Abstract
This paper proposes an improved PWM-OFF-PWM of the brushless DC motor under braking condition which can reduce commutation torque ripple. Compared with other brake modulation methods, PWM-OFF-PWM can restrain non-commutation torque ripple attributed to non-conductive freewheeling, while incapable of suppressing commutation torque ripple at low speeds effectively. Therefore, to solve commutation torque ripple caused by the turn-off time being shorter than turn-on time, an improved PWM-OFF-PWM is proposed in this paper. Moreover, to describe the influence factors of commutation torque ripple, the formulas of the duty cycle of switching tube in the commutation period and that during non-commutation period is derived. We can conclude from mathematical analysis that when duty cycles of outgoing and incoming phases satisfy derived formula, the non-commutation current in commutation interval will be more stable, which can contribute to suppress the commutation torque ripple at low speed under PWM-OFF-PWM mode and to improve the operation stability of BLDCM. The correctness and feasibility of the proposed method can be proved by simulation and experimental results.
Highlights
Brushless DC motor (BLDCM), as characterized by high efficiency, large density of power, and convenient control, has been widely applied in electric vehicle, industrial, and medical equipment [1], [2]
The torque ripple, in particular, the commutation torque ripple attributed to the structure characteristics and commutation method, has restricted the use prospect of BLDCMs in fields with high performance
Numerous studies are conducted on the reduction of commutation torque ripple, with the suppression strategies being roughly classified as three categories below: 1) Equalize various change rates of the currents of the incoming and outgoing phases in the interval of the commutation [3]–[9]
Summary
Brushless DC motor (BLDCM), as characterized by high efficiency, large density of power, and convenient control, has been widely applied in electric vehicle, industrial, and medical equipment [1], [2]. In order to suppress commutation torque ripple, the commonly used methods include modulation method, current prediction method, and introducing new link to adjust DC bus voltage to maintain a specific proportional relationship with back EMF. Compared with other regenerative braking modulation methods, PWM-OFFPWM can restrain non-commutation torque ripple attributed to non-conductive freewheeling, while incapable of suppressing commutation torque ripple at low speeds effectively. An improved PWM-OFF-PWM is proposed to suppress the commutation torque ripple for BLDCM in the braking state. 3) According to the mathematical analysis, we can draw the conclusion that when duty cycles of the outgoing and incoming phases meet the derived formula at commutation stage, the sudden change of non-commutation current will effectively suppress, inhibiting the commutation torque ripple under PWM-OFF-PWM mode and improving the operating stability of BLDCM. CAUSE OF COMMUTATION TORQUE RIPPLE BASED ON TRADITIONAL PWM-OFF-PWM MODULATION
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