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AXIAL-FLUX MACHINESBrian J. Chalmers Introduction FORMATS OF AXIAL FLUX MACHINES The possible formats of axial-flux machines are summarised below. In general, the armature, which carries induced emfs and load currents, may be either stationary or rotating. Alternative armature arrangements include:
Topologies Include:
BRUSHED DC MACHINES WITH AXIAL FLUX Axial-flux brushed dc motors have existed for many years. Typically, their stationary field system comprises one or two sets of permanent magnets mounted on disc-shaped steel endplates, one set of magnets sometimes being replaced by a simple mild steel flux-return plate. The armature is often ironless. Windings may be of printed circuit type, or stamped from copper sheet, or may be wire-wound [1]. Attributes claimed for these motors include fast response, high efficiency, good commutation and high power-to-weight ratio [2]. [ AN ILLUSTRATION OF A MAVILOR PRODUCT MAY BE INCLUDED HERE]SOLID ARMATURE MACHINES The earliest example of an axial-flux machine with solid armature was the Faraday disc. The rotating copper disc `armature` required sliding contacts for current collection and the space utilisation was very poor.The family of eddy current machines, including couplings, brakes and dynamometers, have solid `armatures` in which an iron member carries both flux and induced currents. A high-performance eddy-current dynamometer with axial flux was developed for use in commercial testing of automotive engine systems [3]. This is a double-sided homopolar inductor machine in which a pulsation of unidirectional axial flux is produced by rotation of a central toothed rotor, causing power to be dissipated as eddy-current losses in twin outer loss members of pure iron. Note that neither the `armature` nor the field system rotate. The rotor is a simple toothed member cut from a steel plate. The symmetrical arrangement, with a dc excitation coil and a loss member on each side, minimises magnetic asymmetry and axial forces. Eddy currents flow in the plane face of each loss member adjacent to the airgap and heat is removed very effectively by water flowing in circumferential channels in the faces of the lossplates remote from the air gap. Loadings in these machines are very high. Thus, peak flux density in the air gap is about 1.75 T. Electric loading in the loss members is about 160 A/mm of periphery at the mean radius. Designed loss density at rated power is 100 W/sq.cm. which is several times that occurring in the largest turbine generators. The specific power is exceptionally high, at 10 kW/kg. The range of four production machines had ratings of 70 kW at 12,000 rev/min, 165 kW at 10,000 rev/min, 300 kW at 7,500 rev/min and 500 kW at 4,500 rev/min. The 500 kW machine required an active area of about 0.25sq.m. per side, with a rotor diameter of just 0.61 m. These dynamometers have been applied all over the world. BRUSHLESS MACHINES WITH AXIAL FLUX As for conventional brushless dc motors, the corresponding arrangements of brushless axial-flux machines have stationary armatures and rotating permanent-magnet field systems. A variety of topologies have been developed and may be used as either motors or generators. [AN ILLUSTRATION OF A MAVILOR PRODUCT MAY BE INCLUDED HERE]The double-sided Torus machine, with a central stator and twin outer rotors, was developed at UMIST [4]. A simple toroidal strip-wound steel stator core carries a slotless toroidal winding which may have any chosen number of phases. For dc generator applications, rectifiers may conveniently be mounted on the stator casing. Machines of this type have been developed for a variety of applications [5]. Again, the symmetrical arrangement minimises mechanical unbalance but special attention has to be given to control axial magnetic forces of attraction during assembly of the machine. Analysis has shown that optimal Torus designs have magnet thickness in the range one to two times the winding thickness. A more economic solution may be offered by slotted strip-wound stator cores, which require smaller magnet thickness. Single-sided motors with slotted stator have been developed for motor-in-wheel drives for solar powered vehicles [6]. An extra thrust bearing was provided to support the axial force exerted by the magnets. This successful design had a high efficiency (c. 94%), as necessary for this application, and has been used by many constructors in long-distance solar-powered race vehicles. Multiple disc construction is applicable when it is desired to produce high output and rotational forces impose a limit on rotor diameter. Multiple-disc, high-speed generators have been developed for which special attention was given to mechanical design [7]. The armatures are ironless and magnet flux passes axially through the machine from end to end. INDUCTION MACHINES WITH AXIAL FLUX As usual for induction motors, air gap length must be small and slotting effects must be controlled. Axial-flux induction motors should therefore use slotted strip-wound cores with small slot openings. Slot skew, usually in the rotor, is also desirable and achievable. Single-sided construction necessitates attention to axial forces. Machines of this type are in production.References [1] Campbell,P: "Principles of a permanent-magnet
axial-field dc machine", Proc.IEE, 121, December 1974, pp.1489-1494.
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