Abstract:
An electric motor having a regenerative braking capability is described which has several embodiments, each of which utilize two pairs of brushes, each pair of which has individual brushes positioned on diametrically opposite sides of the commutator. One pair is positioned for preferably optimally running the motor while the other pair of brushes is used solely for braking purposes. For normal motor or running operation, a motor switch assembly interacts with an activating mechanism to physically lift the pair of braking brushes from the surface of the commutator and also ensures that the running brushes come into contact with the commutator. When the motor switch assembly is switched to stop the motor, the switch assembly causes the activating mechanism to first lift the running brushes from the surface of the commutator and then place the braking brushes onto the commutator surface.

Description:
BACKGROUND OF THE INVENTION 
     The present invention is generally directed to series wound motors for use in power tools, and more particularly to such motors that have a regenerative braking capability. 
     Series wound electric motors that are used in many applications, including electrical power tools have an operating characteristic that is generally considered to be undesirable, namely, the motors tend to exhibit a relatively long coast-down time after the power supply voltage to them has been switched off. In some applications such as circular saws, for example, brake stopping time may be relatively long after the motor has been switched off due to the inertia of the motor armature, the gearing, the shaft and the circular saw blade. This coast-down time is not only a nuisance for power tool users, it presents a potential risk of injury to an operator who is careless or impatient when using such a tool. 
     Because the coast-down characteristic has been recognized as a problem for decades, there have been attempts to provide mechanical as well as electrical braking systems for such motors. Known electronic braking systems for universal motors employ some type of regenerative braking technique, which is based upon the fact that all motors can exhibit generator characteristics. When a tool is switched off, therefore, the motor behaves similarly to a generator in that power is generated for as long as the armature keeps spinning and a magnetic field from the stator exists. Universal motors employing wound fields on the stator are not easily braked since the magnetic field quickly collapses upon switch-off, which is why regenerative braking is employed. At switch off, there is only enough residual magnetism to allow generator action to occur for a short time. However, if all or a portion of this initial generated power is fed back into the stator coils, by placing the coils across the generator output, the magnetic field of the stator is “regenerated” for as long as the tool keeps rotating. In placing the field coils across the armature to allow regeneration of the magnetic field, the field coils themselves act as the load which results in the braking torque. It is common for a resistive element to be placed in series with the field coils to limit high current spikes, adjust braking time, and improve the longevity of internal components. 
     One important aspect regarding regenerative brakes deals with the construction of the field coils or with the connection of the field coils to the armature. For regenerative braking to occur, the polarity of the magnetic field must remain the same for braking as it was for normal running (or motoring). This is achieved most commonly with either of two techniques: by interchanging the connections between the field and armature at switch off, or by using a second set of field coils at switch off for the purposes of braking, wherein this second set of coils is oppositely wound in the same stator slots as the normal motoring coils. 
     However, regenerative braking has its disadvantages. Although it was previously stated that all motors exhibit generator characteristics, most motors are poor generators. This is primarily due to the fact that motors and generators are constructed differently. Motors require a lead angle relative to a geometric neutral position against the direction of rotation whereas generators require a lead in the direction of rotation relative to the neutral position. In this regard, the geometric neutral position is a straight line that lies perpendicular to the field poles. This may differ from a magnetic neutral position which is the north/south magnetic axis of the armature which results when power is applied to a pair of brushes spaced 180° apart contacting the commutator. 
     If the same lead angle is employed during running and braking, there can be drastic consequences when a motor with a back lead is forced to brake using conventional regenerative techniques. At tool switch off, when the regenerative action occurs for accomplishing braking, a huge spark is often witnessed at the brush/commutator interface. This spark damages the brushes and the commutator and reduces the life of the motor. The spark occurs because the motor is optimized to run as a motor with a back lead and is then switched run as a generator which requires a forward lead. 
     A compromise may be implemented in a regenerative brake design, by lessening the motor lead in order to obtain acceptable braking. However, by lessening the motor lead, motor performance is sacrificed. It may also be necessary to ensure a stronger brush pressure on the commutator in order to achieve acceptable braking. But again, this has adverse effects on motor performance and motor life. 
