Electric motor

Abstract: An electric motor suitable for incorporation in pipestacks for drilling and pumping applications has a drive shaft (4) carrying spaced rotor discs (6) driven by annular stator windings (7) carried by the motor housing wall. The windings (7) are organized as linear motors operating on a circular path and provision can be made for energization of the windings so as to cause axial movement of the shaft, for hammering or braking action. The shaft (4) can be tubular to provide a passage for drilling mud, or for a fluid being pumped.

Description: The invention relates to an electric motor having particular capabilities, high torque at a relatively low speed, as may be required in certain applications, for example, in drilling and fluid extraction systems. There is known from EP-A-0 208 467 an electric motor of the kind comprising a housing having a shaft mounted therein for rotation around the shaft axis and for movement along the axis, electrical drive means carried by one of the shaft and the housing and co-operating driven means carried by the other of the shaft and housing, the drive and driven means having co-operating faces extending transversely of the shaft axis, and energizing means for energizing the drive means to act on the driven means. Energization of the drive means causes axial movement of the shaft against a return spring to release a brake acting between the shaft and the housing. On de-energization of the drive means, the brake is automatically applied, because of return axial movement due to the spring. US-A-3 048 723 describes an electric motor in which two rotors are similarly moved axially between a central stator and respective brake surfaces. EP-A-0 201 021 describes a synchronous electric motor with permanent magnet rotors of annular form axially spaced along a shaft and each received between sub-assemblies of a stator. The invention provides according to claim 1 an electric motor of the kind described in which the energizing means are operable selectively to energize the drive means to act on the driven means to effect rotation of the shaft and/or reciprocal axial movement of the shaft. Although axial spacing between the drive means and the driven means is desirably small to maximize efficiency, clearance is provided to accommodate the axial movement of the shaft, which can be controlled as to direction and timing so as to be employed for example to effect braking of shaft rotation, or as a hammering action, or may be advantageous when the motor is driving a drill bit. Where the drive means comprises a plurality of drive elements, the translational movement of the shaft and housing can be electrically powered in both directions, but the power stroke can be resisted by a suitable form of spring providing energy for the return stroke. The drive means may comprise windings wound in the manner of linear electric motor windings but extending around the shaft axis in a circular path so as to carry the driven means, which can comprise magnetic material, around the axis. A high torque can be obtained because the motor structure of the invention allows a large effective "motor area" between the motor elements, that is, the drive windings and the co-operating driven element. For most purposes, the invention can be embodied in a motor having a housing of circular cylindrical shape with the shaft journalled along its axis by suitable bearings, so as to function as an output or drive shaft. The driven means is conveniently carried by the shaft and preferably comprises a plurality of axially spaced rotor discs or elements of magnetic material, which may be selected to maximise the drive effect of the energization of the drive means. The drive means then comprises a plurality of annular stator windings carried by the housing so that each is located adjacent at least one side of a respective rotor disc. Each rotor disc can advantageously be received between two of the annular stator winding which are energized in common. A motor embodying the invention can be arranged to operate asynchronously, in simple, robust applications, when the driven means can comprise laminated or solid material, or synchronously to obtain greater efficiency, when the driven means can comprise permanent magnets. Effective cooling may be required, as by forced circulation of a cooling fluid, for example, an oil which may function additionally as a lubricant for the motor bearings. The circulation can be effected by an external pump or by an impeller on the motor shaft. The shaft and the frame or housing can be journalled together for relative rotation by conventional bearings located between them. Alternatively such bearings can be located between the shaft motor elements and the housing. Hydrodynamic and hydrostatic bearings can be integrated with the motor elements and housing, and the motor elements can be arranged and energized so as to provide an electro-magnetic bearing effect. A plurality of the co-operating rotor and stator means can be stacked along the shaft to provide a motor which provides a desired high output torque at a readily controllable speed. The motor can be extremely compact, particularly in its transverse dimension, and is consequently very suitable for incorporation in pipe stacks used in drilling and fluid extraction systems. It can thus be employed for driving drill bits and low speed borehole pumps, for example, screw pumps. The motor can be readily incorporated in pump systems such as are described in EP 0 063 444. A further improvement in compactness can be obtained by employing a hollow motor shaft which can be used for example, for passage of a fluid being pumped or for supply of drilling mud. The invention is further described below, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional schematic front view of an electric motor embodying the invention; and Figure 2 is a like view of a second electric motor embodying the invention incorporated in a pump unit. The illustrated electric motor comprises a frame or housing having a wall 2 of circular cylindrical cross-section, and a motor drive or output shaft 4 extending along the axis 5 of the housing and protruding beyond its ends. The drive shaft 4 carries at equally spaced intervals along it a selected number of like rotor elements 6, each in the form of a co-axial circular disc of magnetic material secured to the shaft. Each rotor element 6 is received axially between two annular stator elements 7 which are concentric with the shaft 4 and secured at their outer peripheries to the housing wall. The two outer stator elements 7 form end walls to the housing 2 and the inner stator elements are stacked together in pairs in contact, with each pair between each adjacent spaced pair of the rotor elements 6. The housing wall 2 and shaft 4 are journalled together for relative rotation about their common axis 4, and, as shown at the upper only of Figure 1, the housing wall supports bearing means 9 operative between it and the peripheral outer edges of the rotor elements 6. Additionally or instead, such bearings could extend between the stator elements 7 and the shaft 4. The stator elements 7 each comprise windings as employed in linear electric motors but with the windings arranged to extend not linearly but in a circular configuration, so as to effect rotation in a selected direction of the elements 6 and thus of the shaft 4 when energized. The stator elements 7 are energised asynchronously, or preferably synchronously, by an AC and/or DC power supply derived, from an external power source 10 by way of a frequency converter 11. The illustrated electric motor is shown with axial spacing between the rotor elements 6 and the stator elements 7 and this spacing permits operation of the motor so that the rotational movement of the drive shaft 4 is replaced or supplemented by an axial reciprocating movement, as when the shaft or the housing is driving a drill bit. To achieve such operation, the energisation of the stator elements 7 is appropriately controlled by means of a control unit 12. The bearing means 9 are such to permit the resulting limited axial movement of the shaft 4. Thus by appropriately energising, for example, only the lower of each pair of the stator elements 7, as shown in Figure 1, the rotor elements 6 and the shaft 4 are moved downwardly, with or without a rotational movement, and by energising only the upper one of each pair of stator elements 7, the shaft is urged upwardly. A hammer action of the shaft 4 can thus be obtained. The axial movement of the shaft 4 can be limited by appropriate stop means, for example, abutments 14 provided on the radial faces of the stator element 7 as shown at the lower part of Figure 1, to avoid physical contact between the rotor and stator elements. The stop means can be arranged to have a braking effect on shaft rotation if required. To remove the heat developed by energization of the windings of the stator elements 6, forced circulation of cooling fluid through the housing can be provided. The cooling fluid can be a lubricant, to lubricate the bearing means 9. The shaft 4 can be hollow to provide a path for inclusion in the fluid circulation circuit. A fluid circulated through the motor can have a bearing function by providing hydrodynamic and/or hydrostatic bearing, and it can absorb or distribute shocks resulting from the hammer action of the shaft. The electric motor 20 of Figure 2 corresponds in essential particulars to the motor of Figure 1, but is incorporated in a borehole pump unit. The pump unit of Figure 2 comprises a Moineau-type displacement element 21 coupled to the lower end of the motor drive shaft 24 so as to be rotated by the motor within tubing 25 of which the motor housing 26 forms a part. The tubing 25 is received within production casing 27. At least the portion of tubing 25 above the housing 26 conveniently comprises electric conductor means providing power for the motor 20. The conductor means can comprise concentric tubular conductors separated by solid insulating sleeving or by dielectric or barrier fluid circulated through passages in which the motor housing may be included. Reference may be made to EP 0 063 444 for further particulars of such conductor and fluid arrangements. The motor shaft 24 can be tubular so as to provide a passage for a fluid being pumped, as indicated by arrows 29,through into the tubing 25, with a further improvement in compactness.

