GB2289994A - Magnetic reluctance motor - Google Patents

Abstract

A magnetic reluctance motor has a salient pole rotor 3 interacting with stator poles 1, 2 to form a machine operating on the magnetic reluctance principle. The machine incorporates a shaded-pole feature by introducing an angle of inclination between laminar structures forming the rotor and stator cores. This performs eddy-current screening restricting magnetic flux transit between poles as they separate. It allows the magnetic attraction between poles during the approach phase to drive the motor. <IMAGE>

Description

MAGNETIC RELUCTANCE MOTORS FIELD OF INVENTION This invention relates to an electric motor which operates by exploiting magnetic reluctance. Reluctance motors are those having ferromagnetic salient pole stators and rotors. They operate by virtue of the stronger ferromagnetic attraction between rotor poles and stator poles as the rotor turns to bring the poles into register. Alternatively, or in addition, they operate by weakening that attraction as the poles separate. The change in the magnetic forces between rotor and stator poles can be achieved in various ways, as by suitable commutation of electric current supplied to windings on the poles when a d. c. power supply is used or as by simple reliance on a phase lag between current and rotor position where a. c. is used to power the pole windings. The latter is typical of the synchronous motor operation of a domestic electric clock.

BACKGROUND OF THE INVENTION It is reported from the research of Robert G Adams of New Zealand, as published in the December 1992/January 1993 issue of Australian magazine NEXUS, that if permanent magnets are used as rotor poles and solenoid-excited soft iron cores form stator poles, with the solenoids excited to oppose the attraction of the permanent magnet for a short period after the poles pass through the in-register position, then the motor runs continuously and is extremely efficient in operation. Indeed, there are reports of such motors running at cool temperatures whilst delivering somewhat more mechanical power than is supplied as electrical input. This is contrary to standard science teaching and several researchers have tried to replicate the Adams machine without corroborating his finding. However, subject to there being such confirmation by rigorous testing which takes full account of the problems of measuring input power, given that the excitation waveform of the current inflow is subject to rapid fluctuation during a cycle of rotation, the scientific presumption is that the phenomenon known as magnetocaloric cooling developed by cyclic ferromagnetic field activity accounts for the necessary thermodynamic energy balance.

It is known to use permanent magnets to provide the drive power, with control somehow deactivating the magnetic attraction between poles.

U.K. Patent No. 547,668 (Inventor: Stanley Isaiah Hitchcock) describes a motor which uses axially orientated magnets interspersed along the motor axis between sets of rotor laminations, with the return magnetic flux path through radially disposed stator pole pieces and an all-embracing casing structure serving as a general bridging yoke. The Hitchcock patent further suggests that the controlled excitation of the stator pole pieces by series connected magnetization coils mounted on each of the pole pieces can serve to oppose the magnetic attraction of the magnets during the pole separation periods of the machine cycle.

The specific implementation of the invention to be described below may resemble in part the form of motor described by Hitchcock, but there are vital differences which characterize what is here claimed and their advantages will be explained.

The specific form of motor described as embodying this invention has a form involving reference to 'shaded poles' and it is, therefore appropriate to mention as background the use of shaded pole techniques in certain electrical induction instruments. Typically, an aluminium disc is mounted to rotate in the air gap of a laminated a. c. electromagnet and a portion of the pole area at the air gap is 'shaded' by a metal conductor which develops a reaction current by induction. Its action is to cause the disc to be driven through the gap in a direction determined by the position of the shaded pole section. Sections of the disc turn into the pole gap, entering on the non-shaded side and leaving on the shaded side.

This action occurs because a copper band wrapped around a shaded portion of the pole area causes the magnetic flux through that area to lag in phase behind the magnetic flux through the other portion of the pole area. The interactions of the current induced in the disc then develop a drive torque. This phenomenon, however, is not one that can be used efficiently in electric motors as such. In the instrument application a restraining brake torque is applied by a permanent magnet acting elsewhere on the perimeter area of the disc, so that the speed of the disc rotation becomes a measure of the rate at which electrical work is supplied to an external system by the current flow through the instrument.

Reference is also made to patent No. GB 2,183,102 of which this Applicant is a coinventor. This a solenoidal excitation winding on a rotor core axis which is part of a rotary transformer built into an alternator.

