Silent phase commutation in a three-phase brushless dc motor

Abstract

A method drives a three-phase motor having first, second, and third coils. The method electrically connects the first coil to a first voltage reference and the second coil to a second voltage reference while leaving the other coil floating during a first driving phase. During a second driving phase, the first coil is electrically connected to the first voltage reference and the third coil is electrically connected to the second voltage reference while the second coil is left floating. During a transition phase that immediately follows the fast driving phase and immediately precedes the second driving phase, the second coil is electrically connected alternately to the first and second voltage references. By alternately connecting the second coil to the first second voltage references and during the transition phase, the method causes the current through the second coil to reduce to zero at a slower rate than prior methods. This enables the variations of the currents that the two phases in commutation, the second and third coils, to happen in a way that maintains their sum constant, as much as possible. This reduces the torque ripple during the phase commutations and its accompanying acoustic noise.