Design of a three phase four quadrant variable speed drive for permanent magnet brushless DC motors

Abstract

The aim of this research project is to design a three phase four quadrant variable speed drive (VSD) for a permanent magnet brushless direct current motor (PMBLDC) that can be applied to an electric bicycle (e-bike). The design is confined to PMBLDC motors with a maximum power rating of 1.5kW. The speed controller operates in current mode at a maximum voltage and current rating of 50V and 30A, respectively. The VSD has the ability to smoothly control the current delivered to the DC motor and therefore controls its torque. The motor’s current is limited in all four quadrants of operation, and its speed is limited in the forward and reverse directions. The performance of the proposed DC motor VSD system is tested on an electric- bicycle.

The PMBLDC motor has three hall sensors embedded into the stator to determine rotor position. A phase switcher module interprets the position signals and produces a switching pattern. This effectively transforms the BLDC motor into a direct current (DC) brushed motor. The unipolar switching scheme used ensures that current flows out of the battery only for motoring operation and into the battery during regenerative braking. The current and torque are directly proportional in a BLDC motor. Torque control is achieved in the BLDC motor using a single channel current controller. The phase switcher current is monitored and used to control the duty cycle of the synchronous converter switches.

The proposed e-bike speed control system provides efficient control in all four quadrants of operation and it is a suitable alternative for a low cost transportation mode.