Abstract:
A motor driven pump comprises a motor stator ( 30 ), a motor rotor ( 10 ) operable to rotate around the outside of the stator ( 30 ) and a pump mechanism ( 40, 50 ) driven by the motor rotor ( 10 ) and disposed at least partly inside the motor stator ( 30 ).

Description:
This application claims priority to International Application No. PCT/GB99/02441 filed Jul. 26, 1999. The International Application was published in the English language on Feb. 17, 2000 as International Publication No. WO 00/08338 and itself claims the benefit of United Kingdom Application No. 9817155.6 filed Aug. 6, 1998 and United Kingdom Application No. 9821795.3 filed Oct. 6, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to motor driven pumps. 
     2. Description of Related Art 
     The usual arrangement when a motor drives a pump is to have the pump inline with the motor unit, as detailed, for example in DE-A-4 123 384. Unfortunately this means that the total length of the unit is at least equal to the length of the motor plus the length of the pump, and the combination may often be too long to use in confined spaces. Also the amount of materials and components needed are no less than for manufacturing the two items separately. 
     In order to reduce the length of pump-motor combinations two types of solution have been devised. The first is to use the rotor of the motor as the rotor of the pump, for example by threading the rotor with machined helical feed channels as in DE-A-3-937 345. This reduces the size of the pump and is a simple design requiring few components, but the pressures achieved by this pump will be very low, merely one or two bar gauge, and there is no possibility of using a more complex design of pump, for example a multiple rotor screw pump, where a driven power rotor acts in combination with one or more ancillary rotors. The design of a multiple rotor screw pump is detailed in EP-A-0 736 667. 
     The second previously proposed design overcomes this problem to some extent. The arrangement uses a screw pump where the central pump rotor does not turn while the ancillary screws or rotors turn with the motor rotor. This type of pump is illustrated in DE-A-3 701 586. This type of pump enables high pressures to be reached, but still contains some significant disadvantages. Two housings are required—one for the pump unit and a second for the stator assembly, which increases the bulk and the cost of the unit. The unit is also disproportionately elongate. 
     SUMMARY OF THE INVENTION 
     The present invention seeks to reduce the size and components of the motor driven pump. The invention provides a motor driven pump comprising: a stator; a rotor, coaxial with the stator, operable to rotate around the outside of the stator and a pump mechanism driven by the rotor and disposed at least partly inside the stator. 
     This invention provides a number of advantages compared to the prior art. In particular the stator may form part of the pump housing, reducing the number of components and the time consuming assembly required. The pumped fluid passes inside the stator and so the fluid flow through the pump may be used to cool the stator thus increasing the efficiency of the motor driven pump. This may be most effectively done when the pump housing abuts the stator, leading to efficient conduction of heat to the fluid flowing through the pump. The integration of the pump within the motor means that the mechanical and hydraulic components of the pump are acoustically shielded leading to low levels of pump audible noise. The unit can also be made to be more compact than an equivalent motor inline with a pump which means that it has many applications in confined spaces, for example under a car bonnet as part of the power steering system. 
     Furthermore it is possible to use the power rotor or the ancillary rotors of the pump as a hydraulic bearing for the rotor. This reduces the number of components necessary, reducing the cost and assembly time of the unit. One further bearing may function as the main bearing for both the motor rotor and the pump to ensure that there is no clash with the stator and no audible noise is caused by the vibration of the rotor. This provides an inherent advantage over the previous designs where the motor rotor was an integral part of the pump. 
