Permanent magnet (PM) brushless DC (BLDC) motors are widely used in various applications demanding speed regulation and control. PMBLDC motors are a typical example with advantages of high efficiency, high power density and maintenance-free operation. However, in some applications with limited space, high temperature or severe vibrations, use of Hall-effect sensors for rotor position detection may encounter hazards or problems. For example, high temperature can affect the characteristics or cause failure of sensors while insufficient space would not be able to accommodate sensors. Other conditions such as severe vibration or in-water operation may involve a risk of sensor destruction. Therefore, sensorless control is considered as an appropriate technique for such applications.
Sensorless control can reduce not only the number of external wiring/connections between the motor and the driver but also the maintenance requirement caused by vibration. This will improve the system reliability and reduce cost.
There have been a lot of studies reporting sensorless control techniques for BLDC motors. Some of them are based on detection of zero crossing points (ZCPs) of back electromotive force (EMF) or terminal voltages. The ZCP techniques often require the voltage of the neutral point or virtual neutral point, which are used for comparison with terminal voltages for commutation. However, neutral or virtual neutral points often contain high-frequency noise and windings of common motors may have no lead-outs for neutral points.
To avoid this problem, some studies indirectly obtained back-EMF ZCPs with a series of signal subtractions and manipulation. However, a thirty electric degree phase shift is still required for correct commutation, and the entire circuitry is complex. Some others directly compared the filtered average terminal voltages to obtain the required signals for commutation. For the above methods, the inclusion of low pass filters (LPFs) may solve the noise problem due to pulse width modulation (PWM) switching but will limit the speed range. In addition, some literature discussed the speed range that can be achieved under heavy load conditions.
Ji Sang Guk, a researcher at the Faculty of Electrical Engineering, has proposed a sensorless technique with a wide speed range and high efficiency using an enhanced commutation method.
This technique uses a low-pass filter with two cut-off frequencies that can be regulated for various speed operations. The higher cut-off frequency is designed for high speed to reduce phase lag, and the lower one is used for start-up to reject the noise.
From the experiments, with loads applied, he proved that the developed technique is capable of increasing the speed beyond the rated speed. For the entire speed range, the proposed technique demonstrated high efficiency even at high torque, high speed conditions.
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