Abstract:
A direct current motor includes a stator and a rotor. The rotor has a shaft; a rotor core fixed to the shaft; a commutator fixed to the shaft; and rotor windings wound about poles of the rotor core and terminated on the commutator. The motor also has a common brush and at least two selectable brushes in sliding contact with the commutator; at least two direct current power sources; and a switching device configured to connect the common brush and a selected one of the selectable brushes with predetermined combinations of the power sources to operate the motor in different speed modes.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This non-provisional patent application claims priority under 35 U.S.C. §119(a) from Patent Application No. 200910108907.9 filed in The People&#39;s Republic of China on Jul. 17, 2009. 
     FIELD OF THE INVENTION 
     The invention relates to a motor, and in particular, to a direct current motor. 
     This invention relates to a motor assembly and in particular, to an assemble having a multiple speed DC motor. 
     BACKGROUND OF THE INVENTION 
     Appliances driven by electric motors are often required to operate at different rotational speeds. For example, when a drill driven by a motor is used for drilling holes, high speed operation is required, and when it is used for turning screws, low speed operation is required. Even though the motor may drive the appliance through a gearbox, sometimes the speed reduction provided by the gearbox is not sufficient or the use of a gearbox is not appropriate. 
     Speed and torque and battery voltage and current 
     SUMMARY OF THE INVENTION 
     Hence there is a desire for a motor which can operate in different predetermined speed modes. 
     Accordingly, in one aspect thereof, the present invention provides a direct current motor comprising a stator and a rotor, the rotor comprising: a shaft; a rotor core fixed to the shaft; a commutator fixed to the shaft; rotor windings wound about poles of the rotor core and terminated on the commutator; a common brush and at least two selectable brushes in sliding contact with the commutator; at least two direct current power sources; and a switching device configured to connect the common brush and a selected one of the selectable brushes with predetermined combinations of the power sources to operate the motor in different speed modes. 
     Preferably, the at least two selectable brushes comprises a low speed brush corresponding to low speed operation of the motor and a high speed brush corresponding to high speed operation of the motor; and wherein the switching device connects the power sources in series between the common brush and the high speed brush, to operate the motor in a high speed mode; and wherein the switching device connects the power sources in parallel between the common brush and the low speed brush, to operate the motor in a low speed mode. 
     Preferably, the at least two direct current power sources comprises a first power source and a second power source, and the switching device comprises three single pole double throw switches and each switch comprises two contact points and one contact arm; the first switch has one contact point connected with the positive terminal of the first power source and the fixed end of the contact arm connected with the positive terminal of the second power source; the second switch has one contact point connected with the positive terminal of the first power source, the other contact point connected with the negative terminal of the first power source and the common brush, and the fixed end of the contact arm connected with the negative terminal of the second power source; the third switch has one contact point connected to the low speed brush, the other contact point connected to the high speed brush, and the fixed end of the contact arm connected with the positive terminal of the second power source. 
     Preferably, the contact arms of the three switches are connected together via a connecting rod. 
     Preferably, the voltages of the power sources are equal. 
     Alternatively, the motor comprises two selectable brushes corresponding to low speed and high speed respectively; when the switching device connects the power sources in series between the common brush and the high speed brush, the motor operates in a high speed mode; and when the switching device connects one of the power sources between the common brush and the low speed brush the motor operates in a low speed mode. 
     Alternatively, the motor comprises two selectable brushes corresponding to low speed and high speed respectively; the power sources comprises a power source with a higher voltage and a power source with a lower voltage; and the switching device selectively connects the power source with the higher voltage between the common brush and the high speed brush, or connects the power source with the lower voltage between the common brush and the low speed brush. 
     Preferably, the switching device is a linked switch connected between the power sources and the selectable brushes. 
     Alternatively, the motor comprises two selectable brushes corresponding to low speed and high speed respectively; voltages of the power sources are different; and the switching device selectively connects anyone of the power sources between the common brush and anyone of the selectable brushes. 
     Alternatively, the at least two selectable brushes comprises a low speed brush and a high speed brush, and wherein the at least one power source comprises a first power source connected between the common brush and the switching device and a second power source connected between the switching device and the high speed brush, the switching device selectably connecting the first power source to the low speed brush for operating the motor in a low speed mode or connecting the first power source in series with the second power source between the common brush and the high speed brush to operate the motor in a high speed mode. 
