Brushless DC motor position sensorless control method
3. Back EMF third harmonic integration method
Since the back electromotive force of BLDCM is a typical trapezoidal wave, it contains the fundamental wave and its higher harmonic components. By simply superimposing the three-phase voltage of the armature, the third harmonic and its odd harmonic can be obtained. The 3rd harmonic component of the counter electromotive force is extracted and integrated, and the switching signal of the power device is obtained when the integral value is zero. There are two ways to obtain the 3rd harmonic signal of the back EMF: a neutral point of the star resistance of the motor and a star resistance connected in parallel with the three-phase winding end of the motor to obtain the 3rd harmonic component of the back electromotive force; The motor derived from the point can use the neutral point of the DC side and the neutral point of the star resistance network to obtain the 3rd harmonic component of the back electromotive force; then filter the obtained signal to filter out the higher order of the 3rd harmonic. The component, due to the lowest fundamental frequency of the higher order component, is 9 times lower for the filter. Therefore, it has a wider operating range than the direct zero-crossing of the counter electromotive force. This method avoids the interference caused by the inverter switch, but the amplitude of the 3rd harmonic is smaller than the amplitude of the back EMF, which is difficult to detect. Especially in the case of low speed, the 3rd harmonic signal is weaker, and it is difficult to obtain the rotor position. signal.
4. Freewheeling diode method
This method determines the commutation timing of the motor power tube by monitoring the conduction of the free commutating diodes connected in parallel across the inverter power tube. One of the BLDCM three-phase windings is always in the off state, so the switching sequence of the six power tubes can be obtained by monitoring the turn-on and turn-off conditions of the six freewheeling diodes. The method can improve the speed regulation range of the motor, in particular, the motor speed lower limit can be widened. However, this method requires that the inverter must work in the PWM chopper mode in the upper and lower power tubes, which increases the control difficulty; in addition, it is not easy to remove the mis-conduction signal caused by the invalid signal and the glitch interference of the freewheeling diode conduction. achieve. This method also has a large detection error, the back EMF coefficient, the winding inductance is not constant, and the back EMF waveform is not a standard trapezoidal wave, etc., which will cause the rotor position error. Since this method requires parallel detection circuitry on the diode, this is difficult to achieve for integrated power devices such as IPM. Because of these shortcomings, this method is not widely used in China, and the technology is not very mature.
