WO1995024067A1

WO1995024067A1 - Electric motor

Info

Publication number: WO1995024067A1
Application number: PCT/GB1995/000432
Authority: WO
Inventors: Christopher Robert Duncan; Michael Edward Gailes
Original assignee: Numatic International Limited
Priority date: 1994-03-01
Filing date: 1995-03-01
Publication date: 1995-09-08
Also published as: AU1818295A; GB9403933D0

Abstract

An electric motor (10) comprises a stator (20) and a rotor. The rotor is accommodated within a rotor cavity defined by the stator and is adapted for rotation in that cavity about a longitudinal axis extending through the cavity, the stator (20) is formed of a magnetic-flux carrying material and carries coils of wire which can be connected to an electrical power supply whereby in use an electric current in the coils generates a magnetic field in the stator (20) and rotor cavity to cause or allow rotation of the rotor. Each lamination (22) of the stator (20) is shaped to define a plurality of external cooling elements (32) which in use are connected directly by the environment for cooling the motor (10).

Description

ELECTRIC MOTOR The' present invention relates to an electric motor and has particular reference to an electric motor for use in vacuum cleaning apparatus . The present invention also embraces a vacuum cleaning apparatus including an electric motor of the present invention.

A typical electric motor comprises a stator and a rotor, the rotor being accommodated within a rotor cavity defined by the stator and is adapted for rotation in said cavity about a longitudinal axis extending through the cavity, said stator being formed of a magnetic-flux carrying material and carries one or more coils of wire which can be connected to an electrical power supply whereby in use an electric current in said coils generates a magnetic field in the stator and rotor cavity to cause or allow rotation of the rotor; such a motor is hereinafter referred to as a "motor of the kind described". The coils are constructed and arranged in a manner well-known to a person skilled in the art such that in operation one or more magnetic poles are created in succession at circumferentially spaced locations around the longitudinal axis of the cavity. The rotor also carries one or more coils of wire. In some embodiments the coils of wire on the rotor can be connected to a DC electrical power supply to generate a plurality of magnetic poles which are fixed in relation to the rotor; in use the poles created by the current in the rotor coils are repelled by the magnetic poles generated by the stator coils and, as the poles of the stator are created in succession around the longitudinal axis of the cavity, the rotor is caused to rotate about said axis. Alternatively, the rotor coils may not be connected to a power supply; in use, the magnetic field created by the current in the stator coils induces an electric current in the rotor coils; the induced current in the rotor coils creates one or more magnetic poles as described above which are repelled by the succession of poles created by the stator coils thereby causing the rotor to rotate.

In some embodiments the rotor and/or stator may comprise pole pieces which are adapted to concentrate the magnetic field generated by the electric current in the coils between the pole pieces of the rotor and the pole pieces of the stator.

A person skilled in the art will be aware however that in a motor from the kind described in which the generated magnetic field is constantly changing in magnitude and polarisation, energy is lost in the form of heat as a result of magnetic hysteresis and eddy currents. Heat is also lost as a result of the finite resistance of the wires forming the coils on the rotor and stator, particularly when the electric motor is running under load such that the counter-emf generated as a result of the rotation of the rotor in the magnetic field within the rotor cavity does not substantially balance the applied emf.

This problem is particularly acute in the case of electric motors used in aspirators such, for example, as vacuum cleaners where in order to generate an effective volumetric rate of gas flow, the rotor is required to turn a fan at a speed up to about 30,000 RPM; in vacuum cleaning apparatus an electric motor of the kind described may be rated at about 800-1200 watts, typically 1000 watts.

The heat generated by an electric motor in vacuum cleaning apparatus may therefore be significant, particularly when the motor is running under load. In order to reduce energy losses through magnetic hysteresis and/or eddy currents, the stator of a motor of the kind described is constituted by a stack of laminations; each lamination is adhered to a neighbouring lamination by a thin layer of an adhesive material. Each lamination is generally rectilinear or circular in plane view and defines a central hollow such that the stack of laminations define the rotor cavity. It will be appreciated by a person skilled in the art that in order to minimise the energy losses by magnetic hysteresis or eddy currents in use, it is desirable to minimise as far as possible the thickness of each lamination.

However, the layer of adhesive material between each lamination and its neighbouring lamination is typically a poor conductor of heat; the only path available for heat to escape the stator is therefore via the external surface of each lamination. By reducing the thickness of each lamination therefore the surface area of each lamination available for heat removal is correspondingly diminished; but in these circumstances, the amount of heat generated is also reduced.

