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Field Weakening of Permanent Magnet Machines Design Approaches
T. A. Lipo and M. Aydin
Electrical and Computer Engineering Department
University of Wisconsin-Madison
1415 Engineering Drive
Madison, WI 53706-1691, U.S.A
Email: lipo@engr.wisc.edu
Caterpillar Inc.
Technical Center TC-G 855
P.O. Box 1875
Peoria, IL, 61656-1875, USA
Email: aydin_metin@cat.com
Abstract - 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 structures 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 reviews 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 tremendously 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 has 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 eliminated. 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 allowable 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 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 which 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 demagnetization 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 capacity 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 flux 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 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 configurations. 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 and 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. Thecurrent phasor must be aligned with the q-axis in order to obtainmaximum output torque for non-salient machines. As for thesalient pole machines, the current phasor is slightly shiftedtowards the d-axis to achieve maximum torque for a given valueof current. At high speeds, flux weakening becomes necessarysince the machine back EMF can cause the stator voltage toexceed the maximum inverter output voltage. Therefore, thevoltage drop jL I becomes negative by adding a negative d-d daxis current, which results in reduced total airgap flux and theexcess back-EMF compensation reducing the machine terminalvoltage.
III. REVIEW OF RADIAL AIRGAP PM MACHINES CAPABLE OF FIELD WEAKENING
The development of relatively low cost rare earth magnetsopened a new era in PM machine design. One relatively earlydevelopment thrust was a novel Double Salient PermanentMagnet (DSPM) machine seen in Fig. 2. DSPM machinetopologies can be realized by introducing high energy magnetsinto doubly salient structure of a synchronous reluctancemachine. They are also good examples of flux control in PMmachines. The permanent magnets can be placed either in thestator or in the rotor. The stator version is illustrated in Fig. 2.In this case there exist both magnets and field winding in thestator structure. Such DSPM machines can be used foradjustable speed drive applications with improved efficiencyand power/torque density. It is one of the true field weakeningPM machine topologies which was developed at the Universityof Wisconsin-Madison [5-7]. The stator is formed by laminatedsteel, stator windings and high energy NdFeB magnets. Rotorhas a simple laminated structure. The machine flux can becontrolled by adjusting the reluctance path of the PM flux. Oneimportant advantage of the DSPM machine is to utilize the highenergy NdFeB magnets. Required airgap flux can be provided through this small size and small magnet thickness. In addition,
this structure introduces flux concentration principle. In other words, airgap flux can be higher than magnet residual flux density by introducing an increased magnet surface area.
Another type of DSPM machine is illustrated in Fig. 3. In this case, PMs are introduced by using ferrite magnets on the inner surface area of the stator and a circumferential DC field winding is placed in the stator core [8]. Stator and rotor structures are composed of laminated steel. The DC field winding produces magnetic flux which is in the same trajectory of the magnet flux. Flux boosting or weakening can be achieved simply changing the direction of the current. One important advantage is that the magnet cost is reduced dramatically in this structure. Also high airgap flux density can still be obtained through the large magnet surface area.
A different DSPM machine configuration suitable for traction application is given in Fig. 4. This machine is the inside-out version of the previous DSPM machine [9]. By reversing the location of rotor and stator, airgap diameter is increased resulting in increased torque capability. This type of PM machines is already in use in the automotive industry.
Another PM machine topology with flux weakening capability developed at UMIST in the UK is shown in Fig. 5 [10]. In this machine, the rotor structure is composed of two sections, one of which is surface mounted part and the other is axially laminated reluctance section, and they are both connected to the same shaft. The main objective of such a design is that the two rotor sections can be design independently so as to acquire a desired ratio of L /L . d q
A new radial flux PM machine with airgap flux weakening is shown in Fig. 6 [11]. This machine has an annular iron mounted on the surface of the magnets. There exist four iron sections and eight flux barriers as seen in the figure. The stator structure is the same as conventional radial flux PM machine. In this structure, the control of airgap flux is achieved by applying I dcurrent, which is not used to lessen the magnet flux but to modify the flux path. The magnet flux linked by the armature winding is decreased with this approach while the flux from the magnets is preserved.
