A Novel Drive System For Brushless DC Motor With Significant Reduced Torque Ripple

In the nearly three decades, the permanent magnetic brushless dc motor(BLDCM) isincreasingly being used for various applications such as household appliances, industrialautomation, office automation, electric automobile, aero-space and medical appliance and soon. This is mainly due to its compact structure, high power density, high efficiency, high starttorque, high control performance, flexible control scheme, lower maintenance cost, and longlifetime. Although the normal control technology for BLDCM has been so mature at homeand abroad, there still are many problems such as electromagnetic torque ripple suppression,position sensorless control and high-performance integrated controller technology and so onwhich are worth being studied and improved. The high-performance speed or positioncontrol can not be achieved, BLDCM is so limited in the applications where requires highprecision and high requirements. So the main purpose of this dissertation is minimizing thetorque ripple.In actual practice, many BLDCM have a nonideal trapezoidal back-EMF, some even arepseudo-sinusoidal waveform. It will produce grisliness torque ripple for BLDCM when120°conduction mode is used, even it’s drived by ideal square-wave current. Consideringthis situation, in this paper, for the BLDCM which has a general crest width of theback-EMF waveform, two kinds of torque ripple condition when drived by sine-wavecurrent as well as by square-wave current are analyzed detailedly and compared, and themodeling of BLDCM drive system along with control system for speed and current has beenpresented using MATLAB/SIMULIK. All kinds of existing control strategies are studied andcompared. It indicates that sinusoidal current driving is more favorable to minimizing thetorque ripple when the crest width of the back-EMF waveform is less than a certain value.In order to achieve sinusoidal current control, current sensor and a high resolutionposition sensor such as rotary encoder or resolver is usually required to obtain exact motorcurrent and rotor position information, the conventional PMSM Vector Control system is aexample. Such sensors cause disadvantages, such as system cost, size, increased rotor inertiaand so on. Therefore, the idea of sensorless contol of PMSM/BLDCM is proposed and developed. However, these sensorless algorithms still exist many problems, they are verydifficult to control BLDCM/PMSM from near-zero to full speed and obtain exact rotorposition information. To soulve this problem, a low cost current-sensorless vector controldrive for BLDCM which has a pseudo-sinusoidal back-EMF waveform with onlyLow-Resolution position sensor is proposed. The position estimation algorithm is based onaverage rotor speed. A sampling resistor, instead of a current sensor, is placed at the DC bus.According to the relationship between DC-bus current and phase currents of SVPWMcontrolled three-phase inverter, the phase currents of the motor can be calculated in software.Therefore, the system cost is lower, the performance is better.Meanwhile, experimental platform of BLDCM is built by using6IRFP450(MOSFET)as three-phase voltage type DC/AC inverter bridge main circuit switching device, and usingTMS320F2812DSP as digital control core. The design of software and hardware is showed.Experimental result proves the correctness of theoretical analysis and the effectiveness of theproposed drive system.