This has laminar stator members which might be said to have poles but only in the sense that an air gap in a transformer core has pole faces.

There is no motion of those pole faces causing cyclic change of the air gap between poles and that disclosure does not concern a reluctance motor, but is mentioned because at the air gap the laminations of the stator and the rotor interface in an orthogonal non-coplanar manner. This is a feature of the machine to be described in support of this invention. The object of the prior disclosure of GB 2,183,102 is to configure the rotor and stator assemblies so that eddy-currents are minimal when there is normal a. c. excitation, but there is no suggestion in that disclosure that this could assist in producing drive torque for a motor operating on the reluctance principle. In fact, none of the above disclosures realize or give the necessary insight into the eddy-current enhancement effects accompanying the in-built pole gap coupling asymmetry feature exploited in the machine to be described.

BRIEF STATEMENT OF THE INVENTION According to the invention, a magnetic reluctance motor comprises a magnetic reluctance motor comprising a stator configured to provide a set of stator poles, a rotor providing a set of salient pole pieces, and magnetization means arranged to provide a magnetic field which drives magnetic flux across the pole gaps between the rotor and stator poles as the poles come into register, characterized in that both the stator and rotor poles are formed by laminated assemblies of transformer steel, the laminar configuration being relatively orientated as between the rotor and stator poles so that, when in the in-register position, the laminar planes of the pole faces of the interacting stator and rotor poles are not coplanar but are mutually inclined.

This inclination of the laminations implies a shaded-pole effect if the flux linking the poles tends to be dragged through the planes of the rotor or stator pole laminations.

According to a feature of the invention, the motor may, however, be characterized in that the laminar configuratio is relatively orientated as between the rotor and stator poles so that, when in the in-register position, the laminar planes at the pole faces of the interacting stator and rotor poles are mutually orthogonal and the stator pole laminar planes are inclined in the forward direction of motor rotation, whereby the stator laminations forming the forward sides of the poles in relation to the direction of motion of the interacting rotor poles serve as pole-shading members by restricting passage of magnetic flux transversely through their planar form to the pole gaps as the rotor and stator poles separate.

According to another feature of the invention, the motor is characterized in that stator poles comprise laminations which are each shaped at the pole faces to provide a progressively decreasing air gap width as between the stator pole and a lamination in the interacting rotor pole traversing across the stator pole face. Such a motor may be further characterized in that pole-shading members are mounted on the stator poles at their edges on the forward side in relation to the direction of motion of the interacting rotor poles, these being at the positions of closest approach of the rotor laminations and the stator poles, at which positions the pole gap widens by becoming that of the adjacent stator pole lamination, said pole-shading members being responsive to magnetic flux fluctuations produced by relative movement between the rotor and stator poles by virtue of induced eddy-current in the pole-shading members that serve to restrict passage of in-plane-of-stator flux between poles as they separate, whereby the action of the magnetization means provides a reluctance motor drive force to bring stator and rotor poles into register and the action of the pole-shading members oppose the counterpart reluctance braking effect as the poles separate.

There are aspects of this invention which are related to the invention described in the specification of GB Patent Application No.

2287134, this being a divisional application based on the same original disclosure. Accordingly, the following description which is common to both is broader in scope than the invention here claimed. The essential distinction is that special shaded-pole members are incorporated in the other application, whereas the subject invention exploits a shaded-pole effect introduced by adopting a special laminar configuration of the rotor and stator core members.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a perspective view of sections of a rotor pole and a stator pole having non-coplanar laminations with a shaded-pole feature on the stator pole.

Fig. 2 shows a side elevation view of the pole system depicted in Fig. 1.

Figs. 3 to 6 show the progressive magnetic flux linkage change for transit across the stator pole face by the rotor pole shown in Fig. 2.

Fig. 7 depicts an end view of a stator pole configuration in a linear representation to show the function of the inclined and stepped arrangement of the laminations forming a multipole structure.

Fig. 8 depicts a plan view showing the use of a changing pole gap width as seen by an individual rotor pole lamination in transit past the stator pole face.

Fig. 9 depicts the rotor pole arrangement of a motor incorporating the invention.

Fig. 10 shows a cross-sectional side elevation view of the central body structure of the motor specifically showing how the stator laminar pole assemblies are positioned.

Fig. 11 shows a sectional side elevation view of the motor as viewed from the section marked by arrows in Fig. 10.

