Sensorless Drive for High-Speed Brushless DC Motor Based on the Virtual Neutral Voltage

Page number 1 Sensorless Drive for High-Speed Brushless DC Motor Based on the Virtual Neutral Voltage Abstract Introduction: In recent years, high-speed brushless dc (BLDC) motor, which due to its high efficiency, compactness, low cost, and maintenance compared with a brush dc motor, is receiving more and more interest in industrial automation, especially on blowers and compressors. Since the rotation speed of a high-speed motor can reach up to tens of thousands revolutions per minute(r/min) due to the development of bearing technology, a proper drive method to ensure low loss and high efficiency in a wide speed range becomes a critical issue. Meanwhile, it is well known that the BLDC motor requires six discrete rotor positions for the inverter operation. These are typically generated by Hall sensors mounted on a motor. However, it is a well-known fact that these sensors increase the cost of the motor and need special mechanical arrangements to be mounted. To break through the aforementioned restriction, many position sensorless methods have been considered as potential solutions, thus prompting the sensorless operation of the BLDC motor to be a hot issue in Copyright 2015. All rights reserved

Page number 2 recent years. Therefore, to cope with these two issues effectively will promote the development of the high-speed BLDC motor. Existing system: Generally, the BLDC motor can be driven by either pulsewidth modulation (PWM) or pulse-amplitude modulation (PAM) techniques. It is well known that the inverter, which applied to most of the BLDC motor drive systems, is controlled by the PWM scheme for varying the voltage. When the PWM control is valid, the dc-link voltage is fixed, and the duty of the inverter is controlled by the speed and load conditions. Also, it is popular in the low-speed BLDC motor control. Unfortunately, for a high speed BLDC motor, the high-frequency and largerange current ripple will inevitably increase the copper and rotor iron losses. The PAM scheme is another popular control mode for the BLDC motor, especially at high speed. For PAM control, 120-degree commutation control, i.e., the so called six-step mode is generally used with lower switching frequency than the PWM control. Meanwhile, the dc-link voltage can be adjusted according to the error between the speed and its reference. Moreover, the conclusion that the PAM control for the BLDC motor can provide lower harmonic content and higher efficiency than the PWM control shows the superiority of PAM control at high speed Copyright 2015. All rights reserved

Page number 3 Proposed system: The front-end phase-controlled rectifier can provide an adjustable dc-link voltage, while the back-end three-phase inverter can achieve the commutation control. In particular, due to the very small phase inductance and resistance, a low initial dc voltage from the phase-controlled rectifier can be beneficial to limit the startup current. Three Y-connected resistors of same value are connected across the motor terminals to obtain the virtual neutral point S. In order to satisfy the commutation control of high-speed BLDC motor in a wide speed range and improve the motor efficiency, a hybrid drive method combining PWM and PAM techniques is proposed. When the motor runs at low speed, the PWM control is adopted with a fixed dc-link voltage that can be easily produced by the rectifier. When the motor speed reaches a threshold value, i.e., the enough back EMF can counteract the dc-link voltage, the PAM control works with an adjustable dclink voltage. However, it should be noted that for a high-speed motor with low phase inductance and resistance, the reason why the PAM control is not employed at low speed is the difficult generation of a low dc-link voltage from the rectifier to drive the motor. Copyright 2015. All rights reserved

Page number 4 Advantages: Simplicity of implementation, robust to electronic noise, and eliminating the need for the motor neutral wire. Applications: Motor drives Block diagram INPUT AC SUPPLY CONTROLLED RECTIFIER FILTER THREE PHASE INVERTER THREE PHASE LOAD 12V DC THREE PHASE DRIVER CIRCUIT Copyright 2015. All rights reserved 5V DC PIC CONTROLLER WITH BUFFER