Abstract:
A pump for pumping liquid has an electric motor driving an impeller disposed within a pump chamber. The motor has a stator, a rotor, and an end cap. The end cap has a printed circuit board upon which are mounted two brushes connected to a pair of motor terminals through a thermistor. Optionally, the motor may include, singularly or in combination, one or more inductors located between the brushes and the motor terminals, a varistor for preventing excessive voltage spikes electrically connected between the motor terminals, and one or more capacitors electrically connecting the motor terminals to ground.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This non-provisional patent application claims priority under 35 U.S.C. §119(a) from Patent Application No. 201310215238.1, filed in The People&#39;s Republic of China on Jun. 3, 2013, the entire contents of which are incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    This invention relates to an electric motor driven pump and in particular, to an electric pump having overcurrent protection. 
       BACKGROUND OF THE INVENTION 
       [0003]    When an electric motor draws too much current, it may suffer damage from overheating. 
         [0004]    One situation in which a motor may draw too much current is when its output is impeded. For instance, in an electric pump, the impeller may be blocked preventing the motor shaft from rotating, causing the motor to stall. In such a stall condition the motor may draw too much current and over heat. 
         [0005]    For example, in an automotive application an electric pump may be used to spray water or a detergent solution on to the windshield or head lamps. In cold environments when the temperature drops below freezing, the water or detergent within the pump may freeze, preventing the impeller from rotating. 
         [0006]    When the pump motor is powered on but the impeller is unable to move, the motor continues to draw power such that it generates a large amount of heat over a short period of time, potentially damaging the motor and creating a fire hazard. In order to solve this problem, a fuse can be located between the motor and its power source, and arranged to break if the motor exceeds a certain temperature. However, replacement of the fuse incurs additional maintenance costs, and can be a cumbersome process. 
       SUMMARY OF THE INVENTION 
       [0007]    Accordingly, there exists a need for an electric pump which will not overheating when the impeller is unable to rotate. In addition, the electric pump may include electrical noise suppression components to prevent the pump from interfering with other electrical components and devices. 
         [0008]    Accordingly, in one aspect thereof, the present invention provides an electric pump for pumping liquid, comprising: a pump housing defining a pump chamber; an impeller disposed in the pump chamber; and an electric motor for driving the impeller, the motor comprising: a stator defining a plurality of magnetic poles; and a rotor configured to rotate relative to the stator, the rotor comprising: a shaft; a rotor core fixed to the shaft; a commutator fixed to the shaft adjacent one end of the rotor core; and a plurality of winding coils wrapped around the rotor core and electrically connected to the commutator; first and second motor terminals configured to be connected to an external power source; first and second brushes in sliding contact with the commutator; and a positive temperature coefficient thermistor, wherein the first brush is electrically connected to the first motor terminal through the thermistor; and the second brush is electrically connected to the second motor terminal. 
         [0009]    Preferably, the brushes, the thermistor, and the motor terminals are mounted on an end cap located on an axial end of the motor. 
         [0010]    Preferably, the brushes and the thermistor are mounted on a printed circuit board mounted on the end cap of the motor. 
         [0011]    Preferably, the pump includes a pump cover incorporating a connector housing accommodating the motor terminals, with a first end of the motor terminals disposed in the connector for connection to a power supply and a second end of the motor terminals being inserted in respective sockets disposed on the end cap. 
         [0012]    Preferably, the pump has one or more inductors, respectively electrically connected between the brushes and the motor terminals. 
         [0013]    Preferably, the one or more inductors comprise a first inductor and a second inductor, such that the first brush is connected to the thermistor through the first inductor, and the second brush is connected to the second motor terminal through the second inductor. 
         [0014]    Preferably, a varistor is connected to the two motor terminals in series with the thermistor. 
         [0015]    Preferably, a first capacitor connects a terminal of the thermistor not connected to the first motor terminal to ground, and a second capacitor connects the second motor terminal to ground. 