Fig. 12 shows an alternative rotor structure for use in the motor, there being a cylindrical permanent magnet mounted concentrically on the rotor shaft.

DETAILED DESCRIPTION OF THE INVENTION Whereas the Adams disclosure referenced above uses 'unshaded' soft iron solenoid-wound stator poles in combination with permanent magnet rotor poles, the invention now described uses a form of stator pole that, though of 'soft iron', is 'shaded' in the sense used above and is characterized by having no a. c. excited windings either on the stator or the rotor.

The Adams motor is a switched reluctance motor and may be represented as having a four pole permanent magnet rotor, which, in operation, relies on a commutator to make periodic connection between an electric battery and the series-connected stator solenoids for a limited range of angular position as the poles separate from their in-register position. This results in d. c. current pulses which produce fields in the cores in opposition to the field action of the permanent magnets and virtually cancel the net field acting on those cores. These opposing fields are what may be termed 'end fields in an open magnetic circuit and they are aided by the magnet-induced demagnetization effects of that open circuit magnetic configuration. It needs little extra field action in the opposing sense to compensate for the flux that penetrates across the rotor pole to stator pole air gaps. With the current pulses active, this allows the rotor to turn from the in-register pole position without being dragged back by the magnets but, as the rotor turns on through 45" of angle and the current is switched off, so the magnetic attraction is eventually reasserted between the rotor poles and the stator poles next in sequence in the forward direction.

The shaded pole effect provided by this invention can serve much the same purpose and so can eliminate the need for a commutator, but for the Adams machine to use the orthodox shaded pole induction principle would mean the application of a. c. electrical drive power to the rotor solenoids during the pole approach and the pole separation phases and this is an inefficient application of motor power.

This invention incorporates features which exclude the need for any a. c. power input and rely upon the magnetic reluctance variations attributable to the rotor and stator salient pole interaction as the means for creating the magnetic flux changes which then induce the reacting current in the pole-shading conductors.

This has enormous benefit in that it simplifies construction and avoids the need for current input to several windings through mechanical current commutation or electronic switching, whilst affording the high torque properties of a reluctance motor and bringing to bear the possible additional technological performance advantages of the Adams motor design.

The motor principle of the subject invention relies on there being at least one primary ferromagnet source as an activating field, whether this is a permanent magnet or an electromagnet. In either case, however, for control and for regulating the motor power, there has to be an electrical input source feeding a control winding associated with that primary ferromagnet source.

Then, assuming there are four stator and four rotor poles, over the 45" range of movement before the in-register pole position, there is a strong mutual attraction activated by the magnetism supplied to the stator poles through the rotor poles and this imparts drive torque to the rotor.

The magnetic attraction is effective across an air gap on the sides of the stator poles that are not shaded. Over the next 45" of rotor movement, as the poles separate, the attracting field is dragged across the air gap area which is shielded by the shaded poles and so, without one having to apply an opposing current to a special winding to reduce the magnetic attraction, there is an automatically induced current in the pole-shading conductors which serves the same purpose. The magnetic force between the poles is weakened as the poles separate from their in-register position and this allows the motor to be driven and so power a load solely by virtue of energy drawn from the magnet and the input power that sustains the primary field.

It will be realised that the induced current reactions associated with the shaded-pole feature will set up back fields which demand more forward magnetomotive force from the primary magnet. This accounts for the energy balance, but there are distinct advantages in having the source and the reaction disposed at well-separated positions around the magnetic circuit of the system. The ferro-magnetism has its own way of developing a response which is characterized by 'leakage flux' and which, in normal design of electrical apparatus with magnetic cores, is seen as unwelcome. However, where one seeks to weaken a magnetic attraction force by deliberate design, this is more easily accomplished if there is poor coupling of magnetic field action as between a primary current and a reacting current. This makes leakage highly desirable because the resulting distortion of the ferromagnetic flux inside the linking ferromagnetic core winding has a way of then developing a thermo dynamic action which is attributable to the reorientation and mutual cross-field effects of fields intrinsic to the ferromagnetic state. The latter only arise where the magnetic flux vectors in domains inside the crystals of the ferromagnetic core are driven by fields strong enough to promote their rotation, as opposed to 1800 reversal or 90" transition between axes of easy magnetization. By weakening the direct coupling between the primary control winding and the air gap pole region, the response of the ferromagnet involves that process of domain flux rotation.

