GB2214724A

GB2214724A - Permanent magnet electric motor

Info

Publication number: GB2214724A
Application number: GB8801924A
Authority: GB
Inventors: Roger Charles Hey Sidey
Original assignee: CUSTOM DEV Ltd
Current assignee: CUSTOM DEV Ltd
Priority date: 1988-01-28
Filing date: 1988-01-28
Publication date: 1989-09-06
Also published as: GB8801924D0

Abstract

An electric motor is disclosed comprising a magnetic circuit formed by a permanently magnetized element 10 spaced from a soft magnetic element 13 by first and second portions of a gap with a magnetic flux flowing in opposite directions in the first and second gap portions and the first and second electrically conductive coils 15, 16 disposed in respective first and second gap portions. This electric motor provides a large increase in efficiency enabling the size and complexity of the motor to be reduced whilst enhancing reliability and providing a high relative power handling capacity. <IMAGE>

Description

ELECTRIC MOTOR The present invention relates to electric motors, for instance of the linear type for providing force or thrust in response to an electrical current.

Applications for such motors include industrial automation, instrumentation, vibration testing, pumping, and loudspeakers.

Figure 1 of the accompanying drawings shows a linear motor of conventional design for instance for use in a moving coil loudspeaker; The main elements of the motor are a permanent magnet 1, a soft iron magnetic circuit comprising the outer shell element 2 and a central pole piece 3, and a movable coil assembly 4. Magnetic flux due to the permanent magnet is forced around the soft iron components and thus crosses the gap between the outer shell and the pole piece in a substantially radial direction. The flux is mainly at right angles to the conductor in the coil assembly and force is exerted on the coil windings in the presence of a current flowing therein.The force is proportional to the intensity of the useful gap flux (the component normal to the conductor), the magnitude of the current, and the effective length of conductor (the length exposed to useful flux), and the direction of the force is at right angles both to the useful flux and to the direction or lay of the conductor. The wound assembly of Figure 1 can thus be made to experience a force tending to move it axially in a direction into or out of the stationary assembly according to the direction of the current.

To make efficient use of the permanent magnet material it is necessary to configure the magnetic circuit such that the magnet is operating close to the point of maximum energy product on its demagnetization curve. This is done by designing the circuit to yield a certain ratio of airgap volume to permanent magnet volume. The circuit is also designed to minimize leakage (flux not crossing the gap in such a way that it cuts the conductors), and the air gap is filled as much as possible with conductor material. It can be shown by analysis of such motors that the heat dissipation in the windings for a given set of operating conditions is inversely proportional to the volume of winding material (i.e. the copper volume).With modern high coercivity magnetic materials, the energy products are large so that efficient designs using such materials should feature larger volumes of conductor material. For the conventional configuration of Figure 1 this presents problems with leakage flux. Once the radial dimension of the gap becomes a significant portion of the magnet length or the pole piece height, the amount of flux lost to leakage rises unacceptably. These losses can be counteracted with this configuration, but only by incurring the penalty of a significant increase in the size of the magnetic circuit elements and hence of the overall motor assembly.

Other disadvantages of the motor of Figure 1 are that there is unrestricted access to the moving coil assembly only at one end of the motor and the degree of thermal contact or heat sinking afforded to the windings is relatively poor. Also, to provide a given radial freedom of movement of the coil assembly relative to the magnetic assembly, gaps between the coil and the centre pole and between the outer pole and the coil are required. Both gaps must be equal to the required radial freedom of movement. These clearances reduce the maximum attainable conductor volume and hence reduce the efficiency of the motor.

Thus, the design of all known types of permanent magnet motors has been strongly influenced by the sensitivity to demagnetization or relatively low coercivity of the available high performance permanent magnet alloys. These characteristics necessitate motor designs with small air gaps and, in the case of the short stroke linear motor, relatively low conductor volumes. This in turn has resulted in linear motors which are relatively large and massive in relation to their force output, and which have restricted power handling capability due to the poor thermal coupling or heat sinking of the windings.

The recent availability of highly coercive permanent magnetic materials, such as the rare earth Samarium Cobalt or the Neodymium Boron Iron alloys, has changed the design approach. These alloys are highly resistant to demagnetization and feature much higher energy products than earlier materials. They are thus capable of working with large air gaps and, in the case of the short stroke linear motor, with large conductor volumes. A considerable reduction in bulk and an increase in electromechanical efficiency have been obtained. However, the conventional design of linear motor cannot readily be adapted to exploit fully the potential of the high coercivity materials.

