Electric motor

Some embodiments are directed to an electric motor that includes a stator including a plurality of circumferentially distributed drive elements for causing an electromagnetic driving force to be applied to a rotor of the electric motor in use. Each drive element includes a wire extending around a metal core to define a plurality of coils for magnetizing the metal core when current flows in the coils. At least one space exists between the metal core of each respective drive element and the coils around it. A cooling device is provided for transferring heat away from the drive elements. Each drive element further includes a heat conductor including a plurality of mutually electrically isolated metallic elements located in the or each respective space between the metal core thereof and the coils around it, for transferring heat from the coils to the cooling device.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a National Phase filing under 35 C.F.R. § 371 of and claims priority to PCT Patent Application No.: PCT/EP2015/061440, filed on May 22, 2015, which claims the priority benefit under 35 U.S.C. § 119 of British Application No.: 1409177.1, filed on May 23, 2014, the contents of which are hereby incorporated in their entireties by reference.

FIELD

This invention relates to an electric motor and in particular to a way of removing unwanted heat from a stator thereof.

BACKGROUND

An electric motor comprises a stator and a rotor. Drive elements of the stator generate varying magnetic fields which cause the rotor to spin about an axis. Such magnetic fields are generated by current flowing through coils in the drive elements. The temperature of the motor in use is related to the magnitude of current flowing through the coils. Each of the materials used within the motor will have a life rated temperature limit for continuous operation. The maximum power output of an electric motor is thus restricted by the temperature limit for one of these materials because above this temperature the service life of this material will be reduced.

SUMMARY

According to an aspect of the present invention there is provided an electric motor comprising: a stator comprising a plurality of circumferentially distributed drive elements for causing an electromagnetic driving force to be applied to a rotor of the electric motor in use, wherein each drive element comprises a wire extending around a metal core to define a plurality of coils for magnetizing the metal core when current flows in the coils, and wherein at least one space exists between the metal core of each respective drive element and the coils around it; and cooling means for transferring heat away from the drive elements; wherein each said drive element further comprises a heat conductor comprising a plurality of mutually electrically isolated metallic elements located in the or each respective space between the metal core thereof and the coils around it, for transferring heat from the coils to the cooling means.

A least one heat conductor may comprise a plurality of metal layers separated by electrically insulating adhesive.

Each metal layer may define a plane having a first edge located further from an axis about which the rotor can spin than a second, opposite, edge.

Notional lines extending between said first and second edges may extend substantially towards the axis about which the rotor can spin.

An angle between each said notional line and the axis about which the rotor can spin may be substantially 90 degrees.

The metal layers may comprise copper or aluminum.

At least one heat conductor may comprise at least one bundle of metal wires, the respective wires of each said bundle being electrically isolated from each other along their respective lengths.

The metal wires of each said bundle may be arranged such that along the length of the respective bundles, each said wire occupies substantially every position in the axial cross-section of the bundle.

The or each said bundle of metal wires may be a Litz wire.

The metal wires in each said bundle may comprise copper or aluminum.

The or each respective space between the metal core of each respective drive element and the coils around it may be defined by a curved portion of said coils which is generally curved in a plane perpendicular to the axis about which the rotor can spin.

The cooling means may comprise a water jacket or a plurality of fins.

According to another aspect of the present invention there is provided a vehicle comprising an electric motor according to any of the heretofore mentioned arrangements for propelling the vehicle.

DETAILED DESCRIPTION

An electric motor comprises a stator and a rotor, which are schematically shown inFIGS. 1 and 2. The stator10has a sleeve11in which a plurality of drive elements12are located. Such drive elements12are circumferentially distributed about the sleeve11and surround a rotor14. The rotor14is arranged to be driven by the drive elements12such that it spins about an axis16extending along the length of the stator10. It will be apparent to persons skilled in the art that, in use, the drive elements12generate varying magnetic fields for causing the rotor14to spin. Persons skilled in the art will also appreciate that, in use, the drive elements12will heat up so a cooling arrangement18(e.g. a water jacket or a plurality of fins for convection to air) is provided around the stator10to displace heat from the drive elements12. The stator10is generally annular in profile.

Details of the drive elements12will now be set out, one of which is schematically depicted inFIG. 3.