There are two very important points developing the above theme which need to be stressed as features distinguishing the invention from prior art proposals. Firstly, the Hitchcock patent disclosure uses coplanar rotor and stator laminations. The magnetic flux produced by the axially mounted magnets between rotor sections has to close through the solid structure of the motor casing. This must lead to high eddy-current losses which will confine the flux oscillations to a restricted crosssectional area in the casing, the effect of which is quite the reverse of that found in the subject invention. Owing to the fact that the flux in the bridging yoke between the rotor sections is primed at a base polarization level by the action of the magnets, as when the motor is at rest, any flux oscillation induced in that yoke by running the motor will tend to resist change. Pole attraction on pole closure will be opposed as will the diminution of that attraction as the poles separate.

In contrast, in the Applicant's embodiment of the invention the induction of eddy-current in the laminated stator is confined to a specific portion of the closure path bridging the two rotors where it serves a special commutating function. Both the Adams motor and that of Hitchcock require a commutator.

Secondly, and in this same connection, there is, concerning the 'shaded pole' feature, a major difference in relation to the prior art.

Shaded pole operation is normally inefficient because the induction of eddy-current involves loss and the process would not work unless those eddy-currents are strong enough to assure significant phase shift of magnetic flux in the inducing flux path. In contrast, the invention under discussion aims to use eddy-current action expressly to preclude entry of flux in a certain portion of the stator core path, the particular portion affected as the rotor and stator poles move apart. It is not the primary aim to exclude or distort flux that has already penetrated that path. The result is that such loss as there is no more than that needed to sustain current in a magnetizing winding aimed at suppressing the flux, but because the eddy-current induction effect is self-commutating this has substantial advantages not realized by prior art methods.

This introduction will, therefore, serve to explain the phenomenological basis and advantages of motors incorporating the subject invention.

The remainder of this description will be directed at specific structure and its design function.

Referring to Figs. 1 and 2, a laminated stator pole 1 having one side screened inductively by a solid conductor edge piece 2 is shown in relationship with a laminated rotor pole 3 which is moving in the direction of the arrow.

Note that the laminations of the two poles are in mutually orthogonal planes as their pole gap interface closes to bring them into register.

Figs. 3 to 6 show progressive relative positions of the two poles.

In Fig. 3, with the rotor pole 3 positioned in advance of stator pole 1, the magnetic flux emerging from the edges of the laminations in the rotor pole traverses the air gap to enter into the sides edges of the laminations in the stator pole. As with normal reluctance motor operation, there is a mutual attraction as the field intensity gets stronger with gap closure and so a drive force is exerted on the rotor pole in the forward direction.

In Fig. 4 the poles are in the in-register position and the magnetic pull is directed at right angles to the drive direction and so does not assist or retard the drive.

In Fig. 5 the rotor is now partially attracted to the stator. The magnetic flux emerging from the edges of its laminations in its trailing portion can still traverse the gap and penetrate into the magnetic circuit through the stator pole laminations. However, though this will develop some small amount of retardation, this is minimal because the flux crosses the gap nearly at right angles to the motion. Note then that the conductor edge piece 2 is serving in its shielding or shading capacity and so asserts a blocking action or obstructive effect on the passage of a changing magnetic flux. This weakening costs some power in that there are I2R ohmic resistance losses in the conductor, owing to the current induced, but these can be minimal if the motor operates at high speed and the conductor edge piece is of solid form and sufficiently thick. Note that it is, in the embodiment being described, not a conductive band around a ferromagnetic core portion that permits passage of phase-shifted magnetic flux. There will be appreciable 'skin effects' which make the losses minimal while allowing the conductor to serve as an effective shield against flux penetration. Therefore, as the poles move to the Fig. 3(d) position, by blocking much of the retardation otherwise caused by pole separation, the shading element as the conductor edge piece 2 can demand less input power than would otherwise be needed to separate the poles.

The energy imparted as drive power has come primarily from the ferromagnetic core system and the shaded-pole has prevented that energy from being returned, leaving the ferromagnetic core to recover some of its energy from magnetocaloric cooling of the core.

Fig. 7 shows how a sequence of stator poles can be arranged to allow the passage of a rotor pole without forcing excessive oscillations in magnetic flux intensity. The view perspective is that applicable to Fig.