While the problem of heat removal from an electric motor is not usually a problem for electric motors for operation at relatively low speeds, it is essential that adequate provision is made for each removal from electric motors used in e.g. vacuum cleaning apparatus. If heat is not removed effectively from the stator of such an electric motor, the motor will eventually, on continued use, over-heat" causing the wires of the coils to fuse; the motor will, under such circumstances, "burn-out" and will need replacement. An electric motor of the kind described therefore typically includes a heat-conducting sleeve which is constructed and arranged such that an inner surface of the sleeve intimately contacts the external surface of the stator pack; the external surface of the sleeve is shaped to define a plurality of cooling fins which, in use, are contacted by a gas surrounding the motor to remove heat from the motor. Typically, in a vacuum cleaning apparatus the air flow generated by the fan may be caused or allowed to flow in juxtaposition with the external surface of the sleeve in order to effect "forced cooling" of the motor by the conduction of heat through the cooling fins.

The air flow may be caused or allowed to flow in the longitudinal direction with respect to the motor; the cooling fins may extend longitudinally with respect to the motor so as to permit substantially laminar flow of the air in juxtaposition with the motor between the cooling fins.

It will be appreciated by a person skilled in the art however that it is expensive to manufacture the heat- conducting sleeve with a sufficiently small tolerance to form a substantially intimate contact with the external surface of the stator pack. Furthermore, a layer of adhesive material is normally used to adhere the sleeve to the stator. The efficiency of heat transfer between the laminations of the stator pack and the heat conducting sleeve is therefore generally impaired by the insulating effect of the adhesive material and/or small, unwanted air gaps between the sleeve and the stator pack.

There is therefore a general requirement for an improved means of removing heat from within an electric motor, particularly an electric motor for use in an aspirating apparatus such, for example, as a vacuum cleaning apparatus.

In accordance with one aspect of the present invention therefore there is provided an electric motor of the kind described characterised in that each lamination is shaped to define a plurality of external cooling elements which, in use, are contacted directly by a gas surrounding the motor for cooling the motor.

According to the present invention therefore each of the laminations constituting the stator pack may be shaped to define an external surface having a surface area sufficient to allow adequate cooling of the stator laminate in use by contacting directly the surrounding gas. The present invention therefore seeks to obviate 'the requirement for a heat-conducting sleeve intermediate the stator laminate and the surrounding gas. In other words, the applicants have found that adequate cooling of the stator laminate in use may be achieved without the use of a heat-conducting sleeve as hereinbefore described by shaping each laminate to provide a plurality of external cooling elements which define an integral cooling surface.

The periphery of each lamination may be generally circular or rectilinear. In some embodiments the external surface of each lamination may be configured to define a plurality of castellations constituting said external cooling elements.

In some embodiments each lamination may be generally annular in plan view; said cooling elements may be circumferentially spaced around the lamination.

Said external cooling elements may be rectilinear in cross-section through the longitudinal direction of the stator; alternatively the external surface of each cooling element may be arcuate in said cross-section. In one embodiment, each external cooling element may be generally rectangular in cross-section.

Each lamination may include between 4-20 cooling elements; in some embodiments between 8-16 elements may be provided, typically 12. The external cooling elements on each lamination may be the same or different one from the others; typically however the external cooling elements on each lamination may be uniform and equally spaced to provide a regular external cooling surface.

The length of each cooling element in the plane of the lamination between the outer extremity of the element and the point where the element meets the body of the lamination may be between 1-5% of the diameter of the lamination, typically about 3%; the provision of said external cooling elements in accordance with the present invention may therefore increase the external surface of each lamination by about 10-200%, typically about 90%.

Said stator laminate may comprise about 10-50 laminations, typically about 30. Said laminations may be the same or different one from the others.

In one embodiment said laminations may be identical one to the others. The laminations may be arranged in the laminate such that each lamination is aligned with its neighbouring laminations such that the cooling elements on each lamination are disposed in register with the cooling elements on the neighbouring laminations. The cooling elements may therefore define a plurality of circumferentially spaced ribs extending in the longitudinal direction with respect to the stator. Said laminations may be manufactured by techniques well- known to a person skilled in the art. In particular, each lamination may be punched or cut from a sheet of a metal material adapted to carry a magnetic field.

In some embodiments said electric motor may further comprise one or more end caps; each end cap may be fitted on a respective end of the stator laminate. Typically each end cap may comprise a crown portion and a skirt portion, the skirt portion being shaped to form a tight fit about the external surface of the end of the stator laminate.

In another aspect, the present invention includes an aspirating apparatus such for example as a vacuum cleaning apparatus comprising an electric motor in accordance with the present invention and gas displacing means arranged to be driven by said electric motor; wherein in operation said air displacing means displaces a gas to generate a gas stream for use in aspirating or vacuum cleaning.

Typically, the gas displacing means may be a fan which may be driven by the rotor of said electric motor. In some embodiments the gas stream displaced by the fan may be caused or allowed to flow over the external surface of the stator thereby to increase the rate of cooling of the electric motor. Typically, the laminations may be arranged as hereinbefore described to provide a plurality of longitudinal extending ribs; in use, said gas may exhibit substantially laminar flow between said ribs.

