This paper presents the method of reducing torque ripple of brushless direct current (BLDC) motor. In the BLDC motor, the torque ripple is decided by the back-electromotive force (EMF) and current waveform. If the back-EMF is constant in the conduction region of current, the torque ripple depends on the current ripple. The period of freewheeling region in the conduction region can be acquired by circuit analysis using the Laplace transformation and the torque ripple can be also reduced by varying input voltage to reduce the current ripple. The suggested method to reduce the torque ripple is confirmed by the dynamic simulation with the parameters of 500W BLDC motor.
1. BLDC motor
2. Current ripple
3. Torque ripple
4. Varying input voltage
Fig. 1. PWM inverter and equivalent circuit of BLDC motor
EXPECTED SIMULATION RESULTS
Fig. 2. Back-EMF of 500 W BLDC motor at 6660 rpm.
Fig. 3. Current waveform of 500 W BLDC motor at 6660 rpm. (a) Experimental data. (b) Simulation data.
Fig. 4. Current and torque waveform in simulation. (a) Constant input voltage.
(b) Various input voltage..
This paper presents the method of reducing torque ripple of the BLDC motor by varying the input voltage after circuit analysis using the Laplace transformation. In the simulation confirmed by experiment, the torque ripple is reduced to 10%. The 500WBLDC motor used for simulation and experiment dose not have a trapezoidal back-EMF waveform but a sinusoidal back-EMF waveform. So the torque ripple is not reduced conspicuously, although the current ripple is reduced conspicuously, and produced torque ripple waveform is similar to the back-EMF waveform of 500 W BLDC motor.
 J.-G. Lee, C.-S. Park, J.-J. Lee, G. H. Lee, H.-I. Cho, and J.-P. Hong, “Characteristic analysis of brushless motor condering drive type,” KIEE, pp. 589–591, Jul. 2002.
 T.-H. Kim and M. Ehsani, “Sensorless control of the BLDC motor from near-zero to high speeds,” IEEE Power Electron., vol. 19, no. 5, pp. 1635–1645, Nov. 2004.
 J. R. Hendershot Jr. and T. Miller, “Design of brushless permanent magnet motor,” in Oxford Magna Physics, 1st ed., 1994.
 P. Pillay and R. Krishnan, “Modeling, simulation, and analysis a permanent magnet brushless dc motor drive,” in Conf. Rec. 1987 IEEE IAS Annu. Meeting, San Diego, CA, Oct. 1–5, 1989, pp. 7–14.
 R. Carlson, M. Lajoie-Mazenc, and J. C. dos Fagundes, “Analsys of torque ripple due to phase commutation in brushless dc machines,” IEEE Trans. Ind. Appl., vol. 28, no. 3, pp. 632–638.