     SUMMARY OF THE INVENTION 
     An electric motor having a regenerative braking capability is described which has several embodiments, each of which utilize two pairs of brushes, with each pair having individual brushes positioned on diametrically opposite sides of the commutator. One pair is positioned for preferably optimally running the motor while the other pair of brushes is used solely for braking purposes. 
     For normal motor or running operation, a motor switch assembly interacts with an activating mechanism to physically lift the pair of braking brushes from the surface of the commutator and also ensures that the running brushes come into contact with the commutator. 
     When the motor switch assembly is switched to stop the motor, the switch assembly causes the activating mechanism to first lift the running brushes from the surface of the commutator and then place the braking brushes onto the commutator surface. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an electrical schematic diagram of a preferred embodiment of the motor embodying the present invention. 
     FIG. 2 is an electrical schematic diagram of an alternative embodiment of the motor embodying the present invention. 
     FIG. 3 is an electrical schematic diagram of a second alternative embodiment of the motor embodying the present invention. 
     FIG. 4 is a plan view of structure that can be used to engage and disengage pairs of motor brushes used in the circuitry shown in FIGS.  1  and  2 ; 
    
    
     DETAILED DESCRIPTION 
     The use of one pair of brushes during running and a separate pair for braking provides some distinct advantages compared to known prior art systems, including German Patent No. DE 195 40 740.7-32 which describes a system that uses a pair of brushes that can be oriented in such a way during running to achieve desirable performance; and when the tool is switched off, this same brush pair is then reoriented in a different position for obtaining desirable braking performance. Since one set of brushes is used for both running and braking, the brushes are subject to wear from conditions associated with both running and braking so overall brush life is lower than with the quad brush design embodying the present invention. 
     Since separate pairs of brushes are employed in the present invention, each brush pair can be optimized independently for both running and braking. Different brush geometries, brush compositions, brush spring pressure, angular placement or lead angle can be chosen to fully optimize brake and motor performance. In this regard, during running a lead angle relative to a geometric neutral position against the direction of rotation whereas generators require a lead in the direction of rotation relative to the neutral position. In this regard, the geometric neutral position is a straight line that lies perpendicular to the field poles. A lead angle for the running brushes can vary for optimum performance which will depend upon many design characteristics, and is generally within the range of about 7 degrees to about 40 degrees relative to geometric neutral in a direction opposite the direction of rotation. A lead angle for the braking brushes can also vary for optimum performance which will depend upon many design characteristics, and is generally within the range of about 7 degrees to about 40 degrees relative to geometric neutral in the direction of rotation. 
     This means that motor performance will not be sacrificed by the addition of a regenerative braking capability. Also, the spark which is typically seen at braking using conventional regenerative brakes is virtually eliminated, which leads to longer brush and commutator life. Having separate brushes used for braking contributes to longer brush life for both sets of brushes. 
     Another advantage is that brushes can be independently designed for either running or braking; so that optimal brush material, brush pressure, brush geometry, among other factors, can be chosen for each mode of operation. 
     Turning now to the drawings and particularly FIG. 1 which illustrates an electrical schematic diagram of a preferred embodiment of the present invention which is a universal motor having an electronic regenerative brake which is configured to have four brushes that are adapted to be brought into selective contact with the commutator of the armature of the motor. The motor, indicated generally at  10 , includes a rotatable armature having a commutator  12  of conventional design. A first pair of brushes  14 ,  16  are located on opposite sides of the commutator  12  and a second pair of brushes  18 ,  20  are also present. The pair of brushes  18 ,  20  operating in a regenerative braking circuit portion whereas the brushes  14 ,  16  are at a different lead angle relative to the outer periphery of the commutator  12  and the first and second sets of brushes are angularly positioned relative to one another in a manner that can produce optimal efficiency in the running of the motor as well as providing effective regenerative braking when desired. 
     In this regard, and referring to FIGS. 1 and 4, the motor of the present invention is particularly suited for use in a portable hand tool, such as a drill, router, circular saw, miter saw, saber saw or other motorized hand tool which has a switch  22  for energizing the motor. The switch  22  has switch contacts  24   a,    24   b,    26   a  and  26   b  as well as a brush pair selection mechanism, indicated generally at  28  (FIG.  4 ), which is also interfaced with the trigger switch  22  and operates in a manner that will be hereinafter discussed in detail. The selection mechanism  28  is of the type shown and described in U.S. Pat. No. 4,539,500, which is specifically incorporated by reference herein. 