         [0016]    Preferably, the capacitors are connected to a ground terminal that is connected to a motor housing of the motor. 
         [0017]    Preferably, the pump is configured to pump water or a detergent solution onto an automobile windshield. 
         [0018]    Preferred and/or optional features are set forth in the dependent claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    A preferred embodiment of the invention will now be described, by way of example only, with reference to figures of the accompanying drawings. In the figures, identical structures, elements or parts that appear in more than one figure are generally labeled with a same reference numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are generally chosen for convenience and clarity of presentation and are not necessarily shown to scale. The figures are listed below. 
           [0020]      FIG. 1A  is a perspective view of an electric pump having a motor in accordance with the preferred embodiment of the present invention; 
           [0021]      FIG. 1B  is a sectional view of the pump of  FIG. 1A ; 
           [0022]      FIGS. 2A and 2B  illustrate an end cap assembly for the motor of  FIG. 1A ; 
           [0023]      FIG. 3  illustrates a circuit board assembly used in the end cap assembly of  FIGS. 2A and 2B ; 
           [0024]      FIG. 4  is a schematic diagram of the circuit board assembly of  FIG. 3 ; and 
           [0025]      FIG. 5  is a graph showing the electrical properties of a thermistor used in the motor of  FIG. 1A . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0026]      FIGS. 1A and 1B  illustrate an electric pump  10  used for pumping a liquid, in accordance with the preferred embodiment of the present invention.  FIG. 1B  is a longitudinal sectional view taken along the axis of the pump, which is coaxial with the axis of the motor. In this embodiment the liquid pump  10  is used in a vehicle to spray water, a detergent, or other type of liquid, onto a windshield. It is understood that in other embodiments, pump  10  may be used in any type of application that involves the pumping of a liquid. 
         [0027]    Pump  10  comprises a motor  20 , a pump chamber  11 , and an impeller  14  connected to and driven by motor  20 . The impeller is accommodated within the pump chamber. Pump chamber  11  has an inlet  52  connecting the pump chamber to a source of liquid (e.g., a water or detergent container) and at least one outlet  54  through which, in use, the liquid is discharged. During operation, motor  20  rotates impeller  14  creating a centrifugal force so that liquid flowing in from the inlet  52  is expelled through outlet  54 . The pump chamber is defined by the space formed between a pump housing  12  and a pump plate  13 . 
         [0028]    In the embodiment shown in  FIG. 1A , the pump has two outlets  54  interconnected by a diverter valve  17 . In this embodiment, the direction of rotation of the impeller determines through which outlet the liquid will be pumped. 
         [0029]    As shown in  FIG. 1B , motor  20  comprises a stator  21 , a rotor  22  rotatably mounted to stator  21 , and an end cap assembly  30 . Stator  21  comprises a motor housing  24  and a plurality of magnets  25  fixed to an inner wall of motor housing  24 . Magnets  25  are permanent magnets, although it is understood that magnets  25  may comprise any type of component capable of generating a magnetic field and/or defining a magnetic pole, including electro-magnets. Preferably, motor housing  24  is substantially cylindrical. Most preferably, the motor housing is a deep drawn cup-shaped metal part having one open end and one closed end. 
         [0030]    Rotor  22  comprises a shaft  26 , a commutator  27  and a rotor core  28  attached to shaft  26 , and a plurality of winding coils  29  wound around rotor core  28  and connected to commutator  27 . One axial end of shaft  26  is rotatably attached to one end of motor housing  24 , preferably the closed end by a bearing  23 . The other axial end of shaft  26  is rotatably attached to end cap assembly  30  by another bearing  23 . Bearings  12  may be ball bearings, bearing sleeves, or any other components that provide mechanical coupling between moving and stationary parts, allowing rotor  22  to rotate relative to stator  21 . 
