Abstract:
A system and method for connecting supply power to motor control components includes use of a motor control center subunit with moveable supply power contacts. After a motor control center subunit is secured into a motor control center compartment, the supply power contacts may be advanced to engage supply power buses. For disconnection, the supply power contacts may be retracted and isolated from the buses before physical removal of the subunit. A free wheeling mechanism prevents supply power contacts from advancing and retracting past a preset travel range.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates generally to motor control systems, and more particularly, to a motor control center subunit having a clutch system which governs the limits of extension and retraction of power circuitry within the motor control center subunit. In one embodiment, the system and method described herein allow translation of power circuitry along a threaded drive after full installation of the motor control center subunit into the motor control center. Translation of the power circuitry occurs between a position wherein the power circuitry is engaged with a power bus and a position wherein the power circuitry is retracted from the power bus. Once power circuitry reaches either the engaged or retracted position, the clutch system causes the drive to spin freely and thus prevents movement past either the engaged or retracted position. 
   A motor control center is a multi-compartment steel enclosure with a bus system to distribute electrical power, on a common bus system, to a plurality of individual motor control units mountable within the compartments. The individual motor control center subunits are commonly referred to as “buckets” and are typically constructed to be removable, pull-out units that have, or are installed behind, individual sealed doors on the motor control center enclosure. These buckets may contain various motor control and motor protection components such as motor controllers, starters, contactor assemblies, overload relays, circuit breakers, motor circuit protectors, various disconnects, and similar devices for electric motors. The buckets connect to the supply power lines of the motor control center and conduct supply power to the line side of the motor control devices, for operation of motors. Motor control centers are most often used in factories and industrial facilities which utilize high power electrical motors, pumps, and other loads. 
   Typically, when installing or removing motor control center buckets, the power supply lines are connected. To remove such a bucket, a deadfront door of the bucket or of the motor control center is opened and an operator manually pulls on the bucket to separate the primary disconnects, or “stabs,” from the bus system, thereby disconnecting the power supply. Installation of a bucket is accomplished in a similar manner, wherein the operator manually pushes the bucket into a compartment of the motor control center to engage the bucket stabs with the bus system, thus connecting the system to supply power. The line connections or stabs may be difficult to maneuver manually when an operator is supporting the entire bucket or when the stabs are not visible. 
   Attempts have been made to improve upon the manual installation and disconnection of motor control center buckets and supply power connections from live supply power lines, risers, and/or a vertical bus of a motor control center. Other systems have employed pivotable handles inside the buckets to pivot line connectors to and from supply lines. However, many of these systems require that the bucket or compartment door be open to manipulate the handles and line stabs. Additionally, these systems can subject the pivot line connectors and other components to overdrive and/or overtorquing, as the system includes solid stops when manipulating the connectors between racking in and extracting out positions. 
   It would therefore be desirable to design a motor control center bucket assembly that overcomes the aforementioned drawbacks. Thus, it would be desirable to provide for remote connection or disconnection of the line stabs of a bucket to the power supply lines or bus of a motor control center from a distance. In the event of an arc or arc flash, any heated gas, flame, and/or the arc itself should preferably be contained behind the bucket compartment door or “deadfront.” Furthermore, as connection or disconnection of the line stabs is to be completed remotely, it would be desirable to provide a means to prevent overdrive or overtorquing during remote connection or disconnection of the line stabs. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The present invention provides a system and method for installing a motor control center subunit or bucket into a motor control center and electrically connecting motor control components of the bucket to a power supply. The system and method utilize moveable line stabs to engage the power supply (such as bus bars in parallel) after the bucket has been secured in the motor control center, in order to contain potential arc flashes. A clutch system defines the limits of movement of the line stabs within the motor control center. 
   Therefore, in accordance with one aspect of the present invention, a motor control center subunit includes a housing, a drive mechanism attached to the housing and configured to translate a plurality of selectively moveable conductive contacts between a retracted position and an extended position within the housing, and a freewheeling mechanism attached to the drive mechanism and configured to define the retracted position and the extended position of the conductive contacts. The conductive contacts are moveable when the housing is seated in a motor control center and a front panel of the housing is in a closed position. 
