IE52521B1 - Improvements in step-by-step motors with multipolar magnet - Google Patents

Abstract

Stepping motor comprising a multipolar rotor (1), a stator (601,602) combined with the rotor and means for generating, for each phase, a magnetomotive force. The invention comprises a single coil to generate the magnetomotive force, and means (603) configured for dividing this magnetomotive force which is applied to the poles of the rotor.

Description

The present invention relates to a step-by-step motor comprising a multipolar rotor, a stator associated with the rotor and means for producing, for each phase, a magnetomotive force.

Known motors of this type generally comprise, coaxially associated with the rotor, an annular stator housing containing the energizing coil, for each phase, and comprising a plurality of polar teeth or tongues which may lie between each other and be adapted to cooperate magnetically with the rotor.

The number of steps that may be made by the rotor of such a motor in the course of a complete revolution being a function of the number of polar teeth of the stator, there are numerous applications, particularly in the domain of servo-control and automatic devices where, to obtain the greatest possible precision in the positioning of certain servo-controlled members (measuring systems, printers, etc.), the greatest possible angular resolution, i.e. motors which can make a large number of steps per revolution of the rotor, must be available.

To obtain this high resolution, conventional practice consists in increasing as much as possible the number of -3polar teeth of the stator and oorrelatively, of course, the number of rotor dipoles.

However, for a given motor of which the geometric polar structure has been adapted optimally with the characteristics of the magnetomotive source, it is observed that the operation consisting in increasing the number of stator poles can only be carried out to the detriment of the motive power available on the rotor, i.e. with an appreciable decrease of the motor torque even though it is sought, within the tolerable limits of heating and of demagnetization of the rotor, to compensate this loss of motive power by an increase in the electrical energizing power.

In fact, the increase in the number of steps, and consequently the decrease of the angular value of each of these steps, necessarily brings about a decrease of the width of each stator polar tooth, as well as a reduction in the space separating two consecutive teeth. Thus, the polar teeth being of supposedly unchanged thickness and height, their mutually opposite lateral edges consequently remaining identical, the reluctance of the stator circuit decreases in proportion to the space between teeth to the point that the increase in the leakage flux resulting therefrom brings about magnetic saturation of the teeth of which the section has been reduced.

This phenomenon of saturation of the stator polar teeth -4therefore constitutes a limit to the increase in the motive power of a given motor of which it is desired to increase the number of steps.

When, despite this limit, it is desired that the available motive power be greater than that which it allows to be attained for a determined number of steps, it is then necessary, in known manner, in the first place, to reduce the height of the stator polar teeth of the given motor in order to postpone the appearance of the detrimental phenomenon of saturation then, in the second place, this reduction in polar height allowing the decrease in the axial height of the motor, to compensate for the loss of available motive power by coupling on the same shaft as many identical motors as are necessary for obtaining the desired torque.

This known method obviously presents the drawback of high production costs since the number of the technological elements constituting the step-by-step motor unit procuring the desired motive power must be multiplied by as many times as it is desired for example to double the number of steps.

It is precisely an object of the present invention to eliminate this drawback by providing a technologically simple and economical means for maintaining at its original value the available motive power of a step-by52521 -5step motor with multipolar magnetized rotor whose resolution it is desired to increase by multiplication of the number of polar steps.

To obtain this result, the invention relates to a step-by5 step motor comprising a multipolar rotor, a stator associated with the rotor and means for producing a magnetomotive force for each phase, characterized in that said motor comprises one coil only, for producing the magnetomotive force, and means arranged to divide this magnetcmotice force which is applied to the poles of the rotor.

The invention will be more readily understood on reading the following description with reference to the accompanying drawings, in which: Fig. 1 shows the diametrical section through a stepby-step motor which, according to the known technique, comprises a double stator.

Figs. 2 and 3 show how, on a developed diagram of stator multipolar system, the reluctance and section of the polar teeth of a known motor evolve when the angular value of a step is reduced by half.

Fig. 4 shows, partly in perspective, the structure of the stator multipolar system associated with the rotor of -6a step-by-step motor improved according to the invention.

Referring now to the drawings, the step-by-step motor of Pig. 1, whose structure is well known, comprises a cylindrical rotor 1 whose shaft 2 to which it is connected by a rim 3, pivots in the bearing 4 fast with a support 5.

The rotor, which is, for example, made of hard ferrite, is magnetized so as to present, along the peripheral generatrices, a succession of alternate magnetic dipoles. The rotor 1 is surrounded by a first stator housing 6, generally annular in form, which is fast with the support 5 and which contains an energizing coil 7 with multiple windings to allow supply from a generally polyphased electrical source.

The magnetic circuit, constituted by the stator housing 6, is interrupted on its inner cylindrical wall so as to present a multipolar system 60 constituted by a plurality of teeth which lie between one another and determines, along the generatrices, air spaces which are the seat of a leakage flux produced by the magnetomotive force of the coil and peripherally oriented by the direction of the energizing current, so as to cooperate with the flux of the magnetized dipoles of the rotor 1.