         [0031]    Motor  20  is attached to pump plate  13  which forms a wall of pump chamber  11 . Motor  20  and pump plate  13  are disposed inside pump housing  12  and a pump cover  60  closes the open end of the pump housing. Pump cover  60  is preferably splash proof to give motor  20  protection for the environment. Pump cover  60  has a mounting projection  64  and forms a connector  62  for connection a power supply to motor terminals A shaft seal  18  seals shaft  26  to pump plate  13 . O-ring seal  19  seals pump plate  13  to pump housing  12  to provide a water tight pump chamber  11 . 
         [0032]      FIGS. 2A and 2B  illustrate an end cap assembly  30  of the preferred embodiment. End cap assembly  30  comprises a plurality of brushes  31  arranged to make sliding contact with commutator  27 . When motor  20  is powered, brushes  31  supply electrical power to winding coils  29  through commutator  27 , causing rotor  22  to rotate within stator  21  and drive impeller  14 . 
         [0033]    End cap assembly  30  comprises an end cap  32  and a circuit board assembly  34  fixed to end cap  32 . As illustrated in  FIG. 3 , brushes  31  are connected to circuit board assembly  34 , and positioned on opposite sides of commutator  27 , such that, when motor  20  is assembled, brushes  31  are able to maintain sliding contact with a surface of commutator  27 . The illustrated brushes are of the leaf brush type, having a brush terminal  31   a  fixed to the circuit board, a brush body  31   b  arranged to make sliding contact with the commutator and connected to the brush terminal, electrically and mechanically, by a resilient brush arm  31   c.  The brush body is of a carbon based material and the brush arm urges the brush body against the commutator. However, other types of brushes may be used. 
         [0034]    Circuit board assembly  34  may comprise a printed circuit board (PCB)  36 , and a plurality of electrical components, including a pair of motor terminals  38   a/b,  a pair of chokes or inductors  40   a/b,  a pair of sockets  42   a/b,  a varistor  46 , capacitors  50   a/b,  and a positive temperature coefficient thermistor (PTC)  44 .  FIG. 5  is a graph illustrating the electrical properties of PTC  44  in accordance with the preferred embodiment. 
         [0035]    In some embodiments, PCB  36  is configured such that some of the electrical components (e.g, inductors  40   a/b,  and thermistor assembly  44 ) are arranged on one side of PCB  36 , while the opposite side of PCB  36  carries a plurality of electrical traces connecting the electrical components in accordance with the circuit diagram illustrated in  FIG. 4 . Brushes  31 , inductors  40   a  and  40   b,  sockets  42   a  and  42   b,  varistor  46  and PTC  44  are connected to PCB  36  by soldering. 
         [0036]    Motor terminals  38   a/b  are arranged to be connected to an external power source (not shown), such as a battery, generator, or outlet. Motor terminals  38   a/b  connect to the electrical components on PCB  36  by being pressed into sockets  42   a/b.  In the preferred embodiment, sockets  42   a/b  are substantially U-shaped, each comprising a pair of clamping ends  43 . At least one end of each pair of clamping ends  43  is substantially V-shaped and arranged such that a minimum gap between the ends of each pair of clamping ends  43  is smaller than a thickness of the motor terminals, allowing for motor terminals  38   a/b  to be inserted into corresponding sockets  42   a/b  and held in place by clamping ends  43 . Preferably, both clamping ends of each pair of clamping ends are V-shaped and contact each other at the apex of the V in the relaxed state. As illustrated in  FIG. 4 , motor terminal  38   a  is electrically connected to one of the bushes  31  through PTC  44  and inductor  40   a,  while motor terminal  38   b  is connected to the other brush  31  through inductor  40   b.    