   In accordance with another aspect of the invention, a motor control center includes a motor control center frame having at least one compartment and a motor control center subunit housing constructed to seat in the at least one compartment of the motor control center frame. The motor control center also includes an actuating mechanism attached to the motor control center subunit to control movement of a plurality of conductive contacts and a clutch configured to define a range of movement of the plurality of conductive contacts, with the range of movement having ends at a fully engaged position and a fully retracted position. 
   According to a further aspect of the invention, a motor control center subunit includes a control module housing having a plurality of conductive contacts therein and an actuating mechanism to control movement of a plurality of conductive line contacts when a front panel of the housing is in a closed position. The motor control center subunit also includes a means for preventing the actuating mechanism from translating past a maximum extended position and a maximum retracted position. 
   Various other features and advantages of the present invention will be made apparent from the following detailed description and the drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The drawings illustrate one preferred embodiment presently contemplated for carrying out the invention. 
     In the drawings: 
       FIG. 1  is a partial perspective view of a motor control center subunit installed in a motor control center. 
       FIG. 2  is a perspective view of a motor control center subunit of  FIG. 1 , removed from the motor control center. 
       FIG. 3  is a top view of the motor control center subunit of  FIG. 1  showing a number of stabs in a retracted position. 
       FIG. 4  is top view of the motor control center subunit of  FIG. 3  showing the stabs in a test position. 
       FIG. 5  is a top view of the motor control center subunit of  FIG. 4  showing the stabs in an extended position. 
       FIG. 6  is a cross-sectional view of the motor control center subunit of  FIG. 3  taken along line  6 - 6  of  FIG. 3 . 
       FIG. 7  is a detailed view of a portion of the motor control center subunit of  FIG. 6  showing an arc shield, line contact, and supply conductor thereof. 
       FIG. 8  is a detailed view of the motor control center subunit of  FIG. 7  showing line contact extension. 
       FIG. 9  is a plan view of a control handle of one embodiment of the present invention. 
       FIG. 10  is a side view of the control handle of  FIG. 9 . 
       FIG. 11  is a plan view showing the control handle of  FIG. 9  rotated ninety degrees. 
       FIG. 12  is a side view of the control handle of  FIG. 11 . 
       FIG. 13  is a side view showing the control handle of  FIG. 11  depressed into a motor control center subunit. 
       FIG. 14  is a plan view showing the control handle of  FIG. 11  rotated ninety degrees. 
       FIG. 15  is side view of the control handle of  FIG. 14 . 
       FIG. 16  is a partial perspective view of the motor control center subunit of  FIG. 1  showing an alternate embodiment. 
       FIG. 17  is a partial perspective view of the motor control center subunit of  FIG. 3  showing a clutch assembly. 
       FIG. 18  is a cross-sectional view of the clutch assembly of  FIG. 17  taken along line  18 - 18  of  FIG. 17  and showing the clutch assembly in a fully engaged position. 
       FIG. 19  is a cross-sectional view of the clutch assembly of  FIG. 17  taken along line  18 - 18  of  FIG. 17  and showing the clutch assembly in a fully retracted position. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The following description makes reference to supply power, supply power lines, motor power, load power, line power, and the like. It is appreciated that such terms may refer to a variety of both common and uniquely conditioned voltage and current characteristics, including but not limited to, three phase AC power, single phase AC power, DC power, multiple DC power lines, or any combination thereof. Such power characteristics will be generally referred to as being provided on a bus, supply line, or riser of a motor control center. However, it is appreciated that the present invention may find applicability in other power connectivity configurations, adapted or apart from motor control centers. An example of supply power commonly used in motor control centers is 480V three-phase AC power distributed over three separate supply bus bars. In addition, references to “motor control components” shall be understood to include the various types of devices and control components which may be housed in a motor control center bucket for connection to the supply power. Such devices and components include contactors, relays, motor controllers, disconnects, circuit protective devices, and the like. 