According to the invention, there is provided an electric motor comprising : a magnetic circuit formed by a permanently magnetized element spaced from a soft magnetic element by first and second portions of a gap with magnetic flux flowing in opposite directions in the first and second gap portions; and first and second electrically conductive coils disposed in the first and second gap portions, respectively.

Preferably the soft magnetic element is tubular and surrounds the permanently magnetized element, which may be disposed on the axis of the tubular soft magnetic element.

The permanently magnetized element may be fixed to the soft magnetic element and the first and second coils may be movable with respect thereto. Alternatively the first and second coils may be fixed to the soft magnetic element and the permanently magnetized element may be movable with respect thereto.

Preferably the permanently magnetized element comprises a permanent magnet and first and second polepieces extending from the north and south poles, respectively, of the permanent magnet.

Preferably the magnetic circuit is arranged so that the permanent magnet operates at or adjacent the point of maximum energy product of the demagnetization curve thereof.

Preferably coil drive means are provided for supplying drive currents to the first and second coils such that the resulting forces acting on the first and second coils are in the same direction. Preferably, in order to determine the position of the permanently magnetized element with respect to the first and second coils, inductance measuring means are connected to the first and second coils.

It is thus possible to provide a motor with a magnetic circuit in which relatively large volumes of conductor material can be accommodated without incurring the previous penalty of high leakage flux and which uniquely exploits the capabilities of highly coercive permanent magnet materials. Such a motor is compact and electrodynamically efficient, and a very high relative power handling capacity is attainable. Such motors are simple to produce and can implement simply and reliably a measurement of the displacement produced by the motor.

The invention will be further described, by way of example, with reference to Figures 2 to 4 of the accompanying drawings, in which Figure 2 is a cross-sectional diagrammatic view of an electric motor constituting a first embodiment of the invention Figure 3 is a cross-sectional diagrammatic view of an electric motor constituting a second embodiment of the invention; and Figure 4 is a schematic circuit diagram of a driving and position measuring arrangement for the electric motor of Figure 3.

The linear electric motor shown in Figure 2 comprises a permanent magnet 10 whose poles are extended by polepieces 11 and 12 to provide a permanently magnetized element. The permanently magnetized element is cylindrical in shape and is disposed coaxially with and inside a tubular element or sleeve 13 of material of high permeability, such as magnetically soft iron. The polepiece 12 and the sleeve 13 are rigidly connected together by a non-magnetic member 14.

An annular air gap is provided between the permanently magnetized element and the sleeve 13. The air gap has first and second annular portions containing first and second coils 15 and 16, respectively, which are fixed to each other and to a member 17 for transmitting force or displacement and made of a nonmagnetic material.

Magnetic flux generated by the permanent magnet 10 flows through the polepiece 11, radially outwardly across the second air gap and through the second coil 16 into the sleeve 13, radially inwardly from the sleeve 13 through the first air gap into the polepiece 12, and back to the magnet 10. The magnetic flux thus flows radially in opposite directions in the first and second air gaps and, in use, the directions of the windings of the coils 15 and 16 and their connections are such that, when current flows through the coils, the resulting forces acting on the coils are in the same direction.

For the sake of simplicity, the suspension arrangement for the coils 15 and 16 and the member 17 have not been illustrated. Suitable suspension arrangements may be provided in accordance with the application of the electric motor and may, for instance, be provided by other parts or devices to which the motor is connected so that no additional suspension is necessary.

in use, with current flowing through the coils 15 and 16, there is a relative force acting between the permanently magnetized element and the coils 15 and 16 which urges the coils longitudinally to the left or right, depending on the direction of current and the winding direction of the coils. The sleeve 13 is longer than the permanently magnetized element so as to divert flux leaving the ends of the polepieces 11 and 12 and thus extend the air gap or regions where the flux has a radial component. The coils extend longitudinally beyond the ends of the permanently magnetized element so as to provide an increased working stroke over which the motor can produce force.

The permanent magnet 10 is made of a magnetizable material of high coercivity, such as a Samarium Cobalt or Neodymium Boron iron alloy and the magnetic circuit is arranged such that the magnet operates at or adjacent its point of maximum energy product on its demagnetization curve. The whole of the permanently magnetized element is surrounded by the coils so that the flux set up by the magnet is efficiently "collected by the coils. The effective gap seen by the magnet is twice the radial gap between the sleeve 13 and the permanently magnetized element so that a large winding volume can be accommodated without a large leakage flux.

