GB2246481A - Stator pole construction in an electric motor with improved torque ripple - Google Patents

Abstract

A commutatorless direct current motor has a substantially cylindrical, external permanent magnet rotor having a plurality of north poles (12) and south poles (13). The rotor surrounds a substantially cylindrical stator (14) provided with a plurality of main poles (10) between which auxiliary poles (15) are interspersed. An annular gap (20) is defined between the rotor and the stator. To reduce magnetic or reluctance caused fluctuation of the motor torque each main pole (10) of the stator has an angular extent which substantially corresponds to that of a rotor pole (12, 13). Furthermore, the stator periphery is provided with protuberances (17) arranged such that permanently magnetically caused components of torque ripple are reduced. Contour corrections 16 are also provided. <IMAGE>

Description

AN ELECTRIC MOTOR WITH IMPROVED TORQUE RIPPLE The present invention relates to an electric motor, for example, to a commutatorless direct current motor, particularly arranged for driving hard disk stores and drives.

Existing motors of this type are generally manufactured as permanent magnet-excited dynamos with a cylindrical air gap and slotted stacked sheets or plates.

They normally have an external rotor, but may also be provided with an internal rotor.

One or more coils are wound on to the multipolar stator, which may also have commutating poles. There is a sequential current flow through these coils across semiconductor switches, normally transistors, and rotor movement is brought about by the interaction with the permanent magnet poles. The rotor position must be determined for a clearly defined switchover of the coils, and generally this takes place in a countactless manner.

For example, magnetogalvanic sensors, particularly in the form of Hall generators, are available for this purpose.

However, the rotor position can be determined using only existing motor elements and without additional sensors. In particular, in the case of a moving rotor, the voltage induced in the coils can be used to determine the actual rotor position. However, an associated evaluation electronic circuit is required for such a motor commutation.

A reduction of the periodic torque fluctuations, which are caused by the interaction between the design of the ferromagnetic stator material and the course of the rotor magnetic field intensity over the rotation angle, can be brought about by the stator design as described in DE-OS 37 23 099 Al and US 4,998,032. The procedure described therein makes it possible to provide an overall torque with low ripple. However, as a function of the angular position of the rotor, in the motor starting phase there are divergences from the average starting torque.

Another method for reducing torque fluctuations of the aforementioned motors is described in EP-291 219.

According to the procedure described in this specification, a number of stator poles, 3(2n+l), and a number of rotor poles, 3(2n+l)±1, are provided for a polyphase motor, that is, the number of stator poles is kept approximately the same as the number of rotor poles. This also reduces torque fluctuations during starting or idling and is indicated by the upper envelope curve of Figure 7.

However, in general this method requires at least nine wound coils per motor, whereas the first described arrangement only requires, for example, six coils and is therefore much less expensive from this standpoint. In addition, in the method of the European specification there are normally periodically occurring radial forces, which have different directions and can cause disturbing motor or engine noise.

Furthermore, in this method the winding direction of the coils is not unitary which means additional costs for the manufacture of the motors.

Another arrangement for reducing electromagnetically caused torque ripple is described in DE-OS 34 32 372, which has a stator arrangement with six main poles and six commutating poles, as well as a rotor with eight magnet poles. This torque behaviour is shown by the (phase-tophase) induced voltage of both phases. However, this arrangement leads to a disturbing ripple of the permanent magnetically generated torque.

The present invention seeks to provide electric motors, and in particular motors of the aforementioned type, having a uniform, e.g. angular position-independent overall torque, both during starting and at the rated speed. This means that the electromagnetically produced torque must have a low ripple, i.e. few fluctuations about a mean value in these two operating states.

In addition, the motors should be inexpensive to manufacture and also have a low noise level. Finally, on supplying current to the stator only force couples (torques) without excess radial forces must be generated.

According to the present invention there is provided an electric motor having a stator and a magnetic rotor, wherein the stator has a number of main poles and commutating poles, the contour shapes of the poles being arranged such that magnetic or reluctance-caused fluctuation of the motor torque is minimized as a function of the angular rotation and wherein the angular extent of each said main pole of the stator substantially corresponds to that of a rotor pole.

The applicants have found that an effective reduction of the electromagnetically caused torque ripple is possible through an appropriate modified design of stator. For this purpose stator commutating poles are provided, whilst the ratio of the stator pole number to the rotor pole number is selected in such a way as to obtain a so-called full-pitch motor winding.

In order to also eliminate permanently magnetically caused components of the torque ripple, a special design of the stator periphery and therefore of the air gap is provided, particulary having cam-like protuberances on the stator poles.

