Torque ripple analysis of a permanent magnet brushless DC motor using finite element method

Three-phase trapezoidal back-EMF PM machines are widely used. As a function of the rotor position, the torque produced by these machines has a pulsating component in addition to the DC component. This pulsating torque has a fundamental frequency corresponding to 6 pulses per electrical revolution of the motor. The shape of the torque waveform and thus the frequency content of the waveform can be influenced by several factors in the motor design and construction. This paper addresses the various motor design parameters that influence the torque wave shape. It is shown that in addition to the basic induced back-EMF wave shape, accuracy in the skewing is also a key factor in determining the torque wave shape. Computer simulation using the finite element technique has been conducted to study the torque waveform. Simulation results successfully duplicated the torque waveforms measured in experiments under different excitation currents.