Sensorless Control Strategy Of Brushless DC Motor

The position sensor in the brushless DC motor reduces the anti-interference of the system,increases the failure rate and limits the application of the brushless DC motor in the limited space.Therefore,the use of the positionless sensor operation mode has important engineering application value and significance.In the positionless sensor operation mode,pulse width modulation is generally used to adjust the voltage and current.However,due to the highfrequency switching noise in this modulation method,the collected terminal voltage contains a large number of high-frequency harmonics,which affects the motor commutation.Accuracy.This article first explains the basic structure and working principle of the brushless DC motor.The mathematical models of the back-EMF detection zero-crossing method and the line voltage difference detection zero-crossing method are established to verify that the line voltage difference has the same zero-crossing point as the back-EMF.Since the method of determining the rotor position by detecting the back-EMF zero crossing point in the traditional way needs to reconstruct the neutral point,high-frequency noise in the pulse width modulation mode will cause the virtual neutral point voltage to fluctuate,which is not conducive to voltage signal acquisition and reconstruction The neutral point requires increased resistance,which increases costs.Therefore,compared to the back-EMF detection method,the line voltage difference method can more clearly observe the commutation point,which is more advantageous for the commutation error detection.Aiming at the position sensorless brushless DC motor control system using pulse width modulation,the reasons for the precise commutation affected by the filtering process,freewheeling process and imbalance zero crossing are analyzed,and a compensation method is designed in a targeted manner.A position sensorless starting strategy based on the combination of Buck converter and pulse amplitude modulation is designed.This strategy separates the voltage regulation effect of the inverter.The inverter only needs to commutate,which reduces the failure rate.Based on the stator core magnetic saturation effect,the relationship between the equivalent inductance and the rotor position is analyzed,and the rotor position is determined in a certain sector by judging the magnitude of the DC bus current under the action of a short-time pulse.The combination of wide and narrow pulses is used to determine the rotor position while accelerating the rotor.Finally,the method of setting the speed value is used to switch to the self-synchronous operation process to achieve the rotor preset position,motor acceleration phase,and internal and external synchronous operation in three-stage starting Switching to achieve reliable start of the sensorless brushless DC motor.The simulation model of the position sensorless brushless DC motor control system under the pulse width modulation and pulse amplitude modulation modes was established,and the parameter information such as speed,phase current and line voltage were collected.Compare and analyze the phase current and line voltage under the two modulation modes,and verify that the line voltage and phase current harmonic content in the pulse amplitude modulation mode is lower,simplify the subsequent filtering and phase compensation,and analyze the pulse amplitude.Spikes appearing in the line voltage waveform under the value modulation method are analyzed.The experimental hardware platform and software system are set up.The phase current and line voltage experimental waveforms are measured under two modulation modes and different speeds,and the harmonic contrast analysis is performed.The experimental results prove the correctness of the theory.