   Referring to  FIG. 1 , a partial perspective view of a motor control center structure  10  is shown. As discussed above, motor control centers may include compartments or enclosures for multiple control modules or buckets  11 ,  13 ,  14 ,  15 ,  16 ,  17 . Bucket  16  is shown fully installed into motor control center compartment or enclosure  12  such that its front panel  18  is seated securely against the periphery of enclosure  12  and flush with the front panel  20  of bucket  14 , with front panel  18  being a dead front panel. In this regard, bucket  16  includes a number of latching mechanisms  22  on dead front panel  18  so that an operator may lock bucket  16  into place once installed. In some embodiments, dead front panel  18  may be a door having a set of hinges  19  in order to permit access to motor control components within bucket  16  while bucket  16  is installed in enclosure  12  of motor control center  10 . However, even when closed or sealed, dead front panel or door  18  still permits access to circuit breaker assembly  28 , stab indicator  24 , shutter indicator  26 , and line contact actuator  31 . Line contact actuator  31  is a mechanism for engaging line contacts ( FIG. 2 ) with line power from the motor control center  10 . Thus, even when bucket  16  is fully installed in enclosure  12  and latches  22  have been secured, an operator may still use disconnect handle  30  and may open slide  32  to insert crank  34  to move one or more line contacts (not shown) of the bucket  16 . When slide  32  is moved aside to permit access to actuating mechanism  31 , door  18  is prevented from opening, thereby closing off access to components inside bucket  16 . Additionally, a user may desire to padlock the slide  31  in the closed position, to further regulate who may operate actuating mechanism  31  and when. 
   Referring now to  FIG. 2 , a perspective view of a motor control center bucket  16  is shown. It is noted that bucket  16  may have a housing that includes a number of panels surrounding bucket  16  to fully or partially enclose the components thereof. As shown, bucket  16  includes a pair of side panels  52  and a dead front panel  18 , which support motor control devices and internal bucket components. An upper panel and a rear panel have been removed to show the internal components of bucket  16 . Dead front panel  18  is configured to fit snugly and securely within a motor control center such that a rim  38  of the dead front panel  18  seats against the inner periphery (not shown) of a motor control center enclosure. For purposes of dust protection, rim  38  may optionally include a compressible or flexible seal, such as a rubber seal, or other gasket-type component. Once bucket  16  is inserted into a motor control center enclosure, latch mechanisms  22  may be turned with a key, a screwdriver, or by hand so that latch arms  40  abut an inner surface of the outer periphery (not shown) of an enclosure to hold bucket  16  in place and/or prevent bucket  16  from being removed. Similarly, an automatic retention latch  60  is shown in an engaged position. Upon advancement of line contacts or stabs  46 ,  48 ,  50  automatic retention latch  60  is triggered to engage a frame or lip of the motor control center unit in which bucket  16  is installed. 
   When slide  32  of line contact actuator  31  is moved aside, an opening  36  is exposed. Opening  36  preferably has a unique configuration to accept a specialized crank  34  (as shown in  FIG. 1 ). In other embodiments, to be described below, a manually drivable handle may extend through opening  36  or a remotely operable motor may be the actuator  31 . When slide  32  is moved aside as shown, slide  32  extends over a portion of dead front panel  18 . Thus, in embodiments in which dead front panel  18  is a hinged door, moving slide  32  to expose opening  36  will inhibit a user from opening dead front panel  18 . 
   Bucket  16  also includes a number of conductive contacts or stabs  44 ,  46 ,  48 ,  50 . Control power contact  44  is preferably fixedly attached to the rear of bucket  16 , whereas supply power stabs  46 ,  48 ,  50  are moveable with respect to bucket  16 . However, it is appreciated that control power contact  44  may also be moveable in a similar manner to line power stabs  46 ,  48 ,  50 . Control power contact  44  is of a suitable construction to conduct a control power (typically a few volts) to motor control components (not shown) disposed within bucket  16 . In embodiments where control power contact  44  is permanently positioned at the rear of bucket  16 , control power contact  44  will engage a control power supply line or bus upon installation of bucket  16  into a motor control center. 