Said vacuum cleaning apparatus may further comprise an inlet means for admitting a flow of air into the vacuum cleaning apparatus in juxtaposition with a surface e.g. a floor surface to be cleaned and a dust receptacle adapted to receive dust and detritusen trained in said flow of gas.

Following is a description by way of example only and with reference to the accompanying drawings of methods of carrying the present invention into effect.

In the drawings:-

Figure 1 is a side view of an electric motor in accordance with the present invention.

Figure 2 is a plan view of a lamination included in the electric motor of Figure 1.

An electric motor (10) in accordance with the present invention includes a stator (20) and a rotor (not shown). Said stator (20) comprises a laminated stack of laminations (22).

Each lamination is cut or stamped as a single piece from a sheet of a metal which can carry a magnetic field. The lamination (22) is generally annular in plan view as shown in Figure 2. The lamination (22) has an external surface (24) and an internal surface (26) which defines a central hollow (28); said internal surface (26) is formed with one or more pairs of diametrically opposed pole pieces (30). The lamination shown in Figure 2 of the accompanying drawings includes one pair of pole pieces (30); it will be appreciated by a person skilled in the art however that 2, 3, 4 or more pairs may be used in a manner well-known to a person skilled in the art.

The external surface (24) of the lamination (22) is formed with a plurality of outwardly extending fingers (32). In the embodiments shown in Figure 2, the lamination (22) has twelve such fingers (32); it will appreciated however that in accordance with the present invention any number between about four and twenty such fingers may be provided.

The radial length of each finger between its extremity (34) and a point (36) at which the finger (34) meets the body of the lamination (22) is about one to five per cent of the total diameter of the lamination; in the embodiments shown the radial length of each finger is about three per cent of the diameter.

In the embodiment shown in Figure 2 of the accompanying drawings therefore the provision of the cooling fingers (32) increases the surface area of the external surface (24) of the lamination (22) by about ninety per cent as compared with a similar lamination not having any such fingers (32). It will be understood however that by altering the overall radial length and numbers of the fingers (32) the increase in the surface area of the lamination may be between about 10 per cent and about two hundred per cent.

The laminations are laminated to form the stator (20) by a layer of an adhesive material interposed each lamination and its neighbouring lamination. The laminations (22) are aligned one with the others such that the fingers (32) on each lamination overlap with the fingers (32) on the neighbouring laminations to define a plurality of circumferentially spaced ribs (40) which extend substantially orthogonally to the plane of each lamination (22). The stack of laminations (22) defines a rotor cavity which is constituted by the central hollow (28) defined by each lamination.

Said rotor cavity accommodates the rotor. The rotor is adapted to rotate inside the rotor cavity about an axis substantially parallel to each of said ribs (40). Said rotor has an axial spindle (50) which protrudes from said rotor cavity at one end (21) of the stator laminate. The one end of the spindle (50) carries a fan which is adapted to rotate with the rotor.

Said stator (20) carries a plurality of coils of wire (not shown) which can be connected to an AC power supply. In use, the current in the coils of wire generates a magnetic field within the stator and rotor cavity; the magnetic field is concentrated in juxtaposition with said pole pieces (30). The coils of wire are constructed and arranged in a manner well-known to a person skilled in the art so that, in operation, an opposite magnetic pole is generated in juxtaposition with each of said pole pieces (30) and such that the polarisation of the magnetic field is continually reversed such that one of the pole pieces (30) is successively a north pole and then a south pole; while the other pole piece (30) is successively a south pole and then a north pole.

The rotor also carries a plurality of coils of wire; the rotor coils are not connected to a power supply. In use, the magnetic field generated within the rotor cavity induces a current in the rotor coils; the rotor coils are constructed and arranged in a manner well-known to a person skilled in the art such that the magnetic field generated by the induced current therein interacts with the alternating magnetic field of the stator thereby causing the rotor to rotate in the rotor cavity.

In use, as the rotor rotates, the fan is caused to rotate thereby displacing the air or gas surrounding the motor. If the fan is rotated in a first direction, the air is displaced in a rightwards direction as indicated in Figure 1 by arrow (71); if the fan is rotated in the other direction, the air is displaced in the opposite direction (72). In either case, the air or gas surrounding the motor is caused to flow in juxtaposition with the external surface of the laminations (22). In use, heat generated within the stator laminate (20) as a result of energy losses by magnetic hysteresis and/or eddy currents is conducted through each lamination (22) to the external surface (24) where it is transferred to the surrounding air or gas by conduction and is dispersed by conduction and convection; the movement of the air over the external surface (24) of the stator (20) effects "forced" cooling of the stator.

