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Low force ripples from permanent magnet linear motors (PMLMs) are typically essential requirements in a wide range of demanding control applications with high motion quality. Apart from the sources present in rotary PM machines, phase unbalance due to the end effect is another major contributor of the thrust force ripples in the PMLMs. In this paper, the phase unbalance and induced force ripples are investigated in the case of a planar PMLM typically equipped with single-layer concentrated windings and quasi-Halbach magnets. It demonstrates that although the PMLM is quite carefully deigned to minimize the end effect, the phase unbalance still exists, including both the amplitude and phase position unbalances. And the phase unbalances give rise to even order force ripple harmonics, in which the 2 nd -order force ripple is particularly significant. In order to compensate the end effect and the unbalance between the three phases, a novel PMLM topology is presented, in which the width of the two end teeth (instead of the edge teeth) is adjusted to reduce the force ripple. And the impact of the end teeth width on the ripple force magnitude is further revealed. It is shown that by proper design of the end teeth width, the 2 nd -order force ripple of the PMLM can be minimized to quite a low extent. In comparison with the original design, the 2 nd -order force ripple factor of the novel topology has been suppressed from 0.4% to less than 0.03% in the case study. Such a technique has provided a new way of force ripple minimization, and is applicable to other types of linear motors or actuators with appropriate changes.

Direct torque control (DTC) systems for open-end winding permanent magnet synchronous motors (OEW-PMSM) with common DC bus generate zero sequence currents (ZSC) and large torque ripple, resulting in increased system loss and poor operating characteristics. In this paper, a DTC based on five level torque hysteresis (FLTH) is proposed to improve the system performance. Firstly, an online calculation and selection strategy based on switch table is designed to suppress ZSC. By analyzing the composition of zero sequence voltage and using the double vector method to suppress it, the ZSC can totally be suppressed in theory, without affecting the system performance. On this basis, this paper proposes to add FLTH to DTC to reduce torque ripple. FLTH can not only effectively use many voltage vectors, but also reduce torque ripple. In the case study, the torque ripple rate has been reduced by 53%. Through simulation and experiment, the conventional DTC and the FLTH DTC are compared, and the results prove the validity of the proposed strategy.INDEX TERMS Direct torque control (DTC), open-end winding permanent magnet synchronous motor (OEW-PMSM), five-level torque hysteresis (FLTH), zero-sequence current (ZSC), torque ripple.

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Abstract-The partial demagnetization of surface-mounted permanent magnets of PMSM in the three phase symmetrical short circuit of different temperature is analyzed by the finite element method. Then, rules of the back EMF and flux change in different extent of demagnetization are summarized. Finally, the operation behaviors and secondary demagnetization of the demagnetized PMSM are analyzed. Conclusion, which can judge and prevent demagnetization fault, is presented.