Wide Speed Range Operation of Non-Salient Permanent Magnet Motors

Abstract

Non-salient PM machines have excellent characteristics in terms of power density, torque to inertia ratio, and smooth operation with minimum cogging torque and torque ripples. Especially the distributed windings of these machines result in more sinusoidal back EMF and reduced THD. However, these machines are not a favorite choice for high speed operation because of their inherent low flux weakening capability. Flux weakening capability of the PM machines is related with their d-axis inductance. Normally these machines are designed with high value of d-axis inductance for high flux weakening capability. However, non-salient PM machines are constructed with low permeability surface mounted PMs that result in low inductance characteristics of these machines and hence, poor flux weakening capability. A winding switching technique for the flux weakening of PM machines overcomes this problem because it does not depend upon d-axis inductance of the machine. It depends upon slot per pole per phase combination of the machine. Two machine designs are proposed in this research work that can utilize the winding switching technique for the flux weakening of the non-salient PM machines with slot per pole per phase combinations of two and four respectively. There are a lot of applications that require different wide speed range operations of the electrical machines. Special machine design is adopted to fulfill the maximum speed range of each of these applications. Two novel concepts of coil displacement and phase swapping for multiple wide speed range operations from a non-salient PM machine are introduced in this research work. These two concepts enable a PM machine to be used for the applications requiring different speed range operations without redesigning a machine. Winding switching technique requires winding connections change for the flux wakening of the machine. This operation can generate severe transients during winding changeover and can result in failure of the electrical machine or damage the associated equipment. A current regulated voltage source inverters and thyristors based drive system is proposed to avoid these unfavorable outcomes during winding switching. Thyristors avoid instantaneous winding changeover and hence, avoid any voltage spikes across the switches. Whereas, current regulators control the winding currents and keep them within the limits especially during winding switching. Two novel control strategies are proposed to operate the machine over its entire speed range. Both of the control strategies enhance the maximum output power capability of the machine beyond one p.u. as well as enhance the maximum speed range of the machine beyond its flux weakening capability. Machine operation converts into open winding configuration at winding switching. Hence, √3times higher voltage becomes available at the terminals of the machine that enables at least √3times higher speed compared to the flux weakening capability of the machine. Winding switching technique assumes ideal sinusoidal back EMF of the machine. However, real world machines are not harmonic free and contain some harmonic contents. This issue is also addressed in this research work. It is investigated that the winding switching technique is especially sensitive to 3rd order harmonics and a small harmonic contents in the back EMF of the machine are enhanced significantly during the processes of flux weakening. Impacts of these harmonics on the field weakening range of the machine are investigated and is found that the 3rd order harmonics up to 41% at flux weakening stage contribute positively in terms of flux weakening range of the machine. Analytical, simulated and experiment results are provided for all above mentioned issues to validate the proposed concepts. Proposed machine designs and drives are successfully implemented. It is found that the winding switching technique is a promising solution for the flux weakening of the non-salient PM machines. Moreover, the proposed concepts of coil displacement and phase swapping provide multiple wide speed range operations from a single PM machine design. These concepts have a potential to make things simple and easier and use the same machine design for a variety of applications with different speed ranges.