The invention also extends to an electric motor having a stator and a multipolar magnetic rotor, wherein the stator has substantially planar end faces, and at least one of said end faces is greater in dimension than an intermediate cross-section of the stator taken between the end faces, and wherein the stator end faces are magnetically influenced by one or both end faces of the rotor magnet.

Embodiments of the present invention will hereinafter be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 shows the geometrical design of a stator having six main poles and six commutating poles, and of an associated permanent magnet rotor, Figure 2 shows an enlargement of part of Figure 1 detailing the contour of the stator periphery, Figure 3 shows an electrical control circuit for a motor having eight permanent magnet poles and a stator as shown in Figure 1, Figure 4 shows a perspective view of part of the contour of a stator having a sloping stator protuberance ("cam"), Figure 5 shows boundary lines of the magnetization of a rotor in the case of regular and sloping magnetization, Figure 6 shows a perspective view of part of a modified stator having a stator extension plate, Figure 7 is a graph showing electromagnetically caused torque configuration against rotor angular position for a prior art motor, Figure 8 is a graph showing electromagnetically caused torque configuration against rotor angular position for a motor of the invention, and Figure 9 shows a circuit diagram of the connection of windings formed from coils of stator main poles and coils of stator commutating poles.

The specification uses throughout terminology in respect of the torque components of an electric motor as is explained in DE-OS 39 41 553 and US 4,998,032. In particular, these torque components can be differentiated as electromagnetic, permanent magnetic, reluctant or mechanical components.

The arrangement of stator poles and magnetic poles in a motor of the invention is illustrated in Figure 1. As can be seen, a multipolar permanent magnet having north poles 12 and south poles 13 is surround by an associated, external, substantially cylindrical iron yoke 11. The magnet 12, 13 and its associated iron yoke is arranged around, and surrounds a substantially cylindrical, fixed stator 14. An annular air gap 20 is defined between the stator 14 and the permanent magnet. The actual stator 14 is provided with a series of spaced main poles 10 between which auxiliary poles 15 are interposed. The main poles 10 and auxiliary poles 15 are spaced by slots 19. The main poles 10 are each provided with protuberances 17 and contour corrections or extensions 16. The auxiliary poles 15 may also have contour modifications differing from the circular shape if required. The contour shape of a preferred embodiment is shown on a larger scale in Figure 2. At least the main poles 10 are provided with one or more coils wound in bifilar manner. The commutating or auxiliary poles 15 may also be equipped with coils.

A stator main pole 10 extends over substantially the same angle as a magnet pole 12, 13, so that there is a socalled full-pitch winding. This means that there is an almost linear rise or fall of the induced voltage with the angular position of the rotor as can be seen at 83 in Figure 8. This voltage is the phase-to-phase voltage of two phases of the stator, which in all three phases is interconnected in a star connection.

The presence of the auxiliary or commutating poles 15 provides a substantially trapezoidal path 84 for the induced voltage curve, which simultaneously represents a measure for the generated torque against the angular position.

As can be seen in Figure 8, the electromagnetically generated torque from commutating angle 85 to commutating angle 86 only has a limited ripple, the mean value being approximately 98% of the maximum torque, whereas the minimum value is approximately 93% of the maximum torque.

The described, electromagnetically generated torque of the motor is only generated when a current flows through the coils. The alternating moment of the jerking, which has a permanent magnetic origin, is substantially constant.

For the limited ripple of the electromagnetically produced torque, any additional slot jerking, which may be generated by a conventional stator pole shape, constitutes an undesired interference factor. Furthermore, the objective of being able to start the motor from all angular positions with the same movement sequence, may not be attainable.

Accordingly, to reduce slot jerking and to give the required starting characteristics, the stator poles are provided with the contour changes 16 and 17, shown most clearly in Figure 2, to minimize the remaining, permanent magnetically caused torque fluctuations, without producing a detrimental influence on the motor efficiency.

As a result of this interaction of the basic stator shape 14 with the stator contouring shown in Figures 1 and 2, and of the utilisation of an almost rectangular magnetization of the rotor permanent magnet (i.e. a rapid change of the induction sign at the pole boundaries of the rotor), a better electromagnetically generated torque is made available than has hitherto been possible with similar motors.

The motor and its load, for example, an associated, permanently connected or replaceable magnetic memory disk, are accelerated substantially equally well from each starting position. Despite low motor currents, particularly starting currents, the risk of a starting failure is significantly reduced.

