GB2032706A - Step-by-step electric motor - Google Patents

Abstract

An electric permanent magnet step-by-step motor has a stator provided with a plurality of slots 106, Fig. 3, 1' 2' 3' Fig. 5A, and coils adapted to generate an alternating succession of north and south poles N, S Fig. 5A. The coils are formed by n independent windings a, b, c each passing respectively through one slot 106 out of every n slots; the flow directions of the currents in the windings follow a pre-established sequence, Fig. 4, to generate 2p poles rotating with a predetermined step. Furthermore, the rotor comprises 2p permanent magnets 112 arranged radially in housings 111 in an iron body, the magnets being polarised at right angles to the radii and having like poles of successive magnets facing one another. <IMAGE>

Description

SPECIFICATION Step-by-step electric motor The present invention, due to the collaboration of Mr. Yves POIROT, relates to electric step-by-step motors of the kind having permanent magnets and comprising, on the one hand, a stator having a plurality of notches and windings adapted to generate an alternating succession of north and south poles and, on the other hand, a rotor comprising a soft iron armature having housing directed radially around the axis, the permanent magnets being disposed in said housings and being polarized in directions perpendicular to the diametrical planes passing through the centres of the magnets, like poles of two neighbouring magnets being disposed facing one another, the flux coming out of or going into the rotor being concentrated in the middle of the gap separating two successive magnets.

In permanent magnet step-by-step motors manufactured up to now, the rotor is generally formed from a bipolar magnet with longitudinal magnetization and the stator comprises a number of coils, mostly four in number, energized cyclically. Because of the fairly small number of coils, the number of steps of advance of the rotor is also small and is in general four steps per revolution.

Though the torques obtained with such motors are relatively high, on the contrary the reduced number of steps of advance constitutes a handicap which makes these motors unsuitable for applications requiring a relatively high number of steps of advance.

On the other hand, in step-by-step motors with variable reluctance, though the work frequencies are higher than for permanent magnet motors, the product of torque multiplied by rotational speed is smaller.

For this reason, these motors are not suitable in cases of use where the product torque multiplied by speed needs to be high.

The invention has essentially as its aim to remedy the disadvantages of motors known up to present by providing a motor of the kind mentioned in the preamble able to operate with a high work frequency while developing a torque which decreases as little as possible when the speed increases and which presents also a power/volume ratio as high as possible.

With these ends in view, the motor of the invention is characterized in that the coils of the stator are formed by n independent windings passing respectively through one notch out of n, respective currents passing through these windings whose directions are determined to a pre-established sequence to generate 2p poles (alternatively north and south) rotating with a predetermined step and in that the number of permanent magnets of the rotor is equal to that of the poles of the stator.

As far as the electrical supply of this motor is concerned, the switching sequence of the currents of each winding and the value of these currents are provided so that the motor has even or substantially even steps. Each winding in its turn is thus traversed by a current varying by stages (staircase variation between two limits, one positive and the other negative, and whose absolute value is the nominal value of the current. This variation is periodic with a period equal to 2"/p (p being the number of pairs of poles) and the number of steps is then 2n(q - 1) per period (n being the number of windings and q being the number of stages of the current).

With this arrangement, the step-by-step motor of the invention presents extremely favourable performances taking into account the high number of steps of advance which it is possible to obtain. In particular, it possesses a torque/speed characteristic which decreases very little and its power/speed characteristic increases in the normal range of use. The result is an exceptionally high power/volume ratio which it is not possible to obtain with step-by-step motors of conventional design.

In one particular embodiment, either the notches and the teeth of the stator, or the magnets disposed in the rotor have the form of a helix portion whose axially opposed ends are staggered angularly in relation to one another by the value of a step of advance of the rotor. This arrangement allows a torqueangle of rotation function to be obtained which is continuous and increasing, which contributes to improving further the operation of the motor of the invention.