   Supply power stabs  46 ,  48 ,  50 , on the other hand, do not initially engage supply power lines or buses when bucket  16  is installed into a motor control center. Rather, stabs  46 ,  48 ,  50  are initially in retracted position  42 , as shown in  FIG. 2 , disposed inside bucket  16 . One skilled in the art will appreciate that a number of configurations of supply power stabs  46 ,  48 ,  50  may be utilized. In the embodiment shown, stabs  46 ,  48 ,  50  are shaped to grasp about a supply line, bus, or riser of the motor control center  10  of  FIG. 1 . A lien contact or stab assembly  58  (i.e., conductive contact assembly) includes a stab bracket  59  to which the stabs  46 ,  48 ,  50  are attached. Stab bracket  59  holds stabs  46 ,  48 ,  50  in an orientation for subsequent engagement with the supply power lines or buses of motor control center  10  of  FIG. 1 . It is recognized, however, that stab assembly  58  of  FIG. 2  may include any number of configurations, such as for independently moveable stabs, for other than three stabs, or for actuation by other than a shaft, as will be described below. A shutter or isolator assembly  54  is disposed in the rear of bucket  16 , between stab assembly  58  and the exterior of bucket  16 . Isolator assembly  54  includes a number of moveable shutters  56  which operate to either expose or isolate the stabs  46 ,  48 ,  50  from the power lines or buses of the motor control center  10  of  FIG. 1 . 
     FIG. 3  depicts a top view of bucket  16 , with all housing panels removed except for dead front panel or door  18 . As shown, stab assembly  58  has positioned stabs  46 ,  48 ,  50  in a retracted position  42  wherein the stabs  46 ,  48 ,  50  are located inside bucket  16 . Accordingly, shutters  56  of shutter assembly  54  are closed, isolating the stabs  46 ,  48 ,  50  from the supply power bus or line of a motor control center such as shown in  FIG. 1 . As shown in  FIG. 3 , each shutter  56  includes two separate shielding members  62  and  64 ,  66  and  68 ,  70  and  72 . The shutter  56  for stab  46  includes a left shielding portion  62  and a right shielding portion  64 , each being angled toward stab  46 . Likewise the shutters  56  for stabs  48  and  50  include left shielding portions  66 ,  70  and right shielding portions  68 ,  72  respectively, each being angled toward the corresponding stab. However, the shutter  56  for stab  50  includes an additional mechanical connection  74 . That is, a shutter arm  74  is provided to control a shutter indicating mechanism  76  which displays to an operator via dead front panel indicator  26  whether the shutters  56  are open or closed, as will be described in further detail below. Similarly, a cam or bell crank  80  is attached via rod  78  to stab assembly  58  to translate movement of the stab to a microswitch  82 . Microswitch  82  operates to turn on and off the supply of control power from control power contact  44  to motor control components, such as contactors or overload relays (not shown), of bucket  16 . 
   Referring now to  FIG. 4 , the bucket  16  is shown having the stab assembly  58  in a test position  43 . Stabs  46 ,  48 , and  50  have been advanced to a point or test position  43  at which they nearly touch or just touch shutters  56 , but shutters  56  are still closed. Since shutters  56  are closed, stabs  46 ,  48 ,  50  are isolated from supply power buses, thus preventing arcs from occurring between stabs  46 ,  48 ,  50  and the buses. Being in the test position, stab bracket  59  is moved forward such that actuating shaft or drive  84  is visible. Preferably, shaft  84  is a rotary drive shaft and is connected to the socket of opening  36  shown in  FIG. 2  for operation via crank  34 , shown in  FIG. 1 . Referring back to  FIG. 4 , during the advancement of stab assembly  58 , automatic latch  60  has been triggered to engage the enclosure of the motor control center into which bucket  16  has been installed. Also due to the advancement of stab assembly  58 , rod  78  is pulled by stab bracket  59  such that cam  80  has rotated away from microswitch  82 . Microswitch  82  is thus actuated to permit control voltage from the control power contact  44  to a motor control component, such as a contactor or overload relay (not shown). It is appreciated, however, that microswitch  82 , cam  80  and rod  78  are optional. In other words, embodiments of the present invention may simply permit control voltage to pass through control power contact  44  directly to motor control components immediately upon installation of bucket  16  into a motor control center when contact  44  engages a control power bus. 
