GB2194689A - Reluctance motor air gap dimensions - Google Patents

Abstract

In a reluctance motor comprising a stator having a plurality of pole face pairs 10 and a cursor 6 movable through the air gap spaces of the stator and having a plurality of magnetically conductive members 8, the magnetically conductive members 8 are of a thickness corresponding from 0.4 to 0.65 of the width of the total air gap space between facing poles. The magnetically conductive members may consist of metal sheet stacks (Fig. 4) having a relatively low iron saturation factor. The motor may be operated step by step or continuously. Dimensions and form of the magnetically conductive element are specified. <IMAGE>

Description

SPECIFICATION Reluctance motor The present invention relates to a reluctance motor comprising a stator arrangement having at least one winding and including a plurality of pole face pairs each defining an air gap space therebetween, and a cursor having a plurality of magnetically conductive members movable along a path extending between successive air gap spaces.

Motors of this type develop high torque and are therefore particularly useful as step motors.

Already known is a motor of this type in which the cursor is a bell-shaped rotor having a disk-shaped bottom with a plurality of magnetically conductive tongues extending therefrom parallel to its axis. The tongues are movable in an annular air gap of a stator arrangement (US-PS 3,864,588).

In a further known motor of this type, the cursor is in the form of a disk-shaped rotor with a plurality of magnetically conductive tongues extending radially therefrom.

It has already been attempted to improve the acceleration capability of such motors, i.e.

the relationship of force and mass in the case of linear motors, or of torque and inertia in the case of motors with a rotating cursor, by reducing the mass of the cursor. To this purpose it has been tried to connect the magnetically conductive memebrs of the cursor to one another through struts or plastic fillers.

For improving the performance and efficiency of such motors it is also desired to keep the air gap, i.e. the difference between the width of the air gap and the thickness of the cursor, as small as possible, although there are natural limits to this provision imposed by manufacturing tolerances and structural design considerations.

It has now been found tht apart from minimizing the air gap there is still another possibility of improving the motor characteristics, and in particular, the acceleration capability of the cursor.

According to the invention this may be accomplished by the provision that the thickness of the magnetically conductive members is equal to the width of the air gap multiplied by a factor between 0.35 and 0.65, and preferably equal to half the width of the air gap space. It has been found that the force exerted on the magnetically conductive members in the direction of movement is not only dependent on the thickness of the magnetically conductive members, but also on the gap width of the air gap spaces. Principally it should be expected that a maximum acceleration capability of the cursor would be obtained with a magnetically conductive member of the maximum possible thickness and an air gap space of minimum width.Its has been experimentally found, however, that there is a pronounced maximum of the force exerted on the magnetically conductive members in the direction of movement when the gap width of the air gap space is twice the thickness of the magnetically conductive members. The width of the air gap space is to be understood in this context as designating the distance between the pole faces of each pole face pair.

A further considerable reduction of the mass of the cursor without noticeable reduction of the force exerted thereon by the stator may be achieved by the provision that the magnetically conductive members are composed of laminated magnetically conductive metal sheets disposed in planes extending parallel to the field lines in the air gap spaces and separated from one another by non-magnetic interspaces. The thickness of these interspaces may be up to ten time the thickness of the magnetically conductive metal sheets without a reduction of the force to be generated by more than 10 percent.

In one embodiment of such laminated magnetically conductive members the invention provides that the magnetically conductive members are formed by coiling or folding a ribbon-shaped magnetically conductive material in a manner to leave intervening spaces therebetween. As an alternative the magnetically conductive members may consist of a bonded magnetically conductive powder.

Preferred embodiments of the invention shall now be described by way of example with reference,to the accompanying drawings, wherein: Figure 1 shows a diagrammatical perspective view of the cursor of a linear motor cooperating with a pole pair, Figure 2 shows a diagrammatic exploded perspective view of the main components of a linear motor, Figure 3 shows a cross-sectional view of a linear motor according to Fig. 2, taken through the air gap spaces of a stator half in a plane parallel to the field lines of the air gap spaces and to the direction of movement of the cursor, and Figure 4 shows a modified embodiment of a cursor.

Fig. 1 depicts the essential characteristics of a linear motor for explaining the principle underlying the invention. The cursor comprises a rail-shaped non-magnetic support member 1 with a plurality of plate-shaped magentically conductive members 2, also to be referred to as "ferromagnetic zones", embedded therein at spaced intervals. Movement of the cursor causes magnetically conductive members 2 to pass through an air gap space 3 between poles 4 and 5 of a stator, the winding of which is not shown. When a magnetically conductive member 2 is at a location outside air space gap 3, the field lines extending in air space gap 3 on energization of poles 4 and 5 will tend to pull magnetically conductive member 2 into air gap space 3 to the position shown in Fig. 1.At a given field strength the maximum force acting on the magnetically conductive members in the direction of movement of the cursor is proportional to the volume of the magnetically conductive member and dependent on a demagnetization factor which has, however, a practically constant value in the case of a flat disk or the like.

