1、Field Weakening of Permanent Magnet Machines Design ApproachesT. A. Lipo and M. AydinElectrical and Computer Engineering Department University of Wisconsin-Madison 1415 Engineering Drive Madison, WI 53706-1691, U.S.A Email: lipoengr.wisc.eduCaterpillar Inc. Technical Center TC-G 855 P.O. Box 1875 Pe
2、oria, IL, 61656-1875, USA Email: aydin_metinAbstract - Permanent Magnet (PM) machines have been developed for numerous applications due to their attractive features especially after the development of NdFeB magnets. However, their complicated control lets the researchers develop new machine structur
3、es with easy field control. New alternative PM machine topologies with field weakening or hybrid excitation have been introduced in the literature for years to eliminate the effects of problems associated with the cumbersome field weakening techniques used in conventional PM machines. This paper rev
4、iews the field weakening of PM machines covered from machines perspective. Machine structures and features of each structure are clarified for both radial and axial airgap PM machines studied thus far. I. INTRODUCTION Demand for more compact, efficient and cheaper electric machines has grown tremend
5、ously during the last decade. Meanwhile, a great progress has been achieved not only in the development of permanent magnets but in the area of electric machine design and power electronics as well. Therefore, PM machines have been drawing more and more attention. Development of magnet technology ha
6、s allowed increased power/torque density and efficiency of the PM machines. Especially with the use of NdFeB magnets, the PM machines have reached the highest efficiency and power density levels in the 90s. They areusually more efficient because of the fact that field excitation losses are eliminate
7、d. In addition, copper losses in general are reduced in PM machines compared to conventional machines. In other words, due to lower losses, heating of the PM machines will be less, which can result either run the machine at low temperatures or to increase the shaft power so that the maximum allowabl
8、e temperature has been reached. As far as the power electronics is concerned, less power from the converter is required to deliver the same power to the machine because of the high efficiency of the PM machines. Air gap flux control of PM machines can generally be accomplished by two means: control
9、techniques and suitable modification of the machine topology. Conventional PM machines have a fixed magnet excitation which limits the drives capability and becomes a significant limitation. The machines are operated at constant volt/hertz operation up to base speed and constant voltage operation wh
10、ich requires weakening of the field at higher speeds to extend the speed range. Above base speed, vector control techniques are typically used to weaken the air gap flux. However, these techniques cause large demagnetization current to flow in the machine d-axis and results in high losses and demagn
11、etization risk of the magnets. Furthermore, the magnets may be forced to operate in the irreversible demagnetization region which could permanently demagnetize the magnets by not allowing themagnet to return to its original operating point even after the current is removed 1-2. Thus, the torque capa
12、city of the machine is permanently diminished 3-4. It is obvious that the attainable speed range is limited by the largest tolerable demagnetization current specified by the demagnetization characteristics of the magnets. In addition, the capability of the converter sets an additional limit to the f
13、lux weakening range of the PM machine. The search for a means to realize field weakening in PM machines by eliminating the detrimental effects of d-axis current injection has been of great interest to machine designers and new machine structures are currently of great interest. Therepresently exist
14、a number of alternative solutions in order to eliminate this problem in PM machines and the majority of these solutions have been proposed in the 1990s. Advances in material technology such as PMs, magnetic steel and powdered iron composites have allowed researchers to arrive at new machine configur
15、ations. A survey of these flux control capable PM machine topologies is the subject of this paper. II. FLUX WEAKENING OF PM MACHINES The phasor diagram of a typical PM machine drive is shown in Fig. 1 at base and high speeds. The equivalent circuit of this kind of machine comprises the inductance an
16、d the back-EMF voltage which is the product of magnet flux linkage ( ) and the mmachine electrical speed (). The magnet flux lies along the d-axis and the back-EMF phasor which is 90 degree phase advanced lies along the positive q-axis. The machine torque is generated both by the magnets and by the saliency and depends on the angle between the current phasor and the q-axis. Thecur