         [0037]    As illustrated in  FIG. 3 , PTC  44  comprises a thermistor element  44   b  mounted between a pair of metal strips forming the thermistor terminals  44   a  and  44   c.  One thermistor terminal  44   c  is connected to motor terminal  38   a  and the other thermistor terminal  44   a  is connected to inductor  40   a.  This arrangement may be used to improve the physical strength of PTC  44 . In addition, due to the mass of the thermistor terminals the temperature rise of the thermistor may be dampened slightly to allow the PTC to be less sensitive to sudden transient current surges. 
         [0038]    During operation of motor  20 , if rotation of impeller  14  is impeded (e.g., caused by freezing of liquid within pump chamber  11 , the current flowing through motor  20  may rapidly increase, causing the temperature of thermistor  44  to rise. As illustrated in  FIG. 5 , after thermistor  44  exceeds a certain temperature, the resistance of thermistor  44  will increase significantly. The increased resistance of thermistor  44  reduces the amount of current that flows through motor  20 , effectively turning the motor off, thus protecting motor  20  from suffering damage due to too much current draw. When the temperature of thermistor  44  decreases, its resistance will return to normal, allowing motor  20  to resume normal operation once impeller  14  is able to rotate again. 
         [0039]    During normal operation of the pump, the current through the motor tends to fluctuate rapidly due to commutation by the commutator. This results in electrical noise being transferred to the power supply. Electrical noise is undesirable as it may interfere with other electrical products. In order to reduce the electrical noise created, inductors  40  are connected in series between brushes  31  and motor terminals  38 , wherein the inductive properties of the inductors function to stabilize the current flow through winding coils  29 . In the illustrated embodiment, each motor terminal  38   a/b  is connected to a respective inductor  40   a/b.    
         [0040]    To further enhance suppression of electrical noise, the motor terminals may be connected to earth via a capacitor  50   a/b.  To this end, circuit board assembly  34  further comprises a ground terminal  48  to which one terminal of each of capacitors  50   a/b  is connected. The other terminal is electrically connected to the terminals of inductors  40   a/b  which are connected to motor terminals  38   a/b,  respectively. In the preferred embodiment, ground terminal  48  is fixed to PCB  36  and connected to an edge of end cap  32 , such that when end cap assembly  30  is assembled to motor housing  24 , ground terminal  48  is connected to outer shell  24 . 
         [0041]    In the preferred embodiment, circuit board assembly  34  further comprises a varistor  46 , wherein one terminal of varistor  46  is electrically connected to the terminal of inductor  40   a  that is connected to motor terminal  38   a,  while the other terminal of varistor  46  is electrically connected to the terminal of inductor  40   b  that is connected to motor terminal  38   b.  Varistor  46  exhibits a high resistance at low voltages, but a low resistance at high voltages. During operation, if the voltage across the motor terminals experiences a sudden fluctuation, e.g. a voltage spike caused by commutation, excess current caused by the spike in voltage can be shunted through the varistor, limiting the size of the voltage spike, thus further stabilizing the current drawn by motor  20 . 
         [0042]    It is understood that while the illustrated embodiments illustrate a motor  20  having an inner rotor design with rotor  22  accommodated and configured to rotate within stator  21 , other configurations may be used in other embodiments, e.g., a brushless motor, or a motor having an outer rotor design with the stator being accommodated within the rotor. For example, in a brushless motor, inductors  40   a/b  may be connected to one or more stator winding coils instead of electric brushes  31 . 
         [0043]    Although the invention is described with reference to one or more preferred embodiments, it should be appreciated by those skilled in the art that various modifications are possible. For example, the ordering of or spatial relations among many of the described components may be changed. In addition, non-essential features may be omitted entirely or included in various combinations, not specifically described. The specification and drawings are, accordingly, to be regarded in an illustrative or explanatory rather than restrictive sense. Therefore, the scope of the invention is to be determined by reference to the claims that follow. 
         [0044]    In the description and claims of the present application, each of the verbs “comprise”, “include”, “contain” and “have”, and variations thereof, are used in an inclusive sense, to specify the presence of the stated item but not to exclude the presence of additional items.