The invention will be better understood from reading the description which follows of one preferred embodiment, given pureiy by way of illustration and in nowise limiting. In this description reference is made to the accompanying drawings in which: Figures 1 and 2 are respectively sectional and side views of a step-by-step motor in accordance with the invention, Figure 3 is an enlarged view of a part of Fig. 1, showing the distribution of the magnetic field lines around the rotor, Figure 4 is a diagram showing the sequences for supplying electric current to the windings of the motor of Figs. 1 to 3, Figures 5A to 5L are views showing schematically the step-by-step advance of the magnetic fields generated in the stator, and Figures 6 and 7 show characteristic curves of the motor of the invention.

Referring first of all to Figs. 1 and 2, motor 101 comprises a fixed external part 102, formed by a cover 103 surrounding a stator 104.

The stator 104 is formed by a stack of metal plates cut out so as to present, inwardly, an alternating succession of teeth 105 and notches 106.

In the motor shown in Figs. 1 and 2, teeth 105 and notches 106 are respectively 24 in number.

The stator 104 is provided with a threephase coil 107, formed by three windings electrically independent from each other: these windings are disposed in notches 106 so that each of them passes through one notch out of three. Furthermore, they are supplied with current in a very distinct way as will be explained further on.

The currents passing through these windings generate a succession of north and south poles alternately.

Inside stator 104 can be seen a rotor 108 mounted on a shaft 1 09. The rotor is formed by a soft iron polar mass 110 provided with radial housings 111 in which are disposed permanent magnets 112, here eight in number. The choice of material for forming the magnets (ferrite composed of rare earths such as Samarium-Cobalt) depends on the characteristics sought for the motor.

The polarization of each magnet is transversal, i.e. perpendicular to the diametrical plane which passes through its centre, and the magnets are disposed so that like poles of two successive magnets are opposite one another.

In Fig. 3, there is shown the distribution of the magnetic field lines 11 3 which result from this arrangement: the field lines of two adjacent magnets combine and create a resultant field centred on the radius equidistant from the two adjacent magnets.

Thus, the rotor is set so that, for example, one of its zones which is the seat of a maximum field corresponding to a south pole is opposite a tooth 105 of the stator which is the seat of a magnetic field corresponding to a north pole, that is to say that the middle of the gap between two south poles of two adjacent magnets of the rotor is opposite a tooth of the stator.

The operation of the motor of the invention will now be explained with reference to Fig. 4 and Figs. 5A to 5L.

Fig. 4 shows the variation of the currents flowing in windings a, b and cof the coil of the stator during the succession of sequences A, B, C . . ., L.

As for Figs. 5A to 5L (letters A to L corresponding to the sequences A to L of Fig.

4), they represent schematically, in a developed view, the coil of the stator, each winding a, b and c being limited, for sake of clearness, to its last turn which is an open turn. In Figs.

5A to 5L, the teeth of the stator are designated by numbers 1 to 24, only the first 1 3 being shown, and the notches are designated by numbers 1', 2' . . ., notch 1' being be- tween teeth 1 and 2, notch 2' between teeth 2 and 3, etc.

The electric supply for the three windings a, b and C is such that the current that flows in one direction in a winding, reduces to zero then changes direction, and this cyclically for the three windings.

During sequence A, the three currents in the respective windings a, b and c, flow in the same direction, for example, the positive direction, as shown in Fig. 4, sequence A.

Thus, taking into the account the general rules of electromagnetism, in Fig. 5A, the open turn of winding a which passes through notches 1' and 4' generates a north pole centred on tooth 3; the open turn of winding bwhich passes through notches 2' and 5' generates a north pole centred on tooth 4; and the open turn of winding cwhich passes through notches 3' and 6' generates a north pole centred on tooth 5. The result in the aggregate is a north pole centred on tooth 4.

Similarly, the open turn which passes through notches 4' and 7' generates a south pole centred on tooth 6; the turn passing through notches 5' and 8' generates a south pole centred on tooth 7; and the turn passing through notches 6', 9' generates a south pole centred on tooth 8. The result in the aggregate is a south pole centred on tooth 7.

Thus it can be seen that there exists a pole (north or south) centred on one tooth out of three; there are then eight poles distributed peripherally of the stator.