The illustrated drive element12has a metal core19with an electrically conductive wire21extending around it (e.g. copper wire). The metal core19has a substantially T-shaped cross section as shown inFIG. 3aand includes a body section19aextending outwardly from a head section19b. The wire21, which is connected to a power supply, is wrapped or coiled around the body section19aof the metal core19in such a manner that it defines a plurality of coils there around. Thus when current flows through these coils the metal core19is caused to be magnetized. Magnetic fields generated in this manner are what cause a torque to be generated on the rotor14for causing it to spin. A suitable magnitude of magnetic field intensity fluctuations generated by the respective drive elements12and their specific timing in order to drive the rotor14will be apparent to persons skilled in the art.

The metal core19inFIG. 3comprises a magnetic element and could be formed of a plurality of mutually spaced iron laminates for example, whereby adjacent laminates are separated by a layer of electrically isolating adhesive.

With reference toFIGS. 4 and 5, the coils of wire21extending around the body section19aof the metal core19define spaces30adjacent opposite faces of the body section19a. More specifically, the body section19aof the metal core19has two side faces20,22in addition to both front and rear faces24,26. When arranged in the stator10, a notional line extending between the front and rear faces24,26is substantially parallel to the axis16about which the rotor14can spin (seeFIG. 2). The wire21coiled around the body section19aof the metal core19extends along the two side faces20,22but not the front and rear faces24,26. Instead a curved portion28of the respective coils defined by the wire21connects the sections of wire21that are in contact with the two side faces20,22; each such curved portion28being generally curved in a plane perpendicular to the axis16about which the rotor14can spin. A space30thus exists between the curved portions28of the respective coils and the front and rear faces24,26of the metal core19. The spaces30each generally exhibit the shape of a half-cylinder.

A heat conductor32(which could otherwise be referred to as a heat displacer, heat conduction element or heat displacement member) is located in each of the spaces30bounded by the coils around the body section19aof the metal core19. A heat conductor32is illustrated in this position inFIG. 3, although it is not included inFIGS. 4 and 5for clarity purposes since it would obscure the spaces30depicted in these drawings. Therefore fromFIG. 3it will be apparent that the heat conductors32located in the respective spaces30are configured and arranged to conduct heat away from the curved portions28of wire21coiled around the metal core19and to transfer such heat to the cooling arrangement18. This reduces the temperature of the curved portions28of the coils extending around the metal core19in use by providing a direct path for heat to flow along between the curved portions28of the wire21and the cooling arrangement18. Without such heat conductors32, a substantial amount of heat emanating from the curved portions28would instead be conducted into the lengths of wire in contact with the sides20,22of the body section19abefore flowing through the metal core19itself on its way to the cooling arrangement18, which is a less efficient manner of cooling the curved portions28of wire21.

Improved cooling of the curved portions28of wire21coiled around the body section19aof the metal core19provides that a larger current can pass through the coils before they begin to short circuit. As a result, higher intensity magnetic fields can be generated to drive the rotor14and so maximum power output of an electric motor can be increased.

Merely using a block or body of metal for each of the heat conductors32in a drive element12is not an optimal way of cooling the curved portions28of wire21because this would reduce overall electric motor efficiency. In particular, due to Lenz's law, Eddy currents induced in any such blocks or bodies of metal used as heat conductors32would act to oppose variations in magnetic fields generated for the purpose of driving the rotor14. Such Eddy currents would also increase the temperature in the region of the curved portions28of wire21.

Minimizing the effect of Eddy currents generated in the heat conductors32is therefore desirable.

A suitable heat conductor32conceived with the foregoing in mind, which is schematically illustrated inFIG. 3, is a laminated body that comprises a plurality of metal layers separated by adhesive. The metal layers comprise copper or aluminum for example and are electrically isolated from each other. Each metal layer defines a plane having a first edge34located further from the axis16about which the rotor14can spin than a second, opposite, edge36. Notional lines38extending between the first and second edges of the respective metal layers extend substantially towards the axis16about which the rotor14can spin. As a result an angle between each such notional line38and the axis16about which the rotor14can spin is substantially 90 degrees. Furthermore the heat conductor32comprises an outer layer of electrical insulation such that it is electrically isolated from both the metal core19and the wire21coiled around it. The heat conductor32is configured and arranged to be in touching contact with the curved portion28of wire21it is supposed to conduct heat away from and extends radially outwardly along, or in close proximity to, the front or rear face24,26of the metal core19as far as the cooling arrangement18.

Using a laminated body of the type described in the foregoing paragraph as a heat conductor32, the undesirable effects of Eddy currents already explained are reduced. In particular, if the thickness of the respective metal layers is reduced by a factor of ½ then the magnitude of Eddy currents generated in these layers in use are reduced by a factor of ¼.

