Field-weakening regime for brushless DC motors based on instantaneous power theory

Permanent magnet excitation is beneficial in AC machines in which the flux is required to remain substantially constant. However in inverter-fed drives, where it is necessary to reduce the flux in order to operate above base speed with limited supply voltage, constant rotor excitation becomes a serious drawback. The conventional method used to achieve flux-weakening in permanent magnet motors involves phase advancing the stator currents with respect to the back EMF. The paper reviews the instantaneous power theory and investigates the use of the imaginary power component as an alternative means of providing field weakening in brushless DC motors. Apart from better performance at base speed, this regime is also shown to provide a wider speed range than the conventional method, without any significant current distortion.