current harmonic issues which are common to BLAC drives, the effect of inverter nonlinearity is firstly investigated. It is shown that the nonlinearity effect will generate the 6n±1th order phase current harmonics. Based on the analysis of current harmonics, an inverter nonlinearity compensation method is presented utilizing simple signal processing algorithms. The effect of non-sinusoidal back-electromotive force (EMF) on phase current is also analysed. Based on this analysis, an adaptive current harmonics suppression method that can compensate any periodic non-ideal factors is proposed. The feedforward self-tuning compensation voltages are generated without requiring any knowledge of back-EMF harmonic components, inverter parameters or motor parameters. With the proposed method, the current harmonic distortion of BLAC drives is reduced by 86%.
The position detection errors of BLDC drives are comprehensively investigated. The effects of both misaligned and uneven Hall signal errors are analysed, and a Hall signal balancing strategy is developed so that the peak current ripples can be suppressed by 80%. A new pulse-width modulation (PWM) generation scheme is also proposed to eliminate the PWM update delay of BLDC drives.
In addition, the cause of phase delay error and its effect on BLDC drives are analysed, while a compensation strategy based on the d-axis current is proposed. The optimal phase advance angle can be generated by reducing the d-axis current to zero so that the phase current is in phase with the corresponding back-EMF. Thus, the phase delay can be successfully eliminated, and the peak-to-peak current can be reduced by as much as 20% without the knowledge of the motor parameters or the need for any additional hardware.
Finally, wide angle twelve-step BLDC drives are investigated to reduce the current harmonics typically found in traditional six-step BLDC control strategies. A phase delay error compensation strategy based on the stator current is also proposed so that the continuous absolute position signal is no longer needed. By reducing the stator current to the minimum value, the phase delay error can still be compensated. Results show that when the phase delay error is compensated in the wide angle twelve-step BLDC drives, a quasi-sinusoidal current waveform can be generated.
All theoretical analyses are validated experimentally on a prototype high speed motor drive.