LINEAR ELECTRIC MOTOR POWERED BY THREE-PHASE ALTERNATING CURRENT

Abstract: In the linear electric motor (10), permanent magnets (12) are anchored to and uniformly distributed on a fixed part (11) of the motor, and a geometry of a moving coil block (13) is optimised with respect to the magnets (12) so that a very low oscillation occurrence during movement, or "cogging" obtains. The coils (14) of the coil block (13) are mounted on ferrous-magnetic strips constituting cores (15), teeth of which cores (15) are distanced one from another at a pitch which is greater than a pitch of the magnets. Each coil block (13) is made up of 6 coils, or of a multiple of 6, while the magnets (12) of a sector located opposite the coil block (13) are arranged in sequences of equal length, comprising 7 magnets (12) or a multiple of 7.

Description: Description Linear Electric Motor powered hy Three-phase Alternating Current- Technical Field Linear electric motors have recently found application in the field of machine tools. Background Art Linear electric motors are equipped with permanent magnets, arranged uniformly on the fixed part of the motor, and coil blocks, bearing coils mounted on various cores, which coil blocks are mobile in a parallel direction to the fixed magnets. The coil blocks are kept at a constant distance from the magnets by a mechanical arrangement based on the use of special longitudinal sliding guides. In this category of motors, one of the main technical tasks facing designers is how to make sure that the movement of the coil block is as uniform as possible. Consideration has to be made of the fact that the motion of the coil block is greatly influenced by the high force of attraction developed between the fixed part and the mobile part of the motor, thanks to the strong magnetic field generated by the special magnets used. There is in practice a marked tendency to "cogging", the term used to describe a snagging of the moving part at positions of maximum attraction exerted on the moving part (the coil block) by the fixed magnets. In high-precision work this is to be avoided at all costs. Various solutions have been proposed regarding the cogging problem. Many involve a relative arrangement of the coil block and the fixed magnets in which during a run of the moving part, a significant number of cores never directly face a same number of magnets. For example, patent publication no. W099/41825 illustrates a solution which is substantially based on the fact that the moving part of the motor is made up of blocks of nine coils, each of which blocks is eight times the pitch of the fixed magnets and is separated from an adjacent block by a distance equal to 2/3, 1/3 or 1/2 of the magnet pitch. With the above solution, the aim is to limit the above-described drawback by creating a design in which the distance, or pitch, between the cores of the moving part is smaller than the distance between the fixed magnets; this ratio is 8/9. Obviously newer solutions to the above problem are under development and are effective also in relation to other problems, such as constructional simplicity, low moving part inertia, advantageous costs and so on. The main aim of the present invention is to provide a three-phase alternating- current linear electric motor, where the cogging phenomenon (lack of smoothness in advancement of the moving part), typical of this type of motor, is practically eliminated. A further aim of the invention is to provide a three-phase alternating-current linear electric motor having characteristics which are suitable for use in machine tools and automatic machines, and in particular having low inertia in the moving part. The above aims are all achieved with the three-phase alternating-current linear electric motor of the invention, in which a plurality of magnets are anchored to the fixed part of the motor, and are uniformly distributed at a predetermined distance one from another, and in which a plurality of coils are mounted on cores of a coil block, which block is mobile in a direction which is parallel to the fixed magnets and at a constant distance therefrom; the coils being in blocks of 6 or multiples of 6 and being iiniforrnly distanced one from another; the length of each coil block being the overall "pitch" of the coils and in fact being equal to a length of 7 fixed magnets, or multiple of 7, this being considered the overall "pitch" of the fixed magnets. Alternatively, the fixed part can be constituted by the coil block, and the mobile part by the plurality of magnets. Advantageously, the ratio between the pitch of the coils and that of the magnets, expressing this pitch in electrical degrees, can vary between 200° (coil pitch)/180° (magnet pitch) and 220° (coil pitch)/180° (magnet pitch), all considered within the scope of the present invention. The ideal value of the above-mentioned ratio is obviously 210° (coil pitch)/180° (magnet pitch), which is equivalent to a 7/6 ratio, defined above with reference to the number of magnets and the number of coils comprised within a same length of a tract of the linear motor. While the coil/magnet geometry is repeated in multiples of 6 or 7, following the above-mentioned ratio, the sequence of connections between the coils is repeated, preferably in multiples of 12; where, with R, S, T denoting the three supply phases, the coils are ordered thus, for example: R,-R, -T, T, S, -S, -R, R, T, -T, -S, S. According to the above-mentioned constructional characteristics, a linear motor is obtained in which the single pitch between the single cores of the moving coil block is greater than the single pitch between the single fixed magnets, and the ratio is comprised between a specific interval, with the result that there is a considerable reduction in the cogging phenomenon and the motor is therefore particularly suitable for precision work, thanks to the simplicity and low inertia values of the moving group with respect to the solutions where the