The Design And Research Of Brushless DC Motor Direct Torque Control System

Nowadays, global energy consumption is mainly comes from all kinds of motorsin industrial field. Permanent magnet brushless dc motor (BLDC) has higherefficiency and power density, in addition to the other significant advantages like thelow cost, the small volume, the long life, easy maintenance, and stronganti-interference, which receives widespread attention and has become thedeveloping direction of the motor market. Because of the square-wave drive and theexistence of inductance during the commutation process, the current of the brushlessdc motor can’t mutate. Torque ripple is produced, which limits the brushless dc motorto the applications which have higher requirements of the dynamic characteristic andstatic characteristic. how to inhibit the torque ripple caused by commutation has beenthe research hotspot and difficulty in the brushless dc motor field.In this paper, the mathematical model of brushless dc motor are analyzed in detail.On the basis of the existing models, the mathematical relationship between the realthe brushless motor neutral point and the construction of the midpoint is derived.Under the action of brushless dc motor PWM modulation, the experiment of terminalvoltage waveform formation reasons are analyzed, and also the PWM modulationfunction influences the neutral point voltage of motor, in contrast to the neutral pointof permanent magnet synchronous motor, the neutral point voltage of brushless dcmotor can form complex waveforms with square wave and trapezoidal wave.In this paper, during the process of brushless motor commutation state, thereasons for the formation of commutation torque ripple and the mathematicalrelationship is analyzed in detail. Due to the effect of inductance, the phase currentflow in the process of commutation of rise and decline rate is not equal, which canproduce commutation torque ripple. According to the torque ripple problem, bymeans of the method of direct torque control, dynamic torque can be easily controlled.The simulation waveform is obtained by the simulation modeling in simulink, thereasons for the formation of circular-saw magnetic chain are analyzed, the paper putforward the influence of the different hysteresis width on the design of the system, bycomparison with Simulink double closed loop speed control method, the results of direct torque control method reduces the torque ripple and accelerate the motor torquespeed of dynamic response.Under the condition of system requirements analysis, brushless dc motor controlsystem platform is established. The system main circuit like rectifier and invertercircuit and the three layers of structure of the brushless motor controller are set up.Under the condition of the hardware platform, the controller can realize the voltageand current sampling, the generation of PWM, communication, the over-voltage andover-current protection, external interruption, and the capture of the position signal,etc. On the basis of the hardware experimental platform, the use of C language in CCScompile environment is completed with the program of the system software, including,the program of the motor start, the program of PI adjustment, the program of ADsampling and filtering program, the program of commutation interrupt program andsystem protection interrupt, etc. Through the adjustment of pid parameters, speed canbe smoothly to achieve the speed set point, rapidity and stability is well achieved.Finally, by means of building experiment platform, brushless motor measuredwaveform is given the in actual operation, and the experiment results are analyzed.The experimental results verified the DTC in brushless dc motor control system ofthe correctness and effectiveness.