This known arrangement, which is illustrated in Pigs. 2 and 3, does not make it possible, as will be seen -7hereinafter, to increase the number of steps of a given motor without loss of torque. This is why, when it is desired, for example, to double the number of steps of a motor of given size, the elementary stator structure (stator housing 6’, energizing coil 7') must also be doubled as well as, of course, the length of the rotor 1, in the same proportion, as shown in Fig. 1.

To explain this necessity, Figs. 2 and 3 must be referred to, which show how, in the multipolar systems 60 and 60', the reluctance and the section of the stator polar teeth of a given motor evolve when the step is reduced, for example, by half.

The leakage reluctance Rf of the stator circuit in question (Fig. 2) is inversely proportional to the accumulated surfaces of the sides of opposite polar teeth and proportional to the distance e, separating these sides.

For an identical radial thickness of the stator polar pieces, the opposite surfaces of the polar teeth sides are substantially proportional to the lengths L of these sides.

On the other hand (Fig. 3) when the number of steps is doubled, by reducing the width of the polar teeth by half, the useful iron section S at the base of these teeth is -8consequently also reduced by half, as well as the intermediate distance β2· It follows that, the length L of the opposite teeth sides remaining equal, the stator reluctance decreases substantially by half. Thus, the increase in the induced flux 3$ ' inversely proportional to the reluctance, provokes the increase in the induction B = * and therefore, a saturation at the level of the base of the polar teeth of reduced section S and, by the limitation of the leakage flux between teeth which results therefrom, a limitation of the magnetic torque exerted on the rotor.

It will therefore be necessary, in order to eliminate the detrimental phenomenon of saturation and to improve the energy yield of the motor, to reduce the height of the teeth as well as the height of the rotor for a given magnetomotive force furnished by the same volume of copper of the coil.

The lowering of the height of the motor results in an appreciable reduction of the motor torque, since this torque depends particularly on the motor height (about 50% for a doubling of the number of steps) which leads to compensating this loss by the addition on the same shaft of a supplementary motor each time it is desired to double the number of steps. -9It is precisely to avoid this expensive and complicated multiplication of the elements constituting a group of a plurality of motors with a large number of steps that the invention provides, as shown in Fig. 4, the interposition, between two end rings 601, 602 of stator polar teeth of a step-by-step motor with single coil (not shown), of an intermediate ring 603 made of ferromagnetic material. The ring 603 comprises two series of oppositely directed polar teeth 604, 605 and in a number equal to that of the poles of the rotor and of the stator. Each series of polar teeth of the intermediate polar piece has a relative angular setting coinciding with the angular setting of the dipoles of the rotor, as well as with that of the end polar teeth opposite the stator.

This intermediate ring, whose function essentially consists in subdividing the magnetic potential and to some extent in relaying between the end polar teeth of the stator the flux produced by the source of magnetomotive force, or, in other words, in dividing the magnetanotice force applied to the poles of the rotor, makes it possible, by optimalizing the relative length and width of the polar teeth, as has been explained hereinabove, to eliminate the phenomenon of saturation detrimental to maintaining the motor torque at its maximum value.

The intermediate ring 603 is mechanically maintained in position between the end rings on the inner wall of the -losing le coil by an appropriate, conventional technological means such as, for example, by force-fitting, adhesion or insertion by casting of the plastics material constituting the shell of the coil.

Theoretically, there is no limit to the multiplication in coaxial series of the intermediate stator rings 603 in order to increase the torque at the same time as the length of the motor and its constituent elements (single coil, multipolar rotor, stator housing), increases.

Finally, it must be considered that each intermediate ring 603 with double series of polar teeth (whose shape and relative angular setting are determined only by the inclination or angular setting of the rotor dipolar generatrices) constitutes an auxiliary stator energized in series by the magnetomotive force of the single coil.

This arrangement forming the subject matter of the invention, essentially makes it possible to avoid the saturation of the polar teeth of reduced section by the leakage flux circulating in the spaces between teeth.

Under these conditions, it is possible simply and economically to increase the resolution of a step-by-step motor of given type with double direction of rotation, by increasing the number of its stator polar teeth, without affecting the electromechanical characteristics thereof.

Claims (6)

1. Step-by-step motor comprising a multipolar rotor, a stator associated with the rotor and means for producing a magnetomotive force for each phase, wherein the motor

2. The step-by-step motor of Claim 1, wherein the means 10 for dividing the magnetomotive force comprise at least one intermediate polar piece made of ferromagnetic material, interposed at equal distance between two end stator polar pieces of the motor.

3. The step-by-step motor of Claim 2, wherein the 15 intermediate polar piece presents on either side of its median plane a series of polar teeth of shape and dimensions similar to those of the polar teeth of the end stator pieces of the motor, and in equal number.

4. The step-by-step motor of Claim 3, wherein each series 20 of the polar teeth of the intermediate polar piece presents a relative angular setting coinciding with the angular setting of the dipoles of the rotor, as well as with that of the opposite end polar teeth of the stator. -125. The step-by-step motor of one of Claims 1 to 4, wherein the means for dividing the magnetomotive force are mechanically fixed on the inner wall of the single coil.

5. Further comprises a single coil for producing the magnetomotive force, and means arranged to divide this magnetomotive force which is applied to the poles of the rotor.

6. A step-byr-step motor substantially as hereinbefore 5 described with reference to and as illustrated in the accompanying drawings.