Elimination of torque dips due to back EMF non-ideality in brushless DC motors with surface-mounted magnets Online publication date: Sun, 12-Apr-2015
by Osama Mohamed Arafa
International Journal of Industrial Electronics and Drives (IJIED), Vol. 2, No. 1, 2015
Abstract: In this paper, a technique for eliminating torque dips in brushless DC motors (BLDCM) with surface-mounted magnets is introduced. The root causes of dips are the non-ideality of the back EMF waveform and the unequal rates of growth and decay of the commutating currents. The back EMF non-ideality addressed by this paper is limited to having flat intervals shorter than 120 deg. Such non-ideal BLDCM can be modelled using a set of piecewise continuous equations with the shortening angle delta as a characteristic parameter. The relation between such parameter and the magnitude of the torque dip is derived analytically. A current control strategy which proactively shapes the current reference for maintaining dip-free electromagnetic torque is explained. The proposed control strategy turns the back EMF non-ideality into an opportunity to establish a smooth commutation process during which incoming and outgoing currents have better chance to replace one another at same rate within a period of two deltas.
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