- Email: [email protected]
Developments in electrical machines using permanent magnets
Journal of Magnetism and Magnetic Materials 157/158 (1996) 131-132
~H ELSEVIER
journal or magnetism and magnetic materials
Developments in electrical machines using permanent magnets B.J. Chalmers
*
UMIST, P.O. Box 88, Manchester M60 1QD, UK
Abstract ~'?The availability of high-field permanent-magnet materials has created opportunities for the development of electrical machines with advantageous properties including high efficiency, compact size, low weight and brnshless operation. The paper reports the design and performance of a number of motors and generators which have recently been developed and demonstrated. Keywords: Electrical machines; Permanent magnets; Motors; Generators
The availability of high-field permanent-magnet materiais, with remanence of the order of IT and coercivity approaching 1 k A / m m , has created opportunities for the development of electrical machines with advantageous properties including high efficiency, low weight and brushless construction. These material properties enable machines to be designed with customary values of flux density in the magnetic circuit and much reduced space requirements for the excitation field system. An important category, used in synchronous motors and brushless dc motors, is the so-called 'interior type', in which magnets are buried in internal slots within a laminated rotor core. Fig. 1 shows the lamination for a four-pole synchronous motor [i]. The magnets are accommodated in V-shaped slots which are bridged near the rotor surface, to form a single-piece lamination of adequate mechanical strength to withstand rotational forces. Slotting is also provided for a cage winding to enable starting by induction motor action. It is a property of interior-magnet machines that the reluctance on the polar or direct (d) axis is much greater than the reluctance on the interpolar or quadrature (q) axis. This phenomenon of inverse saliency has been shown to yield advantageous performance characteristics, with input power factors very close to unity over a wide range of loads. The experimental 7.5 kW motor [1] had a measured input power factor of 0.99 and efficiency of 92.8% on rated load, compared with 0.89 and 86.3% for a standard induction motor in the same stator frame. The losses in the permanent-magnet synchronous motor are about half of those in the induction motor, owing to the
elimination of rotor 12R losses and reduction of the stator current. Alternatively, these advantages may be exploited in terms of increased output from the same framesize or reduced size for the same performance. Experiences with the above form of synchronous motor raised the question of whether the same type of machine might offer a related advantage when used as an isolated alternator driven at constant speed. An introductory analysis [2] predicted the unusual phenomenon that, when feeding a resistive load, the output voltage would increase with toad, reaching a maximum at a certain value of load. The effect was demonstrated by experiments on a small machine; Fig. 2 illustrates how its terminal voltage phasor V varied in magnitude and phase as the load current I was increased. The particular condition at maximum output voltage is highlighted, showing the magnitudes of the voltage components; at that point the output voltage V exceeds the open-circuit voltage E by 14.7%. Recent experiments on an industrial 7.5 kW machine have confirmed the occurrence of desirable performance with just a 1.4% voltage drop at rated load. These findings demon-
* Fax: +44-161-200-4648; email: [email protected].
Fig. 1. Rotor lamination for four-pole synchronous motor.
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI0304-8853(95)01038-6
B.J. Chalmers/Journal of Magnetism and Magnetic Materials 157/158 (1996) 131-132
132
strate the possibility of design to achieve very low or zero voltage regulation on rated load, thus overcoming a disadvantage of unadjustable permanent-magnet excitation. The properties of high-field magnets yield flux densities similar to those normally utilised in economic machine designs, even when the magnetic circuit contains quite a large air gap. This reopened the consideration of machines with slotless armature windings, otherwise known as airgap windings, which were used in some of the earliest electrical machines. A disc-type arrangement, shown in half-section in Fig. 3, with axial flux in the air gaps between the discs, has been found to give a very compact design that is well suited for integration with coupled machinery. A toroidal strip-wound stator core carries a slotless toroidal winding which may have any chosen number of phases. The rotor comprises mild steel discs carrying axially polarised magnets. In optimum designs, the thicknesses of winding and magnet are typically about equal and the flux density in the air gap is then approximately equal to half the magnet remanence. The machine effectively contains two independent halves, lying either side of the central plane. The active conductor lengths are
Se
re
~
~
Shaft
Air inlet Magnet Rotor disc
Air outlet Fig. 3. Cross-sectional arrangement of the Torus machine.
q-axis I I I I I
Xq!q
T
~
~L.....I i n c r e a s i n g
,',\
/
J
the two radial portions facing the magnets, whose polarities are arranged to induce additive emfs around a stator coil. The axially directed end-winding lengths are relatively short, yielding low resistance. To indicate the toroidal nature of both the stator core and the stator winding, the name Torus was adopted for this arrangement [3]. Following the initial work at UMIST, there has been growing international interest in this arrangement and machines have been developed for a variety of applications as either motors or generators. These include auxiliary power units, wind energy generators, an electric scooter drive and a water-cooled version for an innovative electric city car. In all cases, axial lengths are very short, power/weight is good, and efficiency is in the range 84-93%, depending on the machine size. References
_ __
d - axis
Fig. 2. Variation of output voltage with increasing load current.
[1] B.J. Chalmers, Supermagnets: Hard Magnetic Materials (Kluwer, Dordrecht, 1991) p. 703. [2] B.J. Chalmers, IEE Proc. B 141 (1994) 186. [3] E. Spooner and B.J. Chalmers, IEE Proc. B 139 (1992) 497.