     FIG. 5  depicts another top view of the bucket  16  wherein the stabs  46 ,  48 ,  50  are in an extended/engaged position  45 . In operation, stabs  46 ,  48 ,  50  are advanced or extended from the test position  43  of  FIG. 4  towards shutters  56  and impinge upon angled portions  62 - 72  of the shutters  56 . As the stabs  46 ,  48 ,  50  are forced forward into and against the surfaces of shutters  56 , the stabs  46 ,  48 ,  50  separate the left angled portions  62 ,  66 ,  70  and right angled portions  64 ,  68 ,  72  of the shutters  56  to expose the stabs  46 ,  48 ,  50  to supply power buses  88 ,  90 ,  92 , respectively. Preferably, a biasing or closure force is provided to bias the right angled portions  64 ,  68   72  and the left angled portions  62 ,  66 ,  70  towards one another, so that the shutters  56  automatically close upon retraction of stabs  46 ,  48 ,  50 . It is recognized that numerous other ways of opening and closing shutters  56  are possible and contemplated. For example, rather than employing two shutter portions for each shutter, one shutter portion having one beveled surface could be slid aside by the advancement of the stabs. Or, the shutters could be connected for manipulation by the turning of rotary shaft  84 . Thus, the shutters  56  could comprise one or several sliding panels with or without beveled surfaces. In other words, shutters  56  may be operated to open and close by the movement of the stabs, by the movement of the stab assembly, by the turning of the actuating shaft, by other actuating components, or by a manual control. Regardless, once the stabs  46 ,  48 ,  50  have penetrated through shutters  56 , the stabs  46 ,  48 ,  50  may be advanced or extended to engage power supply bus bars  88 ,  90 ,  92 . 
   Also shown in  FIG. 5  is a second microswitch  94  connected to activate and deactivate circuit breaker  30 . When stabs  46 ,  48 ,  50  reach the fully engaged position  45  with bus bars  88 ,  90 ,  92 , stab bracket  59  of stab assembly  58  actuates microswitch  94 . Microswitch  94  permits closure of circuit breaker  30 , completing the circuit between bus bars  88 ,  90 ,  92  and the line side of motor control components (not shown) in bucket  16 . 
   Likewise, for removal of bucket  16 , circuit breaker  30  is opened, disconnecting supply power to the motor control devices (not shown) of bucket  16 . Stabs  46 ,  48 ,  50  may then be retracted from bus bars  88 ,  90 ,  92  by a reverse motion of rotary shaft  84 . Once stabs  46 ,  48 ,  50  pass shutters  56 , the right and left portions  62 - 72  thereof will automatically close together to isolate the stabs from bus bars  88 ,  90 ,  92 . Preferably, the shutter portions  62 - 72  and all or some of the housing panels, including dead front panel  18  and a rear panel (not shown), of bucket  16  are formed of plastic or another insulating material. After stabs  46 ,  48 ,  50  have been fully retracted, automatic latch  60  will release from engagement with the motor control center, and an operator may then slide bucket  16  out of the motor control center. 
   Referring now to  FIG. 6 , a cross-sectional view of bucket  16  taken along line  6 - 6  of  FIG. 3  is shown. The left angled portion  66  of a shutter  56  is shown isolating the central stab  48 , since stab  48  is in the retracted position  42  of  FIG. 3 . In  FIG. 6 , it can be seen that stab assembly  58  holds stab  48  in position and engages rotary shaft  84 , shown in section. Therefore,  FIG. 6  illustrates the moving components used to actuate a stab  48 . An operator may use a ratchet or crank (not shown) through opening  36  of slide  32  to turn rotary shaft or worm gear  84 . A stab guide  96  includes a thread bearing  100  to transform the rotational motion of rotary shaft  84  into a translational motion of stab assembly  58 . Thus, rotary shaft  84  and stab guide  96  may generally be referred to as a racking-type actuating mechanism for extending and retracting the stabs  46 ,  48 ,  50 , relative to bucket  16 . As stab assembly  58  is racked or otherwise advanced towards the extended or engaged position  45  shown in  FIG. 5  (i.e. a motion to the left, as oriented in  FIG. 6 ) stab  48  will impinge upon shutters  66 . Also, a sloped lip  104  of stab assembly  58  will strike a bottom portion  106  of latch  60  into an upward position wherein bottom portion  106  rests on stab guide  96  and latch  60  extends through a groove  98  of motor control center  10 , shown in  FIG. 1 , to retain bucket  16  therein. 
