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Top1. Introduction
Axial flux ironless permanent magnetic synchronous motor (AFIPMSM) have the advantages of high power density, small size, high overload capability, efficiency, no cogging torque and reluctance torque (Wang Xiaoyuan, Chen Jing, Guo Yu, 2009; Xiaoyuan Wang, Jingjuan Du, 2006). The AFIPMSM with one-stator-two-rotor is shown in Figure 1. Outer rotor is applied in this motor. Nd-Fe-B permanent material and Halbach array is applied to the permanent material. The magnetic field of one side is increased, the other side is decreased. The flux density in circumferential direction is approximate sinusoidal distribution, which makes the motor’s back-EMF is sinusoidal. The waveformof no-load back EMF is shown in Figure 1 (b).
Figure 1. Basic structure and the wave of Back EMF
Although the AFIPMSM has more than a series of advantages,but as the coreless structure and the long air gap, the value of winding inductance is very small, which makes the current cannot be continued with the PWM control based on VSI, resulting in the large torque ripple. On the other hand, the armature reaction is so small that the effect to no-load magnetic field may be neglected. It is difficulties in motor flux weakening. The limitation of the DC-link voltage constrains the application of the motor that working at above the base speed.
The other inverter topology is the current source inverter (CSI). It has the advantages of fast dynamic response, convenience generative braking, simple converter topology, inherent four-quadrant operation, natural shoot-through protection capability in motor drive applications. In addition, field-oriented control (FOC) schemes are widely employed to achieve improved system dynamics and reliability by controlling the flux and torque independently. A PWM CSI induction drive using GTOs was described by M. Hombu, S. Ueda, A. Ueda, and Y. Matsuda (1985). CSI drives for medium and high-power induction machines were studied by Y. Yin and A. Y. Wu (1998), V. D. Colli, P. Cancelliere, F. Marignetti, and R. D. Stefano (2005), A. Weber, P. Kern, and T. Dalibor (2001), N.R. Zargari, S.C. Rizzo, Y. Xiao, H. Iwamoto, K. Satoh, and J. F. Donlon (2001). The phase angle current control algorithm of CSI for induction motor drive was proposed by A. Klonne and F.W. Fuchs (2003). An instantaneous current control method for CSI drives was proposed by M. H. Bierhoff and F. W. Fuchs (2009). The field-oriented control method for CSI was studied by M. Salo and H. Tuusa, (2005), J.D. Ma, B. Wu, N. R. Zargari, and S. C. Rizzo (2001), L. A. C. Lops and M. F. Naguib (2010), D. Banerjee and V. T. Ranganathan (2009), A. R. Beig and V. T. Ranganathan (2006), where significant improvements have been achieved. CSI was used in the drive system of vehicle applications by A. R. Beig and V. T. Ranganathan, (2006), M. Qiu, Y. W. Li, B. Wu, D. Xu, N. Zargari, and Y. Liu, (2007), Z. Wu and G. Su, (2008), G. J. Su, L. Tang, and Z. Wu, (2009), which extended the constant-torque and constant-power operation regions through the optimization of the circuit topology, and the control algorithm, but the torque-output capability was decreased in the field-weakening region.