One of the attractive radial flux PM machine structures witheasy flux weakening feature is theConsequent Pole Permanent Magnet (CPPM) machine developed at the University ofWisconsin-Madison [12-13]. The actualmachine pictureincluding a zoomed stator view and the machine view is given in Fig. 7. The machine stator and rotor have two sections. The stator is composed of a laminated core, iron yoke and 3 phase conventional winding. A circumferential DC winding is placed in the middle of the stator core. The rotor pole is divided into two sections, one of which has radially magnetized magnet and the other has laminated iron pole. This machine structure has several advantages in comparison with conventional PM machines. Firstly, an easy and a wide range of flux control can be achieved with this machine using airgap flux control technique. The ampere-turn requirement of the field winding is claimed to be low. Secondly, the magnetic configuration of the machine permits airgap flux control with no demagnetization risk of the rotor magnets because the control is realized by the iron pole pieces. Moreover, a simple DC field current control is used in this machine and there is no need for brushes or slip rings. However, the extra DC winding reduces the power density of the machine. The space required for the field winding increases the machine volume. Additionally, airgap surface associated with the field winding does not contribute the energy conversion. Also, 3D flux distribution introduces extra losses.
A new hybrid electric machine proposed is illustrated in Fig. 8 [14-16]. The PM machine is formed with a stator and rotor which is composed of two sections called first and second field magnets. Both field magnets are opposing with the magnet stator pole with a mechanism for varying a phase of magnetic pole. The two rotor concept could be applied to any surface magnet or interior magnet structures. The first field magnets of the rotor is alternately arranged with opposite magnetic poles and the second one has the same structure and is capable of causing relative angular displacement relative to the first one in order to achieve field weakening. It should be mentioned that the same concept was proposed in [17] for surface magnet machine in 1998.
In addition to the techniques mentioned above there exists some mechanical methods to accomplish field control in radial flux machines. A mechanical technique was introduced in [18]. A brushless PM machine with a fixed radial airgap is operated to a higher speed than the normal speed by reducing the magnet strength or average flux per pole. This is achieved by increasing the amount of axial misalignment of the PM rotor resulting in providing axial misalignment between the rotor poles and stator reducing the effective flux over a rotor pole or flux entering the stator as seen in Fig. 9. An integral constant velocity linear bearing is used to couple the moveable rotor and fixed position machine shaft. The constant velocity linear bearing lets the machine shaft, radial bearing, cooling fan, position encoder and output coupling remain in a constant position.
IV. REVIEW OF AXIAL AIRGAP PM MACHINES CAPABLE OF FIELD WEAKENING
Axial flux PM machines have drawn a lot of attention for more than a decade. They provide certain advantages over conventional PM machines such as higher power/torque density and efficiency, easily adjustable airgaps, low noise and vibration levels etc. By the virtue of its structure axial flux machines can have a variable airgap which may be suitable for some flux weakening applications such as electric traction. Axial flux design and rotor-stator arrangement allow the varying airgap to optimize the machine performance as shown in Fig. 10. This feature affects the machine torque and speed range and makes this technology promising for many applications requiring flux weakening. The other important advantage of this technique is to be able to change the torque constant of the machine which results in variable rotorand stator losses.This technique can be applied to double-rotor-single-stator machines too.
One of the axial flux machines for flux weakening operation is developed at the University of Torino in Italy [19]. The machine structure over two poles is displayed in Fig. 11. This work deals with the design of a new Axial Flux Interior PM (AFIPM) machine with flux weakening capability by the use of soft magnetic materials. The machine is composed of two slotted stators and a single rotor. The slotted side of the stator has tape wound core with series connected stator windings. The rotor structure has axially magnetized magnets, rotor disc and main and leakage poles. There exist two flux barriers in between the leakage and main poles. The position and size of the flux barriers can be designed in such a manner that d-axis and q-axis stator inductances can satisfy the required torque in the flux weakening region.
Another interesting axial flux machine with flux control feature is proposed in [20-21]. This machine uses a field weakening coil to achieve field weakening by directly controlling the magnitude and polarity of a DC current of the field weakening coil. The machine structure and the rotor are displayed in Fig. 12. The rotor is formed by magnet and iron
pole pieces which are mounted in holes in a non-magnetic rotor body. The machine has two slotted stators and AC windings, and each stator has a yoke providing a flux return path. Two field weakening coils in toroidal form are mounted on a machine frame as seen in the figure. The coils encircle the shafand the frame is made of mild steel in order to provide a fluxpath for the DC coils. It should be mentioned that it is nonecessary to control the d-axis or q-axis current components othe PM machine. In addition, under normal control rangedemagnetization of the magnets is not an issue by any means.
