Control of a brushless DC motor using a torque vector method and modulated conduction

The research work described in this paper was carried out on a brushless DC motor with airgap windings and surface permanent magnets. In a previous research programme, robust speed or position control was achieved using a sliding mode method. However, the scheme suffered from certain disadvantages: the hardware scheme was relatively complex, the period for calculation of speed and acceleration from the position encoder signal was long and closed loop current control using a voltage source inverter was relatively slow. These resulted in limitation of the sampling frequency and also pulsations in speed. The solution applied by the authors uses the direct vector voltage control method. A variable structure controller based on a 3-dimensional sliding surface provides robust results and can be tuned for a fast, well damped response. However, with switching of the system state, the sampling frequency becomes increasingly critical and stress on the drive can be important. Torque vector control has been shown to give good response from a simple control scheme and pulsations may be reduced by modulation of the conduction periods of the power switching devices