In recent years, there has been a renewed interest in the brushless dc motor as a research subject. This is due to following reasons; BLDC motors have operating characteristics similar to dc motors as compared with ac motors: It has a merit of mechanical contactlessness.
In this paper we deal with the sources of the torque ripple which is one of the elements causing depreciation of bldc motors. At high speeds, torque ripple is usually filtered out by the system inertia. However, at low speeds torque ripple produces noticeable effects that may not be tolerable in applications such as positioning, hybrid control, or robotics. The most significant causes of torque ripple are a variation in the phase current magnitudes, waveform offset, and unmodeled spatial harmonics arising from imperfections of the physical implementation of the fundamental spatial harmonic.
So we study the scheme of the feedforward compensation of torque ripple based on the abovementioned sources of the torque ripple. First of all, we can set $'T_{out} = A(\theta) \cdot T_{in}+B(\theta')$ that means the torque output divide into a proportional component to the torque input and an offset. So feedforward method which is adjusting $T_{in}$ according to the absolute angle of the motor can compensate the torque ripple. But because of the change of the A($\theta$) and B($\theta$) according to the drift of each phase-current offset we must revise these for repeatability. We also study the revision scheme in this paper.