     Lastly the system housing may contain the reservoir of fluid necessary for the running of the pump, further reducing the noise and vibration of the unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the invention will now be described with reference to the accompanying drawings, throughout which parts are referred to by like references and in which: 
     FIG. 1 schematically shows an external rotor motor driven pump; 
     FIG. 2 schematically gives further detail of a screw pump mechanism in the pump of FIG. 1; 
     FIG. 3 outlines schematically the fluid flow through the entire pump; 
     FIG. 4 shows a schematic cross section of the motor driven pump; 
     FIG. 5 schematically shows the pump in use; and 
     FIG. 6 shows schematic details of an alternative embodiment where the pump is entirely enclosed by the stator of the motor. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 schematically illustrates a motor driven pump comprising an external rotor  10 , a stator  30  and a screw pump housing  50 . Permanent magnets  20  are affixed to the rotor  10  and a position sensor  60  detects the rotational position of the rotor  10 . This information is relayed to control electronics  65  which may thus deduce the relative positions and speeds of the stator and rotor and so switch on winding sets in the stator which relate to the various phases of the motor. The electronic control of d.c. brushless motors is well established and so will not be described here in detail. 
     A significant feature of the motor driven pump is that the rotor  10 , is an external rotor: i.e. it rotates around the outside of the stator  30 . The heat conducting pump housing  50  abuts the stator, which means cooling of the stator may take place as fluid flows through the screw pump, see FIG. 3 below. The power rotor  40  of the pump, shown in further detail in FIG. 2, is driven from the rotor of the motor and acts as the central shaft for the motor rotor. 
     It can be seen in FIGS. 2 and 3 that in this embodiment the central shaft of the pump acts as the power rotor  40 . However it is easy to envisage an alternative embodiment where the central shaft remains stationary and the external rotor  10  drives ancillary pump rotors  70  and  80 . A manifold  90  and an endcap  100  complete the pump unit. 
     The fluid flow through the pump is shown in further detail in FIG.  3 . Low pressure fluid enters the pump from an external system through an inlet  110  in an reservoir  120 . It then is fed to the manifold  90  through a tube  130  which maybe incorporated into the external casing of the pump. Fluid then flows through manifold inlet tubes  140  until it is next to the endcap  100  where it enters the pump. As the central rotor  40  of the pump rotates the ancillary rotors  70  and  80  rotate likewise forming chambers between the threads of the screw which force the fluid down the screw pump to the high pressure outlet  150 . 
     The endcap contains a seal  160  and has a seal  170  around it, which stop fluid escaping into the stator  30  or the part of the motor which contains the magnets  20 . The manifold  90  has two seals  180  and  190  between it and the housing assembly  200 . The seal  190  provides a seal between the low pressure fluid and the stator  30 . The seal  180  provides a seal between the low pressure and the high pressure fluid. The endcap  100  also contains a bearing  210  which acts as the bearing for the motor and the screw pump so that there is no clash with the stator and no audible noise caused by the vibration of the rotor. The manifold  90  contains the high pressure outlet  150 , the low pressure inlets  220  and also a space  230  for the control electronics. The control electronics are protected by a cover  240 . 
     The motor components are the stator  30  around which the rotor  10  rotates. Attached axially around the rotor are permanent magnets  20 , shown in cross section in FIG.  4 . The windings of the stator are wound around T-sections  250 . The magnetic flux is directed down one T section and up an adjacent one; when combined with the current in the windings it causes a torque which turns the rotor  10 . 
     The pump is shown in use in a sample application, a car steering system, in FIG.  5 . The pump continually runs on idle—about 1000 rpm. A position sensor  260  detects when the steering wheel  290  is turned and the pump electronics rapidly ramp up the pump to its working speed of 5000-6000 rpm. The hydraulic fluid is delivered to the steering system  270  at high pressure and returns to the pump  280  at low pressure. After completion of the steering movement the pump motor returns to its idle speed. 
     This embodiment is only one of many possible embodiments of this invention. An alternative embodiment is shown in FIG. 6 where the pump is contained entirely within the stator  30 . Other types of pumps and types of motor are also possible. Many other applications are also possible; the pump would prove useful as an oil or fuel pump in a confined space. 
     The skilled person will appreciate that it is possible to combine many different types of motors and pumps, for example brushed d.c. motors, induction motors or switched reluctance motors with any of a roller vane pump, a geared pump or an internal gear pump in the manner described, even though the particular example detailed above relates to a screw type of pump and a brushless d.c. motor.