   Referring to  FIGS. 17-19 , a clutch mechanism  202  is shown in operation within a bucket  11 .  FIG. 17  depicts a partial perspective view from the bottom of bucket  11  in which clutch mechanism  202  is shown engaged with rotary drive  84 . After passing through an opening  36  in the dead front panel  18  of the bucket  11 , rotary drive  84  passes through a second slot in the slide plate  96  of stab bracket  59  and engages with threaded slot  220  of clutch plate  210 . In operation, clutch or freewheeling mechanism  202  limits the translational movement of stab bracket  59  such that stab bracket  59  is able to translate between two limits: the maximum engaged or extended position  45  (shown with respect to the bucket  11  in  FIG. 5  and with respect to the clutch mechanism in  FIG. 18 ) and the maximum retracted position  42  (shown respect to the bucket  11  in  FIG. 3  and with respect to the clutch mechanism in  FIG. 19 ). 
   Referring now to  FIG. 18 , as threaded drive  84  is rotated and stab bracket  59  advances stab assembly  58  ( FIG. 17 ) towards the power bus (not shown), the threaded segment  214  of rotary drive  84  engages clutch plate  210  of clutch mechanism  202 . As threaded drive  84  continues to rotate, clutch plate  210  also advances axially along threaded drive  84  in the direction of the stab assembly  58  ( FIG. 17 ) and the second set of springs  206  begin to compress. When clutch plate  210  reaches the non-threaded end  216  of the drive  84 , the second set of springs  206  are fully compressed between washer  212  and clutch plate  210 , and stab assembly  58  is in maximum extended (i.e., engaged) position  45  (see  FIG. 5 ). At this point, further rotation of drive  84  results in no further advancement of stab assembly  58 , as the non-threaded end  216  of drive  84  spins freely in clutch plate  210 . When non-threaded end  216  of drive is free-wheeling in clutch plate  210 , an audible and tactile clicking occurs as drive  84  is rotated, thus alerting an operator that the maximum extended position  54  has been reached. 
   Referring now to  FIG. 19 , to retract the stab assembly  58  from the power bus (not shown), drive  84  is rotated in the opposite direction. The second set of springs  206  impart a force between clutch plate  210  and the first thread of the threaded segment  214  of the drive  84 , thus assisting with the engagement of threaded drive segment  214  and the threaded inner surface  220  of clutch plate  210 . As drive  84  is rotated further and stab assembly  58  advances axially along threaded drive  84  towards the maximum retracted position  42 , clutch plate  210  advances along the threaded drive segment  214  towards non-threaded segment  218  and the first set of springs  204  compress. When stab assembly  58  (and stab bracket  59 ) reaches maximum retracted position  42 , drive  84  freely rotates about non-threaded drive segment  218 , the second set of springs  204  is compressed, and further rotation of drive  84  produces no additional translation in the retraction direction. Additionally, further rotation of drive  84  results in an audible and tactile clicking to alert an operator that the maximum retracted position  42  has been reached. To begin axial translation of stab assembly  58  towards the fully engaged position, drive  84  is rotated in the opposite direction and the force resulting from the compression of the first set of springs  204  assists in engaging the threaded inner surface  220  of clutch plate  210  and threaded drive segment  214 . 
   By thus limiting advancement of stab assembly  58  between maximum retracted position  42  ( FIG. 19 ) and maximum engaged position  45  ( FIG. 18 ), clutch mechanism  202  prevents damage resulting from over torquing or overdrive of the rotary drive  84  inside bucket  11 . 
   Although clutch mechanism  202  has been described in detail in connection with the rotary drive  84 , it is appreciated that clutch mechanism  202  can also be utilized in a similar manner with any form of stab racking mechanism such as manually drivable handle  116  (described in detail with respect to  FIGS. 9-12 ) or a motor drive  128  (described in more detail with respect to  FIG. 16 ). It is further contemplated that a separate clutch mechanism could be incorporated with each of the non-threaded ends of the drive, such that a first clutch mechanism would limit over-racking of the stab assembly during engagement of the stabs and a second clutch mechanism would limit over-racking of the stab assembly during retraction of the stabs. Additionally, clutch mechanism  202  can be incorporated into other low voltage assemblies where there is a need to provide an enhanced degree of arc flash safety, such as busway plug-in systems components, panel board system component assemblies, system components used in distribution switchboard assemblies, and other systems where a plug-in assembly is remotely racked in and out from a remote location. 