Figure 6 shows a further possible development of the contour shaping of the stator which improves on the combination of the basic stator shape 14 with the stator correction contours 16 and 17. As shown in Figure 6, one or more stator poles 63 are given a projection 64, 65 at one, or optionally two, lateral faces (bases) of the stator poles by way of one or more special stator plates.

This configuration is arranged in particular to act on the lower, axially directed end 58 of the rotor magnet, especially where the rotor magnet is hollow and cylindrical. Therefore the cylinder jacket face of the stator plates can largely be given a maximum diameter, which leads to minimum air gap dimensions and therefore to a better torque or better power level for the motor.

The motor efficiency is also improved in that the interaction face between the permanent magnet 50 and the stator is increased by the projecting, additional, stator surface 64. The projection can be given random surface contours 65, 66, which are effective for reducing slot jerking. In particular, a different section line can be given to each plate layer of the projection, so as to obtain the desired path of the permanent magnetically caused torque component.

A further improvement to the motor efficiency may be obtained by providing an additional winding and by wiring the commutating poles 15. The connection of the commutating pole windings is illustrated in Figure 9 in which the commutating pole windings 94, 95 are shown connected in series with the main pole windings 92, 93. In the case of a stator having six main poles and six commutating poles, the main poles through which the current jointly flows are at right angles to the commutating poles to which they are connected.

The connection of the commutating poles, may, of course, be combined with one or more or all of the aforementioned features. Where all the measures detailed above are used, optimum motor characteristics are obtained, particularly a low electromagnetic torque ripple and low permanent magnetic torque ripple.

One or more of the measures described above may also be combined with measures for equalizing the motor torque.

In particular, the current may be supplied to the motor in a rotation angle-dependent and power-dependent manner. The correction quantity for the current is, for example, described by a function or by a table filed in a memory.

Although this procedure presupposes additional electronic components, they may already be present for other reasons and consequently cause little or no additional expenditure when using this operational mode. A method of this type, for example, is described in DE-OS 3941553 Al.

It will be apparent that variations and modifications of the motor as described above may be made. For example, the motor may be constructed as an inner rotor or a linear motor. In addition, the number of stator and rotor poles may be increased. The invention is also not restricted to motors with permanent magnetic rotors and can also be used with soft magnetic rotors. The motor may, of course, also be operated as a generator, and/or as a synchro for determining angular positions of the rotor, or as a stepping motor with a comparatively low power-level.

Fig. 10 shows a more precise representation of the embodiment according to Fig. l (or 2), in the correct proportions and with the correct dimensions, taking account of the fact that the central hole 9 has a diameter of 9 mm. The dimensions represent extremely advantageous embodiments, possibly essential for the invention. Fig. 10 clearly shows, e.g. that the main pole core has three times the width of a core of a commutating pole, and that the commutating poles have outwardly concave end faces towards the air gap, the radius of curvature of which is approximately three times the radii provided for a convex curvature on either side of the stator cams 47.These stator cams have the same width in the peripheral direction as the opening of the stator slots, at either side of which, as on the end faces of the commutating poles, there is a concave curvature towards the air gap, this curvature having approximately the same radius as the convex curvatures on either side of the stator cams. This radius is approximately 20 % of the cylindrical air gap diameter.

The substantially cylindrical air gap of the outer rotor having eight poles is bound by 8 rotor poles, the radially directed magnetization of which is distributed in an approximately rectangular or trapezoidal manner in the peripheral direction (direction of rotation) and if possible extends completely on to the geometric pole pitch, i.e. extends over 1800e1 if possible.

The radially very fine air gap extends substantially axially and in the peripheral direction. Two diametrically opposing main poles of one phase are excited in such a manner that the exciting field has the same polarity towards the air gap.

Additionally excited commutating poles may be situated offset by 900mech relative thereto and will also have this same polarity towards the air gap. It simply has to be ensured that the poles of one phase and possibly the associated commutating poles are excited simultaneously. To this end, the windings may be connected in series.

The abovementioned trapezoidal magnetization of the rotor permanent magnet is essential for the combinatorial effect of the invention. E.g. Fig. 11 shows a so-called Back-EMF interlinked at two windings, i.e. in this embodiment, the three windings are interconnected in a star connection and the induced voltage upon rotation of the rotor (Back-EMF) between two leads indicates that the magnetization of the rotor as described hereinabove must have a trapezoidal distribution. If the voltage at one winding were to be tapped between the star point and the lead, the path of the voltage curve would be "more markedly trapezoidal" or possibly approximately rectangular. This would then give an even better representation of the rotor magnetization.

