GB2139823A

GB2139823A - Stator for an electromagnetic machine

Info

Publication number: GB2139823A
Application number: GB08410979A
Authority: GB
Inventors: John David Marsh; Craig Anthony Nazzer
Original assignee: Caterpillar Tractor Co
Current assignee: Caterpillar Inc
Priority date: 1983-05-03
Filing date: 1984-04-30
Publication date: 1984-11-14
Also published as: GB8410979D0; GB2139823B

Abstract

Electromagnetic machines of the type having circular arrays of permanent magnets (3 Fig. 1) arranged on a rotor (2) and a plurality of pole pieces 6 arranged on a stator (1), requires that a maximum number of pole pieces be arranged around the stator in order to attain maximum utilization of the magnetic flux. The instant stator of an electromagnetic machine includes a plurality of U-shaped pole pieces 6 with adjacent ones of the pole pieces opening in opposite radial directions from an axis of the stator. The pole pieces 6a which open radially inwardly are stepped so as to have a relatively thicker portion 8 radially outwardly and a relatively thinner portion 9 radially inwardly. <IMAGE>

Description

SPECIFICATION Stator for an electromagnetic machine This invention relates generally to electromagnetic machines, and more particularly, to pole pieces for motors having permanent magnet excitation.

There are, currently available, motors having circular arrays of permanent magnets arranged on a rotor. The rotor is associated with a stator having a plurality of pole pieces energized by a coil carrying a controlled electric current. Commutation of the electric current produces a rotating magnetic field in the stator, and interaction between the pole pieces and the permanent magnets results in rotation of the rotor. One example of such a motor is described in G B-A-1 541211.

In designing motors of this general type, it is important to maintain the mutual inductance of the coil at the lowest possible level in order to obtain maximum utlization of the available magnetic flux, increasing the efficiency of operation and decreasing energy losses. Location and design of the pole pieces is a critical factor in maximizing the utilization of the magnetic flux. It is important that as many pole pieces as possible be arranged around the stator coil. Coincidentally, interpole magnetic flux leakage needs to be kept as low as possible to reduce leakage inductance.

GB-A-2006540 proposes staggering the arrange ment of the pole pieces so that those pole pieces which open radially inwardly are radially further from the central axis of the machine than those pole pieces which open radially outwardly. Such an arrangement increases the number of pole pieces which can be incorporated in a stator of given radius and prevents any increase in interpolarflux leakage resulting from the increased number of pole pieces.

However, it will be appreciated that the dimensions of the machine are inevitably increased as a result of such staggering and this may be unacceptable in certain circumstances. An alternative proposal put forward in the same patent specification is the shortening of the radially inwardly opening pole pieces, again allowing an increase in the number of pole pieces, without increasing interpolar flux leak age. However, as a result of such shortening, the face area of the pole pieces is reduced, resulting in a reduction in the utilization of magnetic flux.

According to the present invention, a stator of an electromagnetic machine has an annular coil; a plurality of U-shaped pole pieces mounted in strad dling relationship to the coil; adjacent ones of the pole pieces opening in opposite radial directions; and the radially inwardly opening pole pieces each having a radially inner portion and a radially outer portion, the radially inner portion having a cross section thinner than the cross-section of the radially outer portion.

The present invention increases the number of pole pieces that can be arranged around the stator while maximizing the face area of the pole pieces.

Utilization of the magnetic flux is maximized and interpolar flux leakage is maintained as low as possible.

For a better understanding of the present invention, reference may be made to the accompanying drawings, in which: Figure lisa diagramatical view of a prior art device; Figure 2 is an axial end elevation showing a prior arrangement of the pole pieces; and Figure 3 is an axial end elevation showing an arrangement of the pole pieces in accordance with an embodiment of the present invention.