     FIG. 7  is an enlarged view of the stab  48  and shutter  66  area of the cross-sectional view of  FIG. 6 . Conductive stab  48  is coupled to a flexible conductor  130 , such as a cable, via a coupling portion  132  of stab assembly  58 . Flexible conductor  130  is of a construction suitable to conduct supply power, via stab  48 , to the line side of a motor control component (not shown). As shown in  FIG. 8 , when stab  48  and stab assembly  58  are racked or otherwise advanced forward to an extended position  45 , flexible conductor  130  flexes to maintain electrical connectivity with stab  48  via coupler  132 . Accordingly, the motion of stab  48  relative to bucket  16  does not interfere with the connectivity of the stab  48  with a motor control component. 
   Referring now to  FIGS. 9-15 , an alternative stab actuating feature is shown. A manually drivable handle  116  may replace or be used in combination with the crank  34  of  FIG. 1  and the racking mechanism of  FIG. 6 . In such embodiments, the rotary shaft or worm gear  84  depicted in previous embodiments may be replaced with a non-tapped shaft or rod directly connected to stab assembly  58 .  FIG. 9  shows such a handle  116  in a locked, starting position  118  that corresponds to the stabs disengaged position  42  of  FIG. 3 . As shown in  FIG. 10 , handle  116  is separated and biased from dead front panel  18  of a bucket by a spring  120  and extends through stab actuating opening  36 . By rotating handle  116  ninety degrees, as shown in  FIGS. 11 and 12 , handle  116  may be unlocked  122 . In some embodiments, an interlock system may be included to prevent unlocking of handle  116  until bucket  16  is fully installed into a motor control center. Such an interlock may be incorporated into the shaft  84  of handle  116 . Once unlocked, handle  116  may be driven or depressed towards dead front panel  18 , compressing spring  120 , as shown in  FIG. 14 . The depressed position  124  of handle  116  corresponds to the stabs engaged position of  FIG. 5 . Handle  116  may then be rotated another ninety degrees  126 , as shown in  FIGS. 15 and 16 , to lock the handle in the stabs engaged position  124 , against the force of spring  120 . For disengagement of the stabs, handle  116  is rotated to unlocked orientation  122 , pulled outward to the stab disengaged position  42  and turned ninety degrees to a locked position  118 . In a general sense, therefore, embodiments of the present invention may include various configurations of simplified, manual actuation of the stabs, similar to that shown in  FIGS. 9-15 . 
   In other embodiments, it may be desirable to use more automated actuation of the stabs of a motor control center. Accordingly,  FIG. 16  depicts an embodiment of a motor control center bucket  16  in which a motor drive  128  is included. Motor drive  128  may replace or augment a racking mechanism or a manual actuation system, such as described above. Preferably, motor drive  128  is a small DC motor and may be powered by a battery or by an electrical connection with motor control center  10 , such as via the control power contact  44  shown in  FIG. 2  or a similar plug or connection. It is recognized, however, that many other types, sizes, and configurations of motor drive  128  are equivalently applicable. For example, it may be desirable to connect motor drive  128  inside bucket  16 . Further, it is contemplated that motor drive  128  could be remotely operable, and could be either hardwired or wirelessly connected for operation. It is also contemplated that motor drive  128  may be connected remotely via an elongated shaft or cable to operate rotary shaft  84  from a distance. 
   Accordingly, one embodiment of the present invention includes a motor control center subunit includes a housing, a drive mechanism attached to the housing and configured to translate a plurality of selectively moveable conductive contacts between a retracted position and an extended position within the housing, and a freewheeling mechanism attached to the drive mechanism and configured to define the retracted position and the extended position of the conductive contacts. The conductive contacts are moveable when the housing is seated in a motor control center and a front panel of the housing is in a closed position. 
   According to another embodiment of present invention, a motor control center includes a motor control center frame having at least one compartment and a motor control center subunit housing constructed to seat in the at least one compartment of the motor control center frame. The motor control center also includes an actuating mechanism attached to the motor control center subunit to control movement of a plurality of conductive contacts and a clutch configured to define a range of movement of the plurality of conductive contacts, with the range of movement having ends at a fully engaged position and a fully retracted position. 
   According to yet another embodiment of the present invention, a motor control center subunit includes a control module housing having a plurality of conductive contacts therein and an actuating mechanism to control movement of a plurality of conductive line contacts when a front panel of the housing is in a closed position. The motor control center subunit also includes a means for preventing the actuating mechanism from translating past a maximum extended position and a maximum retracted position. 
   The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.