~H ELSEVIER
journal or magnetism and magnetic materials
Developments in electrical machines using permanent magnets B.J. Chalmers
*
UMIST, P.O. Box 88, Manchester M60 1QD, UK
Abstract ~'?The availability of high-field permanent-magnet materials has created opportunities for the development of electrical machines with advantageous properties including high efficiency, compact size, low weight and brnshless operation. The paper reports the design and performance of a number of motors and generators which have recently been developed and demonstrated. Keywords: Electrical machines; Permanent magnets; Motors; Generators
The availability of high-field permanent-magnet materiais, with remanence of the order of IT and coercivity approaching 1 k A / m m , has created opportunities for the development of electrical machines with advantageous properties including high efficiency, low weight and brushless construction. These material properties enable machines to be designed with customary values of flux density in the magnetic circuit and much reduced space requirements for the excitation field system. An important category, used in synchronous motors and brushless dc motors, is the so-called 'interior type', in which magnets are buried in internal slots within a laminated rotor core. Fig. 1 shows the lamination for a four-pole synchronous motor [i]. The magnets are accommodated in V-shaped slots which are bridged near the rotor surface, to form a single-piece lamination of adequate mechanical strength to withstand rotational forces. Slotting is also provided for a cage winding to enable starting by induction motor action. It is a property of interior-magnet machines that the reluctance on the polar or direct (d) axis is much greater than the reluctance on the interpolar or quadrature (q) axis. This phenomenon of inverse saliency has been shown to yield advantageous performance characteristics, with input power factors very close to unity over a wide range of loads. The experimental 7.5 kW motor [1] had a measured input power factor of 0.99 and efficiency of 92.8% on rated load, compared with 0.89 and 86.3% for a standard induction motor in the same stator frame. The losses in the permanent-magnet synchronous motor are about half of those in the induction motor, owing to the
elimination of rotor 12R losses and reduction of the stator current. Alternatively, these advantages may be exploited in terms of increased output from the same framesize or reduced size for the same performance. Experiences with the above form of synchronous motor raised the question of whether the same type of machine might offer a related advantage when used as an isolated alternator driven at constant speed. An introductory analysis [2] predicted the unusual phenomenon that, when feeding a resistive load, the output voltage would increase with toad, reaching a maximum at a certain value of load. The effect was demonstrated by experiments on a small machine; Fig. 2 illustrates how its terminal voltage phasor V varied in magnitude and phase as the load current I was increased. The particular condition at maximum output voltage is highlighted, showing the magnitudes of the voltage components; at that point the output voltage V exceeds the open-circuit voltage E by 14.7%. Recent experiments on an industrial 7.5 kW machine have confirmed the occurrence of desirable performance with just a 1.4% voltage drop at rated load. These findings demon-
* Fax: +44-161-200-4648; email: [email protected].
Fig. 1. Rotor lamination for four-pole synchronous motor.
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. SSDI0304-8853(95)01038-6
B.J. Chalmers/Journal of Magnetism and Magnetic Materials 157/158 (1996) 131-132
132
strate the possibility of design to achieve very low or zero voltage regulation on rated load, thus overcoming a disadvantage of unadjustable permanent-magnet excitation. The properties of high-field magnets yield flux densities similar to those normally utilised in economic machine designs, even when the magnetic circuit contains quite a large air gap. This reopened the consideration of machines with slotless armature windings, otherwise known as airgap windings, which were used in some of the earliest electrical machines. A disc-type arrangement, shown in half-section in Fig. 3, with axial flux in the air gaps between the discs, has been found to give a very compact design that is well suited for integration with coupled machinery. A toroidal strip-wound stator core carries a slotless toroidal winding which may have any chosen number of phases. The rotor comprises mild steel discs carrying axially polarised magnets. In optimum designs, the thicknesses of winding and magnet are typically about equal and the flux density in the air gap is then approximately equal to half the magnet remanence. The machine effectively contains two independent halves, lying either side of the central plane. The active conductor lengths are
Se
re
~
~
Shaft
Air inlet Magnet Rotor disc
Air outlet Fig. 3. Cross-sectional arrangement of the Torus machine.
q-axis I I I I I
Xq!q
T
~
~L.....I i n c r e a s i n g
,',\
/
J
the two radial portions facing the magnets, whose polarities are arranged to induce additive emfs around a stator coil. The axially directed end-winding lengths are relatively short, yielding low resistance. To indicate the toroidal nature of both the stator core and the stator winding, the name Torus was adopted for this arrangement [3]. Following the initial work at UMIST, there has been growing international interest in this arrangement and machines have been developed for a variety of applications as either motors or generators. These include auxiliary power units, wind energy generators, an electric scooter drive and a water-cooled version for an innovative electric city car. In all cases, axial lengths are very short, power/weight is good, and efficiency is in the range 84-93%, depending on the machine size. References
_ __
d - axis
Fig. 2. Variation of output voltage with increasing load current.
[1] B.J. Chalmers, Supermagnets: Hard Magnetic Materials (Kluwer, Dordrecht, 1991) p. 703. [2] B.J. Chalmers, IEE Proc. B 141 (1994) 186. [3] E. Spooner and B.J. Chalmers, IEE Proc. B 139 (1992) 497.