number of coils is greater than the number of magnets. Disclosure of Invention Further characteristics and advantages of the present invention will better emerge from the detailed description that follows of a preferred but non-exclusive embodiment of the invention, illustrated purely by way of non-limiting example in the accompanying figures of the drawings, in which: figure 1 is a frontal view, in section, of a tract of a linear electric motor according to the invention; figure 2 is a front view of a component of the motor of figure 1; figure 3 is a perspective view, partially sectioned, of the structural part of the linear motor of figure 1 ; figure 4 is a front view, in section, of a further embodiment of the linear motor of the invention. With reference to figure 1 of the drawings, a tract of linear motor is denoted by 10. The fixed part 11 exhibits a series of permanent magnets 12, arranged at a uniform distance one from another. The magnets are numbered in the figure illustrating this embodiment, from 1 to 14, and the pitch is denoted by Tm. 13 denotes the coil block, which is the moving part of the motor, located opposite the fixed part 11. This coil block 13 can move in two directions A and B according to the three-phase sequence in the coils 14 mounted on the cores 15 of the moving structure. The cores 5 of the coil block 13 are numbered in the figure illustrating this embodiment from 1 to 12, and the pitch is denoted by Td. The pitch is calculated from the sum of the distance, Tx, between the sides of the median spire of a generic coil 14, plus the distance Ty between the sides of the median spires of two adjacent coils. From the above it can be observed that if the fixed part 11 and the coil block 13 are of equal length, the number of magnets will be 14 and the number of coils will be 12. Thus, considering that the distance, or pitch Td, between the coils is 1/12 of the length, and that the distance, or pitch Tm, is 1/14 of the same length, then the ratio between pitches Td and Tm will be 14/12, i.e. 7/6. This ratio can also be expressed considering the electrical phase displacement between two coils 14 and two magnets 12. While the phase displacement between two adjacent magnets 12 of opposite polarity is obviously 180°, the displacement between two adjacent coils 14, considering the length of the polar sector between two consecutive axes of the coils 14 is (180*7/6)=210; thus, the ratio between the displacement between two adjacent coils and the displacement between two adjacent magnets is 2107180°, and this ratio is obviously 7/6, as previously stated. It is worthwhile noting here that independently of the embodiment herein considered, the validity of the solution remains intact as long as the distance, expressed in degrees of electric displacement, between two contiguous coils in a linear motor according to the present invention, is between 200° and 220°. From experimental evidence it emerges that in linear electric motors in which the coils have been distanced at an electrical angle falling within the above range, any lack of uniformity in motion is of an irrelevant entity. In figure 2 a component of the coil block 13 is illustrated. A ferrous-magnetic strip 16, is part of the nucleus of the coil block. Obviously it is coupled with similarly-made strips and is anchored by screws or similar in the recesses 17 to an upper plate 18 which, as can be seen in figure 3, is fixed to slides 19 which slide along longitudinal guides 20 solidly constrained to the fixed structure 11 of the motor. The cores of the coil block 13, in the present embodiment, which are in effect 12 teeth around which the coils 14 are wound, are supplied with three-phase alternating current, with an order (denoting the phases by R, S and T) which can be as follows: R,-R, -i, T, , - , -R, R, T, -i, -S, or -R, R, T,-T, -S, , R,-R, -T, T, ,-S or -T, T, , -S, -R, R, T, -1 ,-S, S, R, -R or T,-T, -S, S, R,-R, -T, T, S -S, -R, R or S, -S,-R, R, T, -T,-S, S, R, -R, -T, T or -S, S, R,-R, -T, I, S -b, -R, R, T,-T. There are fiirther possible solutions, with cores composed of 6 coils or multiples thereof. One of these examples is illustrated in figure 4; it is made up of 6 coils 14', mounted on 6 cores 15', of a moving coil block 13'. The opposite magnet block, equal in length to the coil block 13', comprises 7 uniformly-distanced magnets 12'. In this embodiment the pitches between the cores of the coil block Td and the magnets of the fixed block Tm are the same as those of the embodiment of figure 1; but these distances could be different without invalidating the solution proposed by the present invention, as long as the ratio was maintained unaltered. In particular, the ideal value of this ratio, as we have seen, is 7/6 or, expressed in electric phase displacement terms, 2107180°; it could also be between 2007180° and 2207180°. The structure of the motor of the invention can differ from the one illustrated by way of example in figure 3; for example, the coil block 13 could be anchored differently to the fixed part 11, with perhaps a greater number of parallel guides used. Other solutions could be applied to the mechanical connections among the various structural organs of the motor, while still conserving the relative geometry between coils and magnets. The moving part could also be made from more than one block made up of 6 coils or multiples thereof, arranged consecutively along the direction of motion. The distance between the blocks could be optimised according to various parameters, depending on the specific field of application of the invention. In any case, the invention offers a considerable reduction in the occurrence of cogging in comparison to other known solutions, as well as giving a more immediate response to electrical commands. Thanks to the special structural configuration, the motor is extremely simple, the moving part being much lighter than in other known motors; production is also more economical.