During sequence B which follows, the current reduces to zero in winding a, the currents in windings b and c continuing to flow in the same direction (Fig. 4, sequence B).

Thus, in Fig. 5B, the open turn of winding a which passes through notches 1' and 4' generates no magnetic field; only the open turns of windings b and cwhich pass respectively through notches 2', 5' and 3', 6' generate north poles centred respectively on teeth 4 and 5. The result in the aggregate is a north pole centred on the gap between teeth 4 and 5, i.e. centred on notch 4'.

Thus, between sequences A and B, all the poles have advanced by half a gap between notches.

During sequence C, a current flows again in winding a, but in a reverse direction to that which it had during sequence A. The currents in windings b and c continue to flow in the same direction as during sequence B (Fig. 4, sequence C).

The result is that, in Fig. 5C, the open turns of windings a, band cwhich pass respectively through notches 4', 2' and 3', on the one hand, and 7', 5' and 6', on the other, are traversed by currents in the same direction which generate, in the aggregate, a north pole centred on tooth 5.

Thus, between sequences B and C, all the poles have again advanced by half a gap between notches.

During sequence D, it is the current flowing in winding b which reduces to zero, the currents of windings a and c continuing to flow in the direction which they had during sequence C; therefore the currents of windings a and cflow in reverse directions with respect to each other (Fig. 4, sequence D).

Using arguments similar to those above, it is established in Fig. 5D that the open turns of windings a and cwhich pass respectively through notches 4' and 3', on the one hand, and 7' and 6', on the other hand, generate, in the aggregate, a north pole centred on the gap between teeth 5 and 6, i.e. centred on notch 5'.

During the following sequence E, the current is re-established in winding b, but it flows in a direction opposite to its preceding direction (sequence A to C), the currents of windings a and c keeping their respective flow directions (Fig. 4, sequence E).

The result is, in Fig. 5e, that all the poles have again advanced by half a gap between notches and that, for example, the north pole considered previously is now centred on tooth 6.

Similarly, the current then reduces to zero in winding cduring sequence F, then changes direction of flow during sequence G, the currents of windings a and b maintaining their same respective directions as during sequence E.

The result is that the poles again advance by half a gap between notches and that in particular the north pole which was centred on tooth 6 during sequence E, is centred on notch 6' during sequence F then on tooth 7 during sequence G.

During sequence H, the current flowing in winding a again reduces to zero; then it changes direction during sequence I, now flowing in the same direction as during sequence A.

Then it is the current flowing in winding b which reduces to zero (sequence J), then which changes direction (sequence K). Finally, the current of winding c reduces to zero (sequence L).

For each sequence, the north and south poles advance by half a gap between notches and, finally, in sequence L, the north pole which, initially was centred on tooth 4, is now centred on notch 9'.

In sequence M, succeeding sequence L, the current of winding C again changes direction and becomes positive again, so that sequence M is identical to sequence A: the same distribution of north and south poles is renewed.

With this arrangement of the motor, the rotor effects a complete revolution while advancing by half a gap between notches for each sequence, i.e. a complete revolution in 48 steps; for each step it effects a rotation of 7"30'.

The performance of the motor of the invention are shown by the characteristic curves of Figs. 6 and 7 relating to a rotor equipped with permanent magnets made from Samarium-Cobalt Sm (Co)5.

The curves 201 to 204 of Fig. 6 give the starting frequency expressed in Hertz (ordinates) with respect to the inertia of the load, expressed in g.cm2 (abscissa), for a given frictional couple. Curves 201 to 204 correspond respectively to friction couples of Om.N, 0.5m.N, 1m.N and 1.5m.N.

For example, a friction couple less than 1.5m.N and with a load presenting an inertia of 5000 g.cm2, the motor can start up directly at 250Hz, i.e. 250 steps per second.

In Fig. 7, curves 205 and 206 give respectively the variation of the torque, expressed in kg.cm, and of the power, expressed in Watts, with respect to the rotational speed (abscissa) expressed in Hertz or revolutions/minute.

It can be seen that the torque decreases very little when the speed increases and that the power increases over the whole range of speed variation.