Another type of suitable heat conductor32which addresses the adverse effects of Eddy currents already explained comprises one or more bundles of metal wire, the respective metal wires in each bundle being electrically isolated from each other along their respective lengths and comprising copper or aluminum for example. The separate strands of metal wire in each bundle are arranged (twisted, braided or woven) so that over the length of the bundle, each strand of wire occupies substantially every position in the axial cross-section of the bundle. Such a bundle of metal wires could therefore be a Litz wire, whereby the heat conductor32comprises one or more Litz wires capable of extending along the length of a space30. More specifically a heat conductor32in accordance with this paragraph is configured and arranged to be in touching contact with the curved portion28of the wire21it is supposed to conduct heat away from and extends radially outwardly along, or in close proximity to, the front or rear face24,26of the metal core19as far as the cooling arrangement18.

Since magnetic fields generated by currents induced in the respective strands of a Litz wire substantially cancel each other out, the undesirable effects of Eddy currents already explained are reduced.

The two suitable types of heat conductor32described in the preceding four paragraphs could be used interchangeably in the stator10. For instance all heat conductors32in the stator10could be of the same type. Alternatively one or more drive elements12could include one type of heat conductor32, whereas the remaining drive elements12could include the other type of heat conductor32. Furthermore, a drive element12could include different types of heat conductors32in the respective spaces30.

An electric motor having a stator according to any variation described herein could be used to propel a vehicle whether land based or otherwise such as an armoured main battle tank, car, truck, lorry, boat, jet-ski or aircraft. Such an electric motor could also be used in various other applications unrelated to propulsion of a vehicle.

The specific manner in which the aforementioned wire21is coiled around the body section19aof the metal core19is not essential and could be varied depending on the particular electric motor design. For instance rather than being wrapped around the body section19ain a form of coils arranged one on top of the other (like inFIG. 5) the wire21could be coiled in the manner schematically depicted inFIG. 6.

InFIG. 6like elements to those already mentioned are denoted with similar reference numerals. The wire21is arranged to have two coil sections. The first coil section21ais where the wire21is wrapped around the body section19aa in a series of coils arranged one on top of the other. The second coil section21bis where the wire21is wrapped such that various coils are horizontally stacked around the body section19aas shown rather than being vertically stacked on top of one another, whereby as a result respective coils collectively define the upper surface21cof the second coil section21b.

A heat conductor32suitable for use in such an embodiment has a stepped portion32aand when the heat conductor32is inserted into the space30the underside of the stepped portion32acontacts and extends over the upper surface21cof the second coil section21b. This provides that heat from a plurality of coils which the stepped portion32ais in contact with can flow into the heat conductor32and towards the cooling arrangement18. The face denoted32bis configured such that when the heat conductor32is inserted into the space30it can be arranged to contact the inside surface of the curved portions28of wire21in a similar manner to that heretofore described.

Various possible wire coiling or winding arrangements and heat conductor shapes will be apparent to persons skilled in the art upon reading the foregoing.

It will be appreciated that whilst various aspects and embodiments of the present invention have heretofore been described, the scope of the present invention is not limited to the embodiments set out herein and instead extends to encompass all arrangements, and modifications and alterations thereto, which fall within the spirit and scope of the appended claims.

For instance although the drive elements12described herein with reference to the drawings are said to have two spaces30defined by curved portions28of wire21, in some embodiments the wire21may be arranged such that only one space30is defined between a metal core19and the wire21coiled around it.

Also in some drive element embodiments the wire21may not be in touching contact with the metal core19at all, whereby the wire21is instead supported relative to the metal core19. Nevertheless in such embodiments one or more heat conductors32could still be provided between a wire21and metal core19for conducting heat away from the wire21to the cooling arrangement18.

The heat conductors32do not need to contact the entire inside surface of the curved portions28of wire21. In view of this, persons skilled in the art will be able to envisage various possible shapes of suitable heat conductors32. For instance part of the outer profile of a heat conductor32comprising a plurality of laminated metal layers may be similarly shaped to the inside surface of the curved portion28of wire21so that it can lie against it. However a heat conductor32comprising a plurality of Litz wires might only contact the inside surface of a curved portion28of wire21at various separate locations.

Although it has been previously mentioned that the aforementioned spaces30each generally exhibit the shape of a half cylinder, this is not strictly essential. At least one of a curved portion28of wire21and the front and rear faces24,26could be shaped such that a space30instead exhibits any of a range of other shapes. Furthermore, the curved portions28need not necessarily be curved along their entire length and could instead comprise curved portions separated by at least one substantially straight section for instance.