In this note, an adaptive controller is presented for the torque loop in brushless DC motor (BLDCM) drives in the presence of time-varying parametric uncertainties. After introducing the dynamical model of BLDCM drives, the error dynamics is derived. Then the control structure taking uncertain periodic back-end signals with a known periodicity into account is obtained. The adaptation rule for periodic signals involved in the controller is improved for digital implementation in switched systems. In order to demonstrate the effectiveness and feasibility of the proposed adaptive controller algorithm, it is tested via various numerical simulations.