Same principle of DC field coil is applied to another axialflux PM machine as seen in Fig. 13 [22]. This axial fluxmachine comprises two stators and one rotor which haspermanent magnets and pole portions. The magnets in the rotorgenerate a first magnetic flux and the consequent rotor poles generate a second magnetic flux. A field coil, which is mounted to the housing and located very close to the rotor, is very effective to vary the second magnetic flux mentioned and therefore the machine provides a controllable output voltage.
Fig. 14 shows an axial flux PM brushless synchronous alternator [23]. This machine combines a variable DC coil excitation in addition to PM excitation. The rotor has two discs mounted on a common shaft. Each disc carries magnets and alternate north and south iron poles which are made of steel. The north poles of the disc-1 are located opposite of the north poles of the second rotor disc which are steel poles. The excitation of the steel poles is provided by the DC excitation coil which surrounds the shaft as seen in the figure and is fixed to the inner side of the stator. NdFeB magnets provide high magnetic loading and creates a compact design. There exists ferrous shim under each magnet in order to reduce the interpolar leakage. The stator is formed by a strip of magnetic steel sheet and slots are punched by index punching machine. Toroidal windings are used in the stator slots. The main advantage of this machine is the capability of the field control via DC field excitation which is achieved with a low reluctance path through the rotor discs, the pole pieces and the shaft. It should be mentioned that the axial length of both stator and rotor is bigger because of the shim under the magnets and the stator yoke to let
the flux travel in the stator. Also, the loss mechanisms are more complicated than the conventional and other axial flux PM machines.
Recently, a new axial flux PM machine topology with a DC field winding has been introduced in order to accomplish easy and inexpensive control at the University of Wisconsin-Madison [24-26]. This new Field Controlled Axial Flux surface mounted PM (FCAFPM) machine concept has been proposed not only to offer a solution to field weakening operation but also to improve the features of the conventional PM machines by introducing a new axial flux machine concept with flux weakening capability. Modifying the multiple-rotor-multiple-stator conventional axial flux PM structures by adding one or two DC field windings depending on the machine type to control the airgap flux and providing a path for the DC flux results in different new axial flux machines with field control capability. Some of these new structures are illustrated in Fig. 15. Both NN and NS type double-rotor-double-stator FCAFPM machine concept are shown Fig. 15 (b) and (s) while the double-stator-single-rotor and MULTI stage concepts are displayed in Fig. 15 (d) and (e).
One derivation of the new concept which is called double-rotor-single-stator NS type FCAFPM machine is used as an example to describe the structure and an actual prototype machine built and tested is illustrated in Fig. 16.
The new NS type FCAFPM structure is composed of a twopart tape wound disc type slotted stator structure oneincorporated into another, two rotor discs with axially magnetized surface mounted magnets and iron pieces mounted on the rotor surface, two sets of 3 phase AC stator windings and a DC field winding which is the main difference between the axial flux PM machine and the new concept FCAFPM machine.In other words, there exist two sources in the machine: constant magnet excitation and variable DC field excitation. Excitation of the DC coil of one polarity tends to increase the consequent poles on both inner and outer portions of the rotor thus strengthening the field. Excitation of the DC coil with opposite polarity decreases the field in the consequent poles in both inner and outer portions of the rotor disc thereby weakening the airgap flux. This topology eliminates the demagnetization risk of the magnets since the DC field Aturns do not directly oppose the magnet Aturns and airgap flux can be controlled in a wide range with the FCAFPM machine. More detailed information about the FCAFPM concepts are provided in [26].
The same flux weakening principle can be applied to single-stator-single-rotor structures seen in and double-stator-single-rotor machines seen in Fig. 17. The new Field Controlled double-stator-single-rotor Axial Flux Internal Rotor (FC-AFIR) PM machine has two stators with two sets of 3 phase stator winding and 2 sets of DC field winding. The basic principle of the FC-AFIR machine is the s