Design Of Sensorless Control System For Permanent Magnet Brushless DC Motor

The advantages of Permanent magnet brushless DC motor (BLDCM) such as itssimple structure, easy to maintain, high efficiency and good speed-control performance,together with the emergence of high performance power semiconductor devices,microprocessor, logic circuit, and the continuous innovation of speed drive controlschemes contribute to its solid foundation in national production and the extensive lifeapplication. The control methods with position sensor are often used on permanent magnetBLDCM field, but due to the fact that under many conditions such as high temperature,high pressure, moisture and harsh environment, the position sensor will not work normally.Therefore, research on the permanent magnet BLDCM sensorless control is of greatsignificance.Firstly, this thesis analyzes five kinds of single tube PWM modulation mode.According to the fact that unexcited phase voltage and the motor neutral point potential arenot always the same which makes the current occurred in unexcited phase winding. Afterthe analysis, we can get the conclusion that only in pwm_on_pwm modulation mode canno diode freewheeling current occurred in unexcited phase winding.Then, the line back electromotive force (EMF) zero-crossing method is adopted inorder to realize the correct commutation for permanent magnet BLDCM. In other words,zero-crossing point of the line back EMF is the motor’s commutation point, it isunnecessary to make phase delay. Calculation formulas of three line back EMF amongthree-phase windings are simplified according to the pwm_on_pwm modulation mode,thus only a line voltage and a phase current detected signals are needed besides somenecessary motor parameters in every simplified formula which make it easy to finishcalculating.Furthermore, in order to make the commutation accurately, the specific commutationconditions are revised for the phase delay condition that some detected signals may beneed to be filtered due to the PWM high frequency noise. Besides, due to the disability of using line back EMF zero-crossing method to achievemotor commutation correctly when the permanent magnet BLDCM running at a standstillor in low speed state, the “three-step” starting method is adopted, and the synchronousspeed method and self synchronous switching conditions are given simultaneously whichmake the starting of the motor rapid and smooth.Finally, the permanent magnet BLDCM sensorless control system is designed whilethe DSP TMS320F2812and the PM50RLA120IPM are conducted as the core chips.Besides, an experiment about a5.5kW permanent magnet BLDCM is carried on to verifythe effectiveness of proposed method. The result both from the simulation and theexperiment presents that the control strategy adopted was effective, it can realize both thecorrect commutation and stable operation of permanent magnet BLDCM.