     During operation, when the motor is running, switch contacts  24   a  and  26   a  are closed and switch contacts  24   b  and  26   b  are open. An AC source  30  is connected to line  32  that extends to contact  24   a  which in turn is connected to line  34  that is connected to contact  24   b  and field coil  36 , with the latter being series connected to line  38 , field coil  40  and line  42 , with line  42  being connected to both contacts  26   a  and  26   b.    
     When contacts  26   a  are closed, energy is provided to line  44  that extends to brush  16 , which together with brush  14  is in contact with the commutator  12 . Brush  14  is connected to the AC source  30  by line  46 . When the switch  22  is disengaged to de-energize the motor, the switch contacts  24   a,    24   b,    26   a  and  26   b  are in the positions shown in FIG.  1  and when in that de-energized position, the brushes  14  and  16  are lifted out of contact with the commutator and brushes  18  and  20  are placed in contact with the commutator. In this instance, contacts  24   b  are series connected to line  48 , resistor  50  and line  52 , which extends to the brush  18 . Brush  20  is connected to line  54  that is connected to contact  26   b.    
     For activating the regenerative braking action, the brushes  18  and  20  are lowered into contact with the commutator  12  and a closed loop through these brushes, the resistor, the field coils  36  and  40  is created which provides the regenerative braking action. It should be appreciated that the schematic diagram of FIG. 1 utilizes the field coils in both the running and braking operation. The resistor  50  in the braking circuit creates a load across the circuit loop which when not running as a motor and still spinning, behaves in a manner similar to a generator in that power is generated for as long as the tool keeps spinning. The resistor supplies a load across the generated power and results in a torque being produced which acts against the direction of rotation and causes the motor to come to a stop much more quickly. The resistor also has the effect of limiting high current spikes as well as adjusting the braking time and increasing the longevity of the internal component relative to what may occur were high current spikes permitted to occur. 
     It should also be understood that the preferred embodiment of FIG. 1 may be modified to remove the switch contacts  24   b,    26   a,  and  26   b  from the circuit. In such a modified circuit, when the brake brushes  18  and  20  are lifted out of contact with the commutator, the braking circuit loop will be open circuited and no current flow will occur through lines  48 , resistor  50 , line  52 , brake brushes  18  and  20 , line  54 , line  42 , coils  36  and  40 . However, if this modification is made, it may be important that the mechanism which lifts the brake brushes  18  and  20  from the commutator be appropriately mechanically timed with the operation of the AC line switch  22  so that switch contacts  24   a  and  26   a  are not closed before the brake brushes  18  and  20  are released from the commutator  12  contact. 
     Another alternative embodiment is shown in FIG. 2 which includes an armature having a commutator  60  around the outer periphery thereof, with running brushes  62  and  64  and braking brushes  66  and  68  being provided in the manner in which the brushes  14 ,  16 ,  18  and  20  were described in connection with FIG.  1 . An AC source  70  is connected via line  72  to switch contacts  74   a,  which are connected to brush  62  by line  76 . The other running brush  64  is connected by line  78  to field coils  80  which in turn are connected by line  82  to the AC source  70 . When the switch contacts  74   a  are closed by engaging the trigger switch  22  that also brings running brushes  62  and  64  into contact with the commutator  60 , the motor will be energized. If the switch  22  is disengaged to deenergize the motor, the running brushes  62 , 64  are lifted out of contact with the commutator  60  and braking brushes  66  and  68  are placed in contact therewith. When this occurs, brush  66  is connected by line  84  to a second set of braking field coils  86  which in turn are connected by line  88  to resistor  90  and line  92  that extends to a second set of contacts  74   b,  which in turn are connected to the brush  68  by line  94 . Thus, the braking loop is activated in operation when the trigger switch  22  is released. In this alternative embodiment of FIG. 2 where separate running and braking coils are provided, the braking coils  86  and running coils  80  are oppositely wound to ensure the same magnetic field polarity at braking as just prior to braking. 
     It should also be understood that with regard to the switch  22 , only the contact configuration is important and not the fact that only one switch is shown; i.e., the same contact arrangement can be achieved with one or more switches. Such implementations are known to one of ordinary skill in the art. 
