NOVEL SENSORLESS CONTROL OF PERMANENT MAGNET BRUSHLESS MACHINES FOR HIGH SPEED APPLICATION

This thesis is focused on sensorless brushless DC (BLDC) and brushless AC (BLAC) drives, particularly for high-speed application.
For BLDC drives, zero-crossing detection (ZCD) of back EMF is the most popular solution for sensorless control. In order to further improve its performance and robustness in the high-speed range, a comprehensive investigation is carried out. Firstly, the oversampling technique is employed to detect the zero-crossing points (ZCPs) of back EMF, which can considerably reduce the sampling delay when the sampling ratio is insufficient. Secondly, two non-ideal factors that will deviate the ZCPs, i.e. asymmetric machine parameters and the resistance tolerance of back EMF measurement circuit, are studied. The corresponding commutation correction algorithms are developed to eliminate the commutation errors caused by these non-ideal factors. Thirdly, the ZCP may be undetectable due to a long freewheeling angle, which is a critical burden of the ZCD based sensorless method operating towards high-speed. Consequently, a PWM pattern resulting in a minimum freewheeling angle is identified and a theoretical technique is developed to predict the maximum torque and speed region for sensorless control.
In the high-speed range, the PWM switching ratio is usually insufficient. It is found that conventional commutation patterns, i.e. regular-sampled commutation (RSC) and natural-sampled commutation (NSC), may lead to commutation delay, unstable sensorless control and abundant sideband harmonics. To suppress these adverse influences, a novel commutation pattern, i.e. carrier-synchronized commutation (CSC), is proposed, which can considerably improve the control performance when the switching ratio is insufficient.
For high-speed BLAC drives, the main challenge is the design of a decoupling current controller and a rotor position observer. To tackle this challenge, a generic discrete-time BLAC model considering the fractional period sampling delay is developed. Based on this model, a new decoupling current controller and a new rotor position observer are designed directly in discrete-time domain, which can guarantee good control performance in the low sampling ratio condition.