These particularly favourable characteristics allow power/volume ratios of the motor to be obtained which are extremely advantageous and much higher than those obtained with step-by-step motors of conventional design.

To further improve the operation of the motor and to make the torque-angle of rotation function continuous and increasing, it is desirable for the notches and the teeth of the stator to be wound along a portion of a helix and for their axially opposed ends to be angularly staggered in relation to each other by a step of advance of the rotor, i.e. by 1 /48th of a revolution in the embodiment considered here.

Of course, this arrangement could just as well be replaced by that consisting in retaining rectilinear teeth and notches, but giving the form of a portion of a helix to the magnets exposed in the rotor.

In the preceding description, it has been assumed that the number of windings of the stator was three, which generates eight poles alternatively north and south, which implies that the stator is provided with 24 notches.

Although it is the arrangement which seems to give the easiest manufacture, particularly insofar as the machining of the notches is concerned, it is nevertheless true that the invention is not limited to a motor fitted with three windings, and a number of windings n different from three may be contemplated; the number of poles alternatively north and south is then 2p. While retaining a sequence of current switching in the n windings similar to that mentioned above and shown in Fig. 4, at least n-1 windings being fed simultaneously, the step of advance remains half a gap per notch, and the rotor makes a complete revolution in 2 x (n X 2p) steps, i.e. 4 np steps.

More generally still, the feeding sequence of the motor may be different from that considered up to now, while however arranging for the switching sequence of the currents in each winding and the value of these currents to be such that the motor advances by even or practically even steps. Each winding in its turn is thus traversed by a current varying by stages (staircase variations) between two limits, the one positive and the other negative, and whose absolute value is the nominal value of the current. This variation is periodic with a period of 2w/p and the number of steps is 2n(q - 1) per period, q being the number of stages of the current.

As is evident and as it follows moreover already from what has gone before, the invention is in nowise limited to those of its modes of application and embodiments which have been more specially considered; it embraces, on the contrary, all variations thereof.

Claims (7)

1. An electric permanent magnet step-bystep comprising, on the one hand, a stator having a plurality of notches and a coil adapted to generate an alternating succession of north and south poles and, on the other hand, a rotor comprising a soft iron armature having housing directed radially about the axis, the permanent magnets being disposed in said housings and being polarized in directions perpendicular to the diametrical planes passing through the centres of the magnets, the like poles of two adjacent magnets being disposed opposite one another, the flux coming out of or going into the rotor being concentrated in the middle of the gap separating two successive magnets, characterized in that the coils of the stator are formed by n independent windings passing respectively through one notch out of n, these windings having passing therethrough respective currents whose directions are determined according to a pre-established sequence to generate 2p poles (alternatively north and south) rotating with a predetermined step, and in that the number of permanent magnets of the rotor is equal to that of the poles of the stator.

2. A step-by-step motor according to claim 1, characterized in that the currents passing through the windings vary by stages (staircase variations) between two limits, one positive and the other negative, and whose absolute value is the nominal value of the current, this variation being periodic with a period 2w/p and the number of steps being 2n(q - 1) per period, (q being the number of stages of the current).

3. A step-by-step motor according to claim 2, characterized in that the number of stages of the currents is three and in that a current, which flows in one direction, reduces to zero in one sequence, then changes direction in the following sequence, and this cyclically by the n windings, each step of advance of the magnetic fields being then half a gap between notches and the rotor making a complete revolution in 4np steps.

4. A step-by-step motor according to claim 3, characterized in that the windings are three in number, in that the rotor comprises eight permanent magnets, in that the stator comprises 24 notches and in that each step of advance is equal to 1 /48th of a revolution.

5. A motor according to any one of the preceding claims, characterized in that either the notches and the teeth of the stator, or the magnets disposed in the rotor have the shape of a helix portion whose axially opposed ends are angularly staggered in relation to each other by the value of a step of advance of the rotor.

6. A step-by-step electric motor substantially as hereinbefore described with reference to the accompanying drawings.

7. Any novel feature or combination of features herein described.