Figure 1 shows, diagrammatically, the basic eiements of a typical motor suitable for use with an embodiment of the present invention. The machine has a stator 1 and a pair of rotors 2, each of the rotors 2 has a series of permanent magnets 3, 3a, arranged equidistantly around the periphery of the rotor 2 on a magnetically permeable steel annulus 4. Alternate ones of the magnets 3, 3a, have their north and south poles facing in radially opposite directions. A south pole on one rotor 2 faces a north pole on the other rotor 2 and vice-versa. The rotors 2 and the stator 1 are positioned along a common central axis 5, and the rotors 2 are fixed on a common shaft (not shown) for rotation relative to the stator 1.The stator 1 has a plurality of U-shaped pole pieces 6 opening alternately radially inwardly and outwardly from the axis 5, and equidistantly arranged around the periphery of the stator 1 at the same spacing as the permanent magnets 3, 3a. The pole pieces 6 are mounted on a stator frame, not shown to enhance the clarity of Figure 1. The pole pieces 6 provide magnetic flux paths between the magnets 3, 3a on the two rotors 2, on opposite sides of the stator 1.

The alternating arrangement of the pole pieces 6 provides a magnetic flux path alternately inwardly and outwardly of an annular coil 7 which is mounted to the frame and runs through the straddling pole pieces 6. In response to repeatedly changing the direction of current through the coil 7, the rotors 2 rotate about the axis 5 relative to the stator 1.

Referring to Figure 2, it will be appreciated that, with pole pieces 6 of equal size and shape, the minimum spacing between the pole pieces 6 is determined by the dimensions of the radially inner portion ofthe pole pieces 6. This dimension defines the maximum number of pole pieces 6 that can be arranged on a stator 1 of given radius.

Referring now to Figure 3, part of a stator arrangement showing a stator frame 10 with alternate pole pieces 6a constructed in accordance with an embodiment of the present invention is shown to the same scale as Figure 2. The pole pieces 6a opening radially inwardly toward the axis 5 have a radially inner portion 9 and a radially outer portion 8. The radially inner portion 9 has a thickness in the circumferantial direction less than that of the radially outer portion 8.

Preferably as shown, the radially outer portion 8 of each of the radially inwardly opening pole pieces 6a has a cross-sectional thickness greater than that of any portion of the pole pieces 6 opening radially outwardly from the axis 5, and the radially inner portion 9 of the radially inwardly opening pole pieces 6a has a thickness less than that of any portion of the pole pieces 6 opening radially outwardly from the axis 5. Each of the pole pieces 6, 6a are substantially equal in length and are positioned substantially the same radial distance from the central axis 5 of the stator along a plane perpendiculartothe axis 5. Slightforshortening ofthe radially inner portion 9 of the radially inwardly opening pole pieces 6a may be advantageous in some cases if it desiredfurtherto reduce leakage inductance, so long as the face area of the pole pieces 6a is not substantially reduced.

The instant invention provides a large face area for the pole pieces 6, 6a, thus achieving efficient utilization of the available magnetic flux, while continuing to maintain flux leakage at low levels. Furthermore, it is not necessary either to radially stagger adjacent pole pieces 6a orto substantially radially shorten them.

Other aspects, objects, advantages and uses of this invention can be obtained from a study of the drawings, the disclosure and the appended claims.

Claims

1. A stator for an electromagnetic machine, the stator having an annular coil; a plurality of U-shaped pole pieces mounted in straddling relationship to coil; adjacent ones of the pole pieces opening in opposite radial directions; and the radially inwardly opening pole pieces each having a radially inner portion and a radially outer portion, the radially inner portion having a cross-section thinner than the cross-section of the radially outer portion.

2. A stator according to claim 1, wherein the radially outer portion of the radially inwardly opening pole pieces has a cross-section thicker than the cross-section of the radially outwardly opening pole pieces, and the radially inner portion of the radially inwardly opening pole pieces has a cross-section thinner than the cross-section of the radially outwardly opening pole pieces.

3. A stator according to claim 1 or claim 2, wherein each of the pole pieces is substantially equal in length and is positioned substantially the same radial distance from the central axis of the stator along a plane perpendicular to the axis.

4. A stator according to claim 1, substantially as described with reference to Figure 3 of the accompanying drawings.