     Advantages of embodiments of the present invention include that different brushes and corresponding combinations of the power sources can be selected by the switching device so that the motor operates in different speed modes. Optionally, effective work time of the motor can be increased by connecting the power sources in parallel when the motor is required to rotate at low speed or with greater torque. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  shows a schematic connection arrangement of the commutator, brushes, power sources and switching device of a motor in accordance with a first embodiment of the present invention; 
         FIG. 2  shows a schematic connection arrangement, similar to  FIG. 1 , of a motor in accordance with a second embodiment of the present invention; 
         FIG. 3  shows a schematic connection arrangement, similar to  FIG. 1 , of a motor in accordance with a third embodiment of the present invention; 
         FIG. 4  shows a schematic connection arrangement, similar to  FIG. 1 , of a motor in accordance with a fourth embodiment of the present invention; 
         FIG. 5  shows a schematic connection arrangement, similar to  FIG. 1 , of a motor in accordance with a fifth embodiment of the present invention; and 
         FIG. 6  shows a schematic connection arrangement, similar to  FIG. 1 , of a motor in accordance with a sixth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  schematically illustrates a direct current (DC) motor comprising a commutator C, a common brush B 1 , a low speed brush B 2 , a high speed brush B 3 , two DC power sources U 1 , U 2 , and a switching device. The brushes B 1 , B 2 , B 3  are in sliding contact with the commutator C. The commutator C is attached to the motor shaft P and comprises a plurality of spaced-apart segments. Windings wound about a core of the rotor are connected to the segments. In this embodiment, voltages of the DC power sources U 1 , U 2  are equal. 
     The switching device comprises three single pole double throw switches K 1 , K 2 , and K 3 . Each switch comprises two contact points and one contact arm with a fixed end and a free end arranged for selectively connecting with one of the two contact points. For the first switch K 1 , the first contact point S 11  is connected with the positive terminal of the first power source U 1 , the second contact point S 12  is left unconnected, and the fixed end of its contact arm is connected with the positive terminal of the second power source U 2 . For the second switch K 2 , the first contact point S 21  is connected to a point between the negative terminal of the first power source U 1  and the common brush B 1 , the second contact point S 22  is connected to the positive terminal of the first power source U 1 , and the fixed end of its contact arm is connected to the negative terminal of the second power source U 2 . For the third switch K 3  the first contact point S 31  is connected to the low speed brush B 2 , the second contact point S 32  is connected to the high speed brush B 3 , and the fixed end of the contact arm is connected with the positive terminal of the second power source U 2 . Contact arms of the three switches K 1 , K 2 , K 3  are connected together via a connecting rod (shown by a broken line in  FIG. 1 ). That is, selecting states of the free ends of the contact arms of the three switches can be changed by pushing the connecting rod. 
     When the free ends of the contact arms of the three switches K 1 , K 2 , K 3  are connected with the first contact points S 11 , S 21 , S 31  by pushing the connecting rod, the power sources U 1 , U 2  will be connected in parallel, between the common brush B 1  and the low speed brush B 2  and the motor shaft P will rotate at low speed, i.e. the motor will operate in a low speed mode. When the free ends of the contact arms of the three switches K 1 , K 2 , K 3  are connected with the second contact points S 12 , S 22 , S 32  by pushing the connecting rod, the power sources U 1 , U 2  will be connected in series between the common brush B 1  and the high speed brush B 3 , and the motor shaft P will rotate at high speed, i.e., the motor will operate in a high speed mode. 
     In the above embodiment, different brushes and corresponding combinations of the power sources can be selected by the switching device so that the motor can rotate in different speed modes. Furthermore, effective work time of the motor can be increased by using the power sources U 1 , U 2  connected in parallel when the motor is required to rotate at low speed. As an alternative arrangement, the three switches could be replaced by a single three pole double throw switch. 