Hitherto it was throught sufficient to keep the width of the air gap in a given stator, i.e.

the sum of the two gaps of the air gap space remaining between the cursor and the adjacent pole faces, as small as possible for generating a maximum motoric force to act on the cursor. It has been unexpectedly found, however, that the maximum possible force is achieved when the width of the air gap space, i.e. the distance between the pole faces of the two poles 4 and 5, is about twice the thickness h of the magnetically conductive members 2.

Other dimensions such as the width b of a magnetically conductive member in relation to the distance b' between successive magnetically conductive members may be selected in a conventional manner as applied to reluctance motors.

Shown in Fig. 2 are the essential components of a linear motor having a cursor including two places 6 and 7 the opposite longitudi nal edges of which are formed with a plurality of magnetically conductive teeth 8. Adjacent teeth 8 of each plate are spaced at equal distance b' from one another.

The stator arrangement consists of twelve stator blocks arranged in two spaced parallel rows of stator blocks 11, 12...16 and 21, 22...26, as shown in Fig. 1. With a view to simplification only three stator blocks of each row are shown, the plates being shown at a position separate from the stator blocks.

In a plane perpendicular to the direction of movement of the cursor each stator block has the shape of two letters "C'' disposed one above the other. There are thus two places in which the air gap spaces are located. In each air gap plane each stator block has three pole pairs 10. The center leg of each stator block is provided with a winding only one of which is shown for each stator block row in Fig. 2.

Fig. 3 shows a cross-sectional view of the linear motor taken in a plane extending through the air gap spaces of the first row of stator blocks 11 to 16. As shown in this figure, the distance between adjacent teeth 8 of plates 6 and 7 is equal to the distance between adjacent poles of stator blocks 11 to 16. The distance between adjacent stator blocks is selected in such a manner, however, that when teeth 8 are aligned with the poles of one stator block the poles of the following stator block are in disalignment with respective teeth 8. A linear motor of this type is suiable for use as a step motor. If for instance the winding 31 of first stator block 11 is energized, the teeth 8 closest to the respective pole pairs 10 are pulled into the air gap spaces to the aligned position shown in Fig.

3. Winding 31 is then deenergized, and the electric current is supplied to the winding 32 of the following stator block 12. As the respective teeth are still in disalignment with the air gap spaces of this stator block, they are now pulled into these spaces, causing plates 6 and 7 to be moved to the right by a corresponding distance. This incremental movement continues as the windings of successive stator blocks are successively energized.

Fig. 4 shows a modified embodiment of a cursor plate in which the individual teeth are replaced by corresponding stacks of magnetically conductive metal sheets 17 disposed at spaced locations. In the embodiment of Fig. 4 each stack 18 is formed of four sheets 17. It has been found that the force exerted on the metal sheet stacks 18 is reduced by no more than about 10 to 15 percent as compared to the employ of solid magnetically conductive teeth as in the embodiment of Figs. 2 and 3, when the iron saturation factor is 10 percent, i.e. when the distance between the sheets in the stack is nine times the thickness of the individual sheets.

As depicted on the righthand side of Fig. 4, the individual sheets 1 7 may also be replaced by a single back and forth folded metal sheet 19 to thereby simplify the construction of the cursor plate. A tooth structure having a low iron saturation factor may of course also be obtained by folding or coiling a metal sheet strip in any other suitable manner.

The invention may be applied to any reluctance motor of the type defined in the generic claus of the main claim, that is, also to motors having a disk-shaped or bell-shaped rotor.

In this case the torque may be increased by increasing the number of polse pairs acting at any given time.

The energization of the winding may be controlled in the conventional manner, depending on whether the motor is to be employed as a step motor or as a continuously operating motor. In the latter case a sensor may be provided for feeding the angular position of the cursor back to the control circuit to thereby cause it to be actuated accordingly.

Claims (6)

1. A reluctance motor comprising a stator arrangement having at least one winding and including a plurality of pole face pairs each defining an air gap space therebetween, and a cursor having a plurality of magnetically conductive members movable along a path extending between successive air gap spaces, characterized in that the thickness (h) of said magnetically conductive members (8, 18) is equal to the width of said air gaps multiplied by a factor between 0.4 and 0.65.

2. A motor according to claim 1, characterized in that the thickness of said magnetically conductive members (8, 18) is equal to one-half of the width of said air gap space.

3. A motor according to claim 1 or 2, characterized in that said magnetically conductive members (18) consist of laminated magnetically conductive metal sheets disposed in planes extending parallel to the field lines in said air gap spaces and separated from one another by non-magnetic interspaces.

4. A motor according to claim 3, characterized in that said interspaces have a thickness up to the twentyfold of the thickness of said magnetically conductive metal sheets.

5. A motor accordng to claim 3 or 4, characterized in that said magnetically conductive members (19) are made by winding or folding a ribbon-shaped magnetically conductive material in a manner to leave intervening spaces there-between.

6. 6. A motor according to claim 1 or 2, characterized in that said magnetically conductive members consists of a bonded magnetically conductive powder.