2. Fig. 8 shows a related perspective from a direction similar to that applicable to Fig. 3 and the flux action of a single lamination 4 in the rotor pole is illustrated as the focus for attention.

It will be realised that the design criteria involve confining the magnetic flux in a pole so that it does not penetrate a planar surface of a lamination. Oscillations of flux intensity, especially those which range between near zero and a strong flux amplitude, mean unwanted magnetization losses. Therefore, ideally, the stator pole structure has to admit asymmetrical shaded-pole drag to occur as the rotor moves on from an in-register pole position, whilst reducing the main core loss effects of a flux collapse and allowing the flux in a rotor pole to begin to work by being attracted to the next stator pole in sequence.

Such magnetization losses are tolerable if confined to the laminations inasmuch as they are similar to those accepted in electrical transformers, the laminations being of transformer steel. However, for a motor operating at speed the cyclic frequency of the flux oscillations increases with the number of poles and it is desirable to offset the extra eddy-current loss by sustaining a uni-directional flux bias in the magnetic circuit, this being provided by the d.c. current supplied to a motor control winding or by a magnet.

By providing the angled pole gap on each of the stator pole sections, as depicted in Fig. 8, each rotor pole lamination will cause a forward drive force to be asserted as it moves into the sectors having a reduced gap width. Upon reaching the end of each such sector there is a step increment of the gap width, but the flux from the lamination 4 finds it is blocked from adhering to the rearward stator pole sector by eddycurrent screening induced in the shading elements 2. Accordingly, there is a forward drive and the power input is that needed to overcome the back-magnetomotive force set up by those eddy-current. By using the control winding 5 (see Fig. 11) to sustain the magnetic field in spite of the flux fluctuations, the motor will operate by the usual reluctance motor principle.

Referring now to Figs. 9, 10 and 11, a reluctance motor is shown to comprise a rotor 6 having a cylindrical body section and a central shaft section. Bearings 7 support the rotor. On each side of the rotor body there are eight laminated rotor poles 3. The laminations have an edgewise-abutting interface with the sides of the rotor body. Each of the two sets of rotor poles is assembled in the manner shown in Fig. 9 and located within a flanged rim on a disc 8. There are non-ferromagnetic locating members 9 between each pair of adjacent rotor poles. These are bolted to the disc 8 and serve to keep the laminations tightly together.

Provision of suitable means for fixing the poles in a lateral sense, that is in the direction parallel with the rotor shaft axis, are provided. These could comprise bolts (not shown) linking adjacent locating members 9 and passing through holes punched in the laminations or, for example, there could be restraining bars as bridge members between the members 9 and fitted in slots 10 in the laminations.

The various members other than the laminations and the rotor are composed on non-ferromagnetic material and, apart from the bolts just mentioned, are not electrically conductive. Electrically insulating sleeves and washers, such as are used in transformer assembly, would need to be used where metals bolts are fitted and, of course, the laminations would have the usual surface layer of insulation to limit eddy-current losses.

The stator shown comprises a magnetizing coil serving as a control winding 5 which is of solenoidal form concentric with the rotor axis. Its function is to develop an axial magnetic field in the rotor body, which field drives magnetic flux through the rotor poles to the pole gaps.

The stator pole members are ferromagnetic and comprise laminations which have angled end shape, as shown in Fig. 8. In order that the motor drive generated by the end-shaping and shaded-pole action should be in the same spin direction on both sides of the motor, these laminations when fully illustrated and not seen in part-section would be trapezoidal in form.

There are nine sets of such laminations forming nine stator poles 3, each being a laminar stack that is assembled in the tangentially-inclined manner depicted in Fig. 10. This is done by bolting the laminations in a support frame (not shown) as part of the same stator assembly as that which locates the magnetizing winding 5.

Copper shading members 2 are clamped into the stator assembly along the edges of the stator poles 3, as shown, and, being at the outer perimeter of the body of the motor, this facilitates the cooling of what is the seat of the main power dissipation in the motor owing to the eddycurrent screening.

A design variant is depicted in Fig. 12 where the rotor body section is shown as a composite assembly of a steel shaft section 11 and a hollow cylindrical permanent magnet 12 polarized along the cylinder axis.

