Study on Field-Weakening Theory of Brushless DC Motor Based on Phase Advance Method

An analysis is made on the field-weakening effect caused by the phase advance method of Brushless DC motor, and it is indicated that when the phase of the phase current leads the phase back EMF, the transformer EMF in the phase voltage equation can restrain the increase of the phase back EMF, which is equivalent to the field-weakening control of DC motor. This is the primary reason of field-weakening operation for brushless DC motor. Through analysis the paper deduces the equations of brushless DC motor phase current during the phase advance control. The paper also deduces the mathematic expression of phase advance angle by utilizing the concept of rms current, and the calculation method of the phase advance angle is established. The field-weakening control of the phase advance can be realized through the lookup table method. The experiment section provides a field-weakening operation curve of the brushless DC motor. The result is relatively coincide with the theoretic analysis. Also the defects of this method are also discussed at the end of the paper.