The motor makes efficient use of soft magnetic material and thus requires a relatively small quantity thereof. The size and weight of the motor can therefore be much less than with previously known arrangements, and hence the cost of the motor is substantially reduced. The construction and manufacture of the motor is also generally simpler and therefore less expensive than previously known types of motors.

The sleeve 13 acts essentially as a low loss channel for magnetic flux. In the absence of effects such as magnetic saturation or departure of the permanently magnetized element from its axially centred position, the sleeve 13 experiences little or no force resulting from operation of the motor.

Figure 3 shows an alternative form of linear electric motor with the same reference numerals being used for parts corresponding to the motor shown in Figure 2. The motor shown in Figure 3 differs from that shown in Figure 2 principally in that the first and second coils 15 and 16 are fixed to the sleeve 13, whereas the permanently magnetized element comprising the permanent magnet 10 and the polepieces 11 and 12 is movable with respect to the sleeve 13. Again, suspension arrangements have not been shown in Figure 3 for the sake of clarity and simplicity.However, whereas the coils 15 and 16 in the motor of Figure 2 experience little or no radial force and therefore require little radial restraint in order to freely move within the air gap, the permanently magnetized element in the motor of Figure 3 is in unstable equilibrium when accurately centred on the axis of the sleeve 13 and requires suspension arrangements in order to prevent the element from being displaced radially by the magnetic attraction between the element and the sleeve. Thus, the permanently magnetized element is either fitted to apparatus in which the moving and stationary components are constrained by bearings or flexures, or the motor itself is provided with means for constraining radial movement.

Although the motors of Figures 2 and 3 operate in essentially the same way, each has certain advantages in certain applications. Thus, the motor of Figure 3 requires only one clearance gap to provide radial freedom of movement of the permanently magnetized element, so that a greater volume of conductor for the same size of motor can be accommodated than in the motor of Figure 2. Because the windings 15 and 16 are attached to the sleeve 13 which will normally be stationary, there is no need to provide delicate or unreliable flying leads or flexures for providing electrical connections to the coils. Also, the coils may be arranged in good thermal contact with the relatively massive sleeve 13 so as to improve heat dissipation.There is no need for an end cap or member 14 in the motor of Figure 3 to position the permanently magnetized element and the sleeve 13, and the motor is therefore symmetrical with unrestricted access at both ends to the relatively driven parts.

An advantage of the motors shown in Figures 2 and 3 is the possibility of obtaining information on the relative displacement of the moving part from the motor itself. An arrangement for driving the motor of Figure 3 and for measuring the relative displacement is shown in Figure 4. The same arrangement could equally well be used with the motor of Figure 2.

The arrangement of Figure 4 comprises input terminals 20 for receiving a motor drive input signal.

One of the terminals is connected to a common line or earth and the other is connected to the positive terminal of a summer 21 and the negative terminal of a summer 22. The negative terminal of the summer 21 and the positive terminal of the summer 22 are connected together and to the output of a generator 23 for generating an alternating voltage of relatively small amplitude e. The outputs of the summers 21 and 22 are connected to the inputs of amplifiers 24 and 25, respectively, which are capable of supplying sufficient current to drive the electric motor 26.

The coils 15 and 16 are connected in series between the outputs of the amplifiers 24 and 25. The circuit node at which the coils 15 and 16 are connected together is connected to the input of a circuit 27 which comprises an amplifier and optionally means for measuring the phase and amplitude of the input signals with respect to the signal generated by the generator 23.

The phases and connections of the coils 15 and 16 are such that, when a current is passed through the series connection of the coils1 the forces produced by the coils and acting on the permanently magnetized element 28 are in the same direction.

In the absence of the alternating voltage provided by the generator 23, the amplitude and polarity of the input signal supplied to the terminals 20 determine the amount and direction of displacement of the element 28 with respect to the coils 15 and 16. The amplitude and frequency of the alternating current through the coils 15 and 16 resulting from the signal generated by the generator 23 are such as not to disturb the normal operation of the motor 26. Thus, the resulting fluctuations in the position of the element 28 are damped by the inertia of the element and by its suspension so that minimal net alternating movement is produced.

The coils 15 and 16 contain an equal number of turns so that1 when the element 28 is centred in the motor, the small signal or incremental inductance of the coils are equal. The reactances of the coils will therefore be equal so that there is no net signal at the node 29.

The circuit 27 therefore provides a signal at output terminal 30 indicating that there is no displacement of the motor.