Claims: Claims. 1). A linear electric motor (10) powered by three-phase alternating current, comprising a plurality of magnets (12) anchored at a uniform and predetermined pitch to a fixed part (11) of the motor (10), a plurality of coils (14) mounted on cores (1 ) of a coil block (13), which coil block (13) is mobile in a parallel direction to the magnets (12) and at a constant distance therefrom; wherein the plurality of coils (14) composes blocks of coils formed by six individual coils or multiples of 6, the coils being uniformly distanced one from another, a length of each block of coils being equal to 7, or a multiple of 7, times a pitch of the magnets (12). 2). A linear electric motor (10) powered by three-phase alternating current, comprising a plurality of magnets (12) anchored at a uniform and predetermined pitch to a fixed part (11) of the motor (10), a plurality of coils (14) mounted on cores (15) of a coil block (13), which coil block (13) is mobile in a parallel direction to the magnets (12) and at a constant distance therefrom; wherein the plurahty of coils (14) composes blocks of coils formed by six individual coils or multiples of 6, the coils being uniformly distanced one from another, and wherein a ratio between a distance between axes of two adjacent coils of a single block of the plurality of coils (14) and a pitch of two magnets anchored to the fixed part of the motor can be expressed in electrical degrees in a range going from 200° (coil pitchyi 80° (magnet pitch) and 220° (coil pitc_ι)/l 80° (magnet pitch). 3). The linear electric motor (10) of claim 1 or 2, wherein the coil block (13) comprises at least a block composed of 12 coils, a length of which is 14 times an individual pitch of the magnets (12). 4). The linear electric motor of claim 3, wherein the coils are supplied with three- phase alternating current in an order as follows: R,-R, -T, T, S, -S, -R, R, T, -T, -S, S or in a completely opposite order for each coil. 5). The linear electric motor (10) of claim 3, wherein the coils are supplied with three-phase alternating current in an order as follows: -T, T, S, -S, -R, R, T, -T,-S, S, R, -R or in a completely opposite order for each coil. 6). The linear electric motor (10) of claim 3, wherein the coils are supplied with three-phase alternating current in an order as follows: S, -S,-R, R, T, -T,-S, S, R, -R, -T, T or in a completely opposite order for each coil. 7). A linear electric motor (10) powered by three-phase alternating current, comprising a plurality of coils (14) mounted on cores (15) of a coil block (13) which coil block (13) is fixed; a plurality of magnets (12) uniformly distanced according to a determined pitch, the plurality of magnets (12) being mobile in a parallel direction to the plurality of coils (14) and being maintained at a constant distance therefrom; wherein coils of the plurality of coils (14) are gathered in blocks each comprising 6 coils, or multiples of 6, uniformly distanced one from another, a length of each block of coils (14) being equal to 7, or a multiple of 7, times a pitch of the magnets (12).

Inventors: ANCESCHI STEFANO, ANCESCHI, STEFANO
Applicants: ANCESCHI STEFANO, MOTOR POWER COMPANY S R L, ANCESCHI, STEFANO, MOTOR POWER COMPANY S.R.L.
Application year: 2001
Main classification: H02K41
IPC codes: H02K41