As, according to the invention, the main pole width corresponds to a rotor pole pitch (1800eel)' the induced voltage in one phase represents the magnetic flow curve of a rotor magnet pitch. The cogging torque associated therewith is equalized by the dimensions and configuration of the stator cams (as shown in detail in Fig. 10 for six main poles and six commutating poles in the stator with eight rotor poles). All similar poles are equidistant. The commutating pole pitch is 600 The el commutating poles can be wound as active auxiliary poles supporting the torque in the manner indicated.

Fig. 12 shows a winding diagram for wound commutating poles of the actual motor according to Fig. IC. Each main pole has 100 windings, and each commutating pole 75 windings. NP is the star point of this 3-phase winding of the inner stator, which has a relatively small yoke height. The phase R shows clearly the series connection, with the same direction of winding, of the main pole 1/2 then the commutating pole 4/5 then the main pole 7/8 then the commutating pole 10/il towards the star point NP.

Star connection is disadvantageous in similar motors having three phases. The invention obviates this disadvantage, as in many applications it only allows for the use of a delta connection.

The invention results in a relatively low starting current and improved efficiency at normal speed.

Claims (19)

1. An electric motor having a stator and a magnetic rotor, wherein the stator has a number of main poles and commutating poles, the contour shapes of the poles being arranged such that magnetic or reluctance-caused fluctuation of the motor torque is minimized as a function of the angular rotation, and wherein the angular extent of each said main pole of the stator substantially corresponds to that of a rotor pole.

2. An electric motor as claimed in Claim 1, further comprising motor windings, the number of windings being less than four and the number of coils being less than seven.

3. An electric motor as claimed in Claim 1 or 2, further comprising motor windings which are overlap-free and concentrated.

4. An electric motor as claimed in any preceding Claim, further comprising stator coils having a uniform winding direction.

5. An electric motor as claimed in any preceding Claim, wherein the air gap defined between the rotor and stator comprises one or more areas having a substantially cylindrical configuration and which extend radially or axially.

6. An electric motor as claimed in any preceding Claim, wherein the stator is provided with cam-like protuberances which for example, slope with respect to the rotor axis or are parallel with respect to the latter.

7. An electric motor as claimed in any preceding Claim, wherein pole clearances of the rotor slope or are parallel with respect to the rotor axis.

8. An electric motor having a stator and a multipolar magnetic rotor, wherein the stator has substantially planar end faces, and at least one of said end faces is greater in dimension than an intermediate cross-section of the stator taken between the end faces, wherein the stator end faces are magnetically influenced by one or both end faces of the rotor magnet.

9. An electric motor as claimed in any preceding Claim, wherein the ratio of minimum value to maximum value of the angle-dependent, electromagnetically or reluctance generated torque has a value higher than 0.85 with a constant current flow through the motor.

10. An electric motor as claimed in any preceding Claim, wherein the stator has a substantially cylindrical surface area which is influenced by a substantially cylindrical surface of the magnetic rotor, and wherein the stator also has one or more surface areas in the form of a perforated cylinder base, said surface areas being in the magnetic field of the rotor emanating from a cylinder base of the magnetic rotor.

11. An electric motor as claimed in Claim 10, wherein said surface areas of the stator in the form of a perforated cylinder base are formed from one or more ferromagnetic sheet metal layers.

12. An electric motor as claimed in Claim 10 or 11, wherein said surface areas are defined by one or more projecting plates, the boundary of the or each said plate being provided with at least one recess or extension, which modifies the radius of said boundary in a continuous and regular manner, or modifies the radius of said boundary in an approximately irregular, corner-possessing manner.

13. An electric motor as claimed in any preceding Claim, wherein the stator has a number of commutating poles which are magnetizable by one or more windings engaging around said commutating poles.

14. An electric motor as claimed in any preceding Claim, wherein the stator carries at least one stator coil which is wound with a two-wire bifilar winding.

15. An electric motor as claimed in any preceding Claim, wherein the induced voltage of two interlinked phases is approximately trapezoidal with respect to angular rotation.

16. An electric motor as claimed in any preceding Claim arranged as a commutatorless direct current motor and having a permanent magnetic rotor.

17. An electric motor as claimed in any preceding Claim arranged to drive magnetic or optical mass memories.

18. A permanent magnet-excited dynamo with cylindrical air gap and slotted sheet stack as claimed in any preceding Claim, wherein the dynamo is operated as a generator or synchro.

19. An electric motor substantially as hereinbefore described with reference to the accompanying drawings.