If the element 28 is then displaced towards, say, the winding 15, the ferromagnetic parts of the element 28 move further into the winding 15 and move partially out of the winding 16. The inductance of the winding 15 therefore increases and the inductance of the winding 16 decreases. The reactance of the coil 15 thus increases and the reactance of the coil 16 decreases, which unbalances the "half bridge" circuit provided by the coils and the amplifiers 24 and 25. Thus, a net alternating signal is produced at the node 29, whose phase and amplitude are determined in the circuit 27 so as to supply at the output 30 a signal indicating the amplitude and direction of displacement of the element 28.

Such an arrangement resembles the known linear variable differential transformer type of displacement transducer. However, because the transducer is provided by a linear electric motor in which the individual winding currents may vary unpredictably, the displacement measurement performance of the present arrangement is unlikely to approach the performance which can be be attained by such known transducers.

However, for many applications, the measurement is of sufficient quality and accuracy to be useful, and has the advantage that the measurement may be provided with no modification of or addition to the motor. Thus, in applications where the limitations of the measurement are acceptable, the combined motor and displacement transducer arrangement provides a compact, reliable and easily manufactured device.

The motors shown in Figures 2 and 3 have many advantages over the known arrangements. The construction and mode of operation are well suited to the characteristics of modern high coercivity magnets and allow motors with low leakage flux and high winding volumes to be produced. Such motors are electrically efficient and can produce relatively high force outputs with low internal heat dissipation. Efficient use is made both of the permanent magnetic material and of the soft magnetic material so that motors of small volume and mass for a given output may be produced. The motor of Figure 3 in particular has advantages in providing access to both ends and the ability to work with a large radial freedom of movement and without electrical connections to the moving element. This motor also has the advantage of allowing highly efficient heat sinking or thermal contact from the coils to the sleeve, so that greatly increased average and peak force and mechanical power outputs can be provided with respect to known types of motors of similar size, such as that illustrated in Figure 1.

The motors of this type may be used in a wide variety of applications and can be substituted with advantage for known types of motors in such applications.

Claims

1. An electric motor comprising : a magnetic circuit formed by a permanently magnetized element spaced from a soft magnetic element by first and second portions of a gap with magnetic flux flowing in opposite directions in the first and second gap portions; and first and second electrically conductive coils disposed in respective first and second gap portions.

2. An electric motor as claimed in claim 1, in which the soft magnetic element is tubular and surrounds the permanently magnetized element.

3. An electric motor as claimed in claim 2 in which the permanently magnetized element is disposed on the axis of the tubular soft magnetic element.

4. An electric motor as claimed in any one of claims 1 to 3 in which the permanently magnetized element is fixed to the soft magnetic element whilst having the first and second coils movable with respect thereto.

5. An electric motor as claimed in any one of claims 1 to 3, in which the first and second coils are fixed to the soft magnetic element whilst having the permanently magnetized element movable with respect thereto.

6. An electric motor as claimed in any one of the preceding claims1 in which the permanently magnetized element comprises a permanent magnet.

7. An electric motor as claim in claim 6, further comprising first and second pole pieces extending from a respective north or south pole of the permanent magnet.

8. An electric motor as claimed in claim 6 or claim 7, in which the magnetic circuit is arranged for enabling the permanent magnet to operate at or adjacent the point of maximum energy product of the demagnetization curve thereof.

9. An electric motor as claimed in any one of the preceding claims, further comprising coil drive means for supplying drive currents to the first and second coils for enabling the resulting forces acting on the first and second coils to be in the same direction.

10. An electric motor as claimed in any one of the preceding claims, further comprising inductance measuring means coupled to the first and second coils for enabling the position of the permanently magnetized element with respect to the first and second coils, to be determined.

11. An electric motor as claimed in claim 9, in which the inductance means comprises first and second adder means having one pair of opposite terminals coupled together and to a generator, the other pair of opposite terminals coupled to an input terminal, the first and second adder means being coupled to a respective first or second coil and the first and second coils being coupled together in series and to an output terminal for enabling the relative position of the permanently magnetized element to be determined.

12. An electric motor as claimed in any one of the preceding claims, in which the permanently magnetized element is fabricated from a material of high coercivity.

13. An electric motor as claimed in claim 12, in which the material comprises Samarium Cobalt.

14. An electric motor as claimed in claim 12, in which the material comprises Neodymium Boron iron alloy.

15. An electric motor as claimed in any one of the preceding claims, in which the first and second coils extend beyond the permanently magnetized element for increasing the efficiency of the motor.

16. An electric motor substantially as hereinbefore described with reference to any one of figures 2,3 or 4.