Certain investigations on torque ripple control of bldc motor

Abstract

Nowadays, the application of Brushless DC (BLDC) motor is rapidly increasing in many areas like aerospace applications, robotics, medical instruments and electric vehicle applications, etc. BLDC motor has many predominant features such as higher efficiency, less maintenance, high torque to weight ratio, higher power ratio and easy to control over a wide range of speed. The structure of a BLDC motor is similar to a Permanent Magnet Synchronous Motor (PMSM). The windings are put on the stator and permanent magnets are placed on the rotor. The inductance effect of winding mainly influences the shape of trapezoidal back EMF and rectangular stator currents of the motor. Due to the deviation of stator current from its ideal rectangular shape causes a ripple in torque [Pillay P and Krishnan R, 1988]. It results in vibration and acoustic noise in the motor. This causes BLDC motor application constrained to some particular areas. It is necessary to control the motor to obtain the smooth response of torque and speed with less torque ripple content [Li et al. 2014]. The BLDC motor torque ripple can be reduced either by the improving control scheme or by improving the motor design. In this research torque ripple control of BLDC motor is analysed by improving the control scheme. This research includes torque ripple control using current control method, torque control with speed control of BLDC motor. Main cause of the torque ripple during commutation is ripple occur in phase current. In commutation period current switch over from one phase to another phase in each 60degree. One phase current will be turn off, by the same time another phase current will be turn on. At that time the turned off phase current decaying toward zero called as falling current and the turned on current increasing toward maximum value called as rising current. Due to motor design parameter and back EMF the falling current slop is not equal to slop of the rising current. Due to this mismatch of slop, current ripple occurs in the un-commutated phase current. This leads to torque ripple during commutation period. newline