Claims: An electric motor comprising a housing having a shaft (4) mounted therein for rotation around the shaft axis and for movement along the axis, electrical drive means (7) carried by one of the shaft and the housing and co-operating driven means (6) carried by the other of the shaft and housing, the drive and driven means having co-operating faces extending transversely of the shaft axis, and energizing means (10-12) for energizing the drive means to act on the driven means, characterized in that the energizing means (10-12) are operable selectively to energize the drive means (7) to act on the driven means (6) to effect rotation of the shaft (4) and/or reciprocal axial movement of the shaft. An electric motor as claimed in claim 1 wherein the drive means (7) comprises a plurality of axially spaced annular windings. An electric motor as claimed in claim 2 wherein the windings comprise linear motor type windings orientated around the axis. An electric motor as claimed in claim 2 or 3 wherein the driven means comprises a plurality of axially spaced discs (6) of magnetic material. An electric motor as claimed in claim 1, 2, 3 or 4 having braking or abutment means (14) on at least one face of the co-operating faces of the drive means (7) and the driven means (6). An electric motor as claimed in any preceding claim wherein the housing comprises a circular cylindrical housing wall (2) having the shaft (4) journalled along its axis, the drive means (7) being mounted on the housing wall and the driven means (6) being mounted on the shaft. An electric motor as claimed in any preceding claim wherein the or each driven means (6) is located between a respective associated pair of the drive means (7). An electric motor as claimed in any preceding claim arranged for synchronous operation. An electric motor as claimed in any preceding claim wherein the shaft (4;24) is journalled in the housing by bearings (9) carried by the drive means (7) and/or the driven means (6). An electric motor as claimed in any preceding claim wherein the energizing means (10-12) are operable to effect axial movement of the shaft (4;24) in one direction and a spring provides energy for axial return movement. An electric motor as claimed in any preceding claim wherein the shaft (24) is tubular to provide a fluid passage therethrough. An electric motor as claimed in any preceding claim having means circulating fluid through the housing (1;26) for providing at least one of cooling, lubrication, hydrodynamic bearing, hydrostatic bearing and shock absorption.

Inventors: MOHN FRANK, MOHN, FRANK
Applicants: FRAMO DEV LTD, FRAMO DEVELOPMENTS (U.K.) LIMITED
Application year: 1989
Main classification: H02K21
IPC codes: H02K16, F04C15, H02K21, E21B4, E21B43, H02K33