     A second embodiment is shown in  FIG. 2 , in which the switching device is a linked switch K, such as a double pole, single throw switch, as shown, or a double pole, double throw switch. The common brush B 1  is connected to the positive terminals of the power sources U 1 , U 2 , and the negative terminals of the power sources U 1 , U 2  are respectively connected with the high speed brush B 3  and the low speed brush B 2  via contacts of the linked switch K. The linked switch K selectively connects the power source U 1  with the high speed brush B 3 , or connects the power source U 2  with the low speed brush B 2 . In this embodiment, the voltage of the power source U 1  is preferably higher than the voltage of the power source U 2 . 
     It should be understood that the switching device can also be a pair of linked switches K 1 , K 2 , as shown in  FIG. 3 . The voltages of the two power sources are not equal. Selectively connecting one of the power sources U 1 , U 2  with one of the brushes B 1 , B 2 , B 3 , such as connecting U 1  with B 2 , U 2  with B 2 , U 1  with B 3 , or U 2  with B 3  can be obtained through the cooperation of the pair of linked switches K 1 , K 2 , thereby the motor can be operated at more different speeds. In addition, when one of the power sources is run out, the other one can be switched on conveniently. 
     Referring to  FIG. 4 , it should be understood that switching device can also be a single pole double throw switch K. Contact point S 1  of the switch K is connected to the low speed brush B 2  and contact point S 2  is connected to the positive terminal of the power source U 2 , and the fixed end of the contact arm of the switch K is connected with the negative terminal of the power source U 1 . When the free end of the contact arm of the switch K is connected to the contact point S 1 , the power source U 1  will be connected with the low speed brush B 2  and the motor will rotate in the low speed mode. When the free end of the contact arm of the switch K is connected to the contact point S 2 , the power source U 1  will be connected with the power source U 2  in series between the common brush B 1  and the high speed brush B 3 , and the motor shaft will rotate at high speed. 
     Referring to  FIG. 5 , it should be understood that the switching device can also be a pair of single pole single throw switches K 1 , K 2  and a single pole double throw switch K 3 . The switches K 1 , K 2  are connected between the negative terminals of the power sources U 1 , U 2  and the fixed end of the contact arm of the switch K 3  respectively. The contact point S 1  of the switch K 3  is connected to the low speed brush B 2  and the contact point S 2  of the switch K 3  is connected to the high speed brush B 3 , The positive terminals of the power sources U 1 , U 2  are connected with the common brush B 1 . Selectively connecting one of power sources U 1 , U 2  with one of the brushes B 2 , B 3 , such as connecting U 1  with B 2 , U 1  with B 3 , U 2  with B 2 , or U 2  with B 3  can be obtained through the operation of the switches K 1 , K 2 , K 3 . In addition, both power sources can be connected in parallel to either selectable brush for additional power in that speed mode. This is particularly desirable for the low speed mode when additional torque is required. 
     As shown in  FIG. 6 , the switching device can also be a single pole double throw switch K 1  and a linked switch K 2 . The fixed end of the contact arm of the switch K 1  is connected with the positive terminal of the power source U 2 , two contact points of the switch K 1  are respectively connected with two terminals of the power source U 1 , the common brush B 1  is connected with the positive terminal of the power source U 1 . The linked switch K 2  comprises four contact points S 1 ˜S 4  and a contact arm with a central terminal and two free ends for selectively connecting with two of the contact points S 1 ˜S 4 . The contact point S 1  is connected with the low speed brush B 2 , the contact point S 2  is connected with the high speed brush B 3  and the contact point S 3  is connected with the negative terminal of the power source U 1 . The center terminal is connected with the negative terminal of the power source U 2 . Through operation of the switches K 1 , K 2 , selective connection of the power sources U 1 , U 2  with the high speed brush B 3  or connection of the power source U 2  with the low speed brush B 2  can be obtained to operate the motor in different predetermined speed modes. That is, low speed brush B 2  can be connected to both power sources connected in parallel, while the high speed brush B 3  can be connected to both power sources connected in series or to just power source U 2  to provide two high speed modes of operation. If switch K 1  has an open position then brush B 2  could be connected to just power source U 2  allowing the two power sources to have different voltages and thus not suitable to being connected in parallel. 
     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. 
     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. Therefore, the scope of the invention is to be determined by reference to the claims that follow. 
     For example, number of power sources can optionally be three or four or more and the number of brushes can optionally be four or more. By selecting different brushes and corresponding combinations of the power sources, the motor can operate in different speed modes.