Analysis of Inverter Circulating Current and Magnetic Potential for Flux-Weakening Drive of BLDCM

Abstract

The permanent magnet brushless DC motor (BLDCM) is typically controlled using the six-step commutation method, and the flux-weakening method is employed to enable the motor to operate at speeds higher than the base speed. Currently, it is considered that the weak magnetic angle range is 0-pi/3, while the range for deep weakening is pi/3-pi/2. In field-weakening control, a forward shift of the commutation point results in a circulating current flowing in the three-phase bridge of the inverter and the stator winding of the motor. This paper analyses the principle of the circulating current formed by the inverter. Through magnetic potential analysis and Simulink simulation, it is concluded that flux-weakening control generates a circulating current in the inverter and motor stator windings. The inverter’s circulating current affects the motor’s magnetic potential, causing it to shift towards the rotating direction of the motor rotor. When the forward shift angle of the inverter commutation point is within the range of 0-pi/6 electrical angle, the phase shift of the inverter circulating current remains below pi/6. This configuration weakens the magnetic field and provides the driving effect. However, when the forward shift angle falls within the range of pi/6-pi/3, the phase shift of the inverter circulating current exceeds pi/6, resulting in magnetic weakening and braking. During the braking effect, a reverse torque is generated, leading to a decrease in motor torque and efficiency. Therefore, the range of the weak magnetic angle should be between 0-pi/6.