The electric motor as hereinbefore described is therefore provided with an integral cooling surface and thus obviates the requirement for a heat-conducting shell. The applicants have found that the elimination of the heat-conducting shell included in the prior art motors of the kind described and the shaping of the laminations of the stator to provide a plurality of integral cooling elements allows more efficient transfer of heat from the stator to the surrounding air or gas. Furthermore, by dispensing with the requirement for a heat-conducting shell, the present invention allows the production of a more compact electric motor as compared with the prior art devices of the same rated power. The electric motor on the present invention therefore provides an increased vo. me around the electric motor; e.g. in a vacuum cleaner; said increased volume may contain for example an increased amount of sound insulating material in order to dampen the sound of the electric motor in operation.

Claims

CLAIMS '

1. An electric motor of the kind described characterised in that each lamination of the stator is shaped to define a plurality of external cooling elements which in use are connected directly by the environment for cooling the motor.

2. A motor as claimed in claim 1 characterised in that each of the laminations constituting the state of practice shaped to define an external surface having a surface area sufficient to allow adequate cooling of the stator laminate in use by directly contacting the environment of the laminate.

3. A motor as claimed in any preceding claim characterised in that the environment of the laminate is a stream of cooling fluid.

4. A motor as claimed in any preceding claim characterised in that the environment of the motor is air.

5. A motor as claimed in any preceding claim characterised in that the periphery of each lamination is generally circular or generally rectilinear.

6. A motor as claimed in any preceding claim characterised in that the external surface of each lamination is configured to define a plurality of castellations constituting the external cooling elements.

7. A motor as claimed in any preceding claim characterised in that the cooling elements are circumferentially spaced about the lamination.

8. A motor as claimed in any preceding claim characterised in that each external cooling element is substantially rectilinear in a cross-section through the longitudinal direction of the element.

9. A motor as claimed in any one of claims 1 to 7 characterised in that the external surface of each cooling element is substantially arcuate in cross- section.

10. A motor as claimed in any one of claims 1 to 7 characterised in that each external cooling element is generally rectangular in cross-section.

11. A motor as claimed in any preceding claim characterised in that each lamination has four to twenty cooling elements.

12. A motor as claimed in any preceding claim characterised in that each motor has eight to sixteen cooling elements.

13. A motor as claimed in any preceding claim characterised in that each lamination has twelve elements.

14. A motor as claimed in any preceding claim characterised in that the external cooling element on each lamination may be the same or different from the others.

15. A motor as claimed in claim 14 characterised in that the external cooling elements on each lamination are substantially uniform and equally spaced and adapted to register with elements on corresponding laminations to provide a regular external cooling surface.

16. A motor as claimed in any preceding claim characterised in that the length of each cooling element in the plane of the lamination between the outer extremity of the element and the point where the element meets the body of the lamination is between 1 to 5% of the diameter of the lamination.

17. A motor as claimed in claim 16 characterised in that the length of each cooling element is 3% of the diameter of each lamination.

18. A motor as claimed in any preceding claim wherein the external cooling elements provided about each lamination increase the external surface of each lamination by between 10 and 200%.

19. A motor as claimed in claim 18 characterised in that the provision of external cooling elements increase the external surface of each lamination by 90%.

20. A motor as claimed in any preceding claim characterised in that the stator laminate comprises 10 to 50 laminations.

21. A motor as claimed in any preceding claim characterised in that the laminations are arranged within the laminate assembly such that the cooling elements of each lamination is aligned with its neighbouring elements such that the cooling elements on each lamination are disposed in register with cooling elements on neighbouring laminations thereby to define a plurality of circumferentially spaced longitudinally extending ribs along the external surface of the stator.

22. A motor as claimed in any one of claims 1 to 21 characterised in that the laminations are arranged within the stator assembly so that each cooling element is in juxtaposition the cooling element on a neighbouring lamination, the arrangement being such that successive elements are off-set one with respect to the next to produce a plurality of circumferentially spaced ribs about the external surface of the stator assembly, which ribs are disposed substantially skew with respect to the longitudinal direction of the stator.

23. A motor as claimed in any preceding claim characterised in that the stator assembly may have at least one end cap fitted to an end of the laminate.

24. A motor as claimed in claim 23 characterised in that each end cap may comprise a crown portion and a skirt portion, the skirt portion being shaped to form a tight fit around the external surface at the end of the stator laminate, said skirt being further configured to accommodate one or more of said cooling elements.

25. Vacuum cleaning apparatus comprising an electric motor as claimed in any preceding claim.

26. Vacuum cleaning apparatus as claimed in claim 25 characterised in that said motor is adapted to drive a displacement fan to generate an air stream within the vacuum cleaner; said air stream being adapted to, at least in part, impinge said stator assembly for the purpose of cooling the motor.