The use of 8 rotor poles and 9 stator poles serves to smooth the motor torque action.

In operation, using the structure without the permanent magnet feature, the switching on of a d.c. power supply to the magnetizing coil, by normal electrical circuit means not shown, will set up a surge which promotes rotation. The motor runs on well established reluctance motor principles, given that the shaded-pole screening serves as the flux switching means. As the speed builds up this screening function becomes more effective and so the motor efficiency improves. To stop the motor one simply switches the supply current off.

In operation, using the structure with the permanent magnet feature, the d.c. power supply must provide current to the magnetizing winding in the direction which produces magnetic flux in the central part of the rotor shaft so as to augment the flux produced by the magnet.

Thus the magnet serves to set up a unidirectional bias flux whilst the d.c. current powering the magnetizing winding serves to absorb the back-magnetomotive forces and control the drive power.

The motor will run, as before, but in this latter case the removal of power will allow the magnet to see the central shaft section of the rotor as a kind of yoke for the magnet. The flux that otherwise would penetrate through the rotor poles is then deflected back though the rotor shaft.

Therefore, the magnetizing winding using the rotor of Fig. 12, serves several purposes. Firstly, it precludes the magnet from using the shaft, if of steel, as a by-pass short-circuit route for its flux closure and, secondly, it adds extra power to give a stronger flus action across the pole gaps. Thirdly, it absorbs the back action of flux variation and feeds in power needed to drive the motor. Fourthly, it provides control of the speed and torque, at least over a range of operational motor speeds. Fifthly, by providing a magnetizing effect in the same direction as the magnet, it serves to reduce any tendency for the magnet to become demagnetized and so gives the machine a longer service life. Sixthly, by virtue of the flux feedback route through the central shaft region, it provides for a more rapid stopping action on power switch-off, in that the motor might otherwise tend to run on for a time driven by residual magnetism effects.

It is submitted that, whereas a. c. induction motors having no rotor windings are well established and whereas the ongoing development of cheaper elect ensure their effectiveness as a short-circuit around a linking flux path.

They are simply solid blocks of a metal such as copper. The assembly of the multi-pole radially-laminated rotor structure, having regard to the fact that it is to rotate at high speed, poses the main assembly problem, but that involves no electrical winding and so is a quite straightforward fabrication exercise for technicians skilled in the electric motor art.

Claims (4)

1. A magnetic reluctance motor comprising a stator configured to provide a set of stator poles, a rotor providing a set of salient pole pieces, and magnetization means arranged to provide a magnetic field which drives magnetic flux across the pole gaps between the rotor and stator poles as the poles come into register, characterized in that both the stator and rotor poles are formed by laminated assemblies of transformer steel, the laminar configuration being relatively orientated as between the rotor and stator poles so that, when in the in-register position, the laminar planes at the pole faces of the interacting stator and rotor poles are not coplanar but are mutually inclined.

2. A motor according to claim 1, characterized in that the laminar configuration is relatively orientated as between the rotor and stator poles so that, when in the in-register position, the laminar planes at the pole faces of the interacting stator and rotor poles are mutually orthogonal and the stator pole laminar planes are inclined in the forward direction of motor rotation, whereby the stator laminations forming the forward sides of the poles in relation to the direction of motion of the interacting rotor poles serve as pole-shading members by restricting passage of magnetic flux transversely through their planar form to the pole gaps as the rotor and stator poles separate.

3. A motor according to claim 1, characterized in that stator poles comprise laminations which are each shaped at the pole faces to provide a progressively decreasing air gap width as between the stator pole and a lamination in the interacting rotor pole traversing across the stator pole face.

4. A motor according to claim 3, characterized in that pole-shading members are mounted on the stator poles at their edges on the forward side in relation to the direction of motion of the interacting rotor poles, these being at the positions of closest approach of the rotor laminations and the stator poles, at which positions the pole gap widens by becoming that of the adjacent stator pole lamination, said pole-shading members being responsive to magnetic flux fluctuations produced by relative movement between the rotor and stator poles by virtue of induced eddycurrent in the pole-shading members that serve to restrict passage of inplane-of-stator flux between poles as they separate, whereby the action of the magnetization means provides a reluctance motor drive force to bring stator and rotor poles into register and the action of the poleshading members oppose the counterpart reluctance braking effect as the poles separate.