     With regard to the activating mechanism  28  shown in FIG. 4, it may be used with any of the embodiments shown in FIGS. 1,  2  or  3 , but it will be described in connection with the embodiment of FIG.  1 . Turning to FIG. 4, brushes  14 ,  16 ,  18  and  20  are arranged around the commutator  12  with each brush being fixed in a brush holder  100 , which is pivotable about a pin  102  and is biased by a spring structure  104  in the direction toward the commutator  12  with the spring  104  thus insuring the contact between a particular brush and the periphery of the commutator  12 . Stops  106  on the motor housing plate  108  limits the maximum stroke of each holder caused by wear of the individual brush. 
     On the side of the commutator ring  12  is the activating mechanism formed by the plate  108  that is rotatable concentrically with the motor, with the plate being extended to form a handle  109  near the bottom thereof which is interconnected with the switch  22  of the tool. By means of the handle  109 , the activating mechanism can be set in two different working positions so that corresponding brushes can be lifted from the commutator  12  by means of cams  110  and  112  that are associated with each brush. Each cam cooperates with an extension  114  on each holder  100  so that, for example, by turning the plate  108  so that the cams  110  are rotated in the clockwise direction, the upper right hand brush  14  and lower left hand brush  16  are lifted from the commutator  12 . The other two brushes released by cam  112  are urged against the commutator ring  12  by means of the spring  104  associated with each holder  100 . With this setting, the motor will be driven in the running mode. When the activating mechanism  28  is displaced in the opposite direction, the associated cams  112  will lift the upper left brush  20  and lower right brush  18  from the commutator  12  whereas the other two brushes  14  and  16  will be moved into contact against the commutator ring. With this setting, the motor will be placed in the braking mode. 
     In yet another alternative embodiment of the present invention, and referring to FIG. 3, it is similar to the embodiment shown in FIG. 1, except that the resistor  50  in the braking circuit is replaced by a switching module  120  shown within a dotted box. All components having reference numbers in FIG. 3 that identical to those shown in FIG. 1 provide the same operation. The module  120  is shown as including a resistor  122  that may or may not be necessary depending upon whether the armature windings and field windings  36 ,  40  provide enough resistance not to destroy the other circuit components in the module  120 . The resistor  122 , if provided is connected to line  124  that extends to the emitter of a NPN transistor  126  and to the collector of a transistor  128 , with the collector of transistor  126  and the emitter of transistor  128  being connected to line  52 . Lines  52  and  54  extend to a pulse width modulated controller  130  that may be an integrated circuit or a microprocessor which has output lines  132  and  134  that extend to the base junction of each transistor  126 ,  128 . The controller operates to switch one of the transistors  126 ,  128  on and off at a rate that can be adjustably controlled. Generally, the higher the duty cycle of pulses produced, the faster the braking time. 
     During a braking operation, the timing of the release of the trigger switch  22  relative to the AC line cycle, affects the operation. When the switch  22  is released, the armature has a voltage across it and the polarity of that voltage depends upon when in the AC cycle you release the switch. So, if the voltage on brush  20  is minus and brush  18  is plus, the line  52  extending from the brush  18  to the transistors together with the output line  132  will switch transistor  126  on and off. It will be pulse width modulated, and during those instances when transistor  126  is on, current will go through that transistor  126 , through resistor  122  (if present), through the normally closed switch contacts  24   b,  through the one field coil  36 , line  38 , the other field coil  40 , through the other normally closed switch contacts  26   b,  and back to the brush  20 . 
     If at the time of release of the trigger switch  22 , the upper brush  20  is positive, current flows around the loop in the opposite direction, including down through the transistor  128  and then back to the brush  18 . Since both of the lines  52  and  54  also extend to the controller  130 , one of them will always be able to provide a positive voltage to power the controller. Thus, the controller  130  is being powered by the voltage being generated by the rotating armature, since releasing the switch  22  removes the AC power to the circuit 
     While various embodiments of the present invention have been shown and described, it should be understood that other modifications, substitutions and alternatives are apparent to one of ordinary skill in the art. Such modifications, substitutions and alternatives can be made without departing from the spirit and scope of the invention, which should be determined from the appended claims. 
     Various features of the invention are set forth in the following claims.