Patent Publication Number: US-2012033351-A1

Title: Motor control center and bus assembly therefor

Description:
BACKGROUND 
     1. Field 
     The disclosed concept relates generally to motor control systems, and more particularly, to motor control centers. The disclosed concept also relates to bus assemblies for motor control centers. 
     2. Background Information 
     Motor control centers are used, for example, in some commercial and industrial applications to distribute electrical power to a variety of loads (e.g., without limitation, relatively high power electrical motors, pumps, and other loads). 
       FIG. 1 , for example, shows a portion of a motor control center  10 . The motor control center  10  includes a multi-compartment enclosure  12  for receiving a plurality of motor control modules  14 , 16 , 18 , 20 , 22 , 24 , commonly referred to as “buckets”. Typically, each bucket (see, e.g., bucket  22  of  FIG. 1 ; also shown in  FIG. 2 ) is a removable, pull-out subunit that has, or is installed behind, a door  26 . The door  26  is preferably coupled to the housing  12  by hinges  28  (shown in phantom line drawing in  FIG. 1 ) to permit access to motor control components of the bucket  22  while it is installed in the enclosure  12 . For example and without limitation, the door  26  permits access to a circuit breaker assembly  30 , a stab indicator  32 , a shutter indicator  34 , and a line contact actuator  36 . When the bucket  22  is fully installed and electrically connected behind the door  26  of the enclosure  10 , an operator may operate a disconnect handle  38 . In a de-energized state of the motor control center  10 , the operator may operate an isolation feature by moving a slide  40  and inserting crank  42  through a hole  43  in the door  26  to access the line contact actuator  36  to move a number of line contacts (see, for example, stab contacts  46 , 48 , 50  of the bucket  22  of  FIG. 2 ) to an isolated position out of (see  FIG. 2 ) electrical contact with a bus assembly  60  (partially shown in phantom line drawing in  FIG. 2 ; see also  FIG. 3 ) of the motor control center  10 . 
     A portion of a circuit diagram for the motor control center  10 , in accordance with the present industry standard practice, is shown in  FIG. 3 . In the example of  FIG. 3 , the motor control center  10  is for a relatively low voltage (e.g., without limitation, up to 600V) system including a main three phase bus assembly  60  having a horizontal bus  62 , which is fed by a main circuit breaker  30  and rated, for example and without limitation, up to 3200 A. More specifically, the motor control center  10  is made up of a series of structures each housing a vertical bus  64  typically rated, for example and without limitation, at 300 A, 600 A, 800 A or 1200 A. Subunits, for example, in the form of motor control starter units  70  or feeder units (not shown) of various different sizes and configurations, are electrically connected to the vertical buses  64  by a suitable electrical connector assembly (generally indicated by reference  66  in  FIG. 3 ) such as, for example and without limitation, a male/female stab connector assembly. In accordance with applicable safety regulations, the bus assembly  60  of the motor control center  10  is rated to withstand arcing events with an associated short circuit current level of about 42 kA, about 65 kA and about 100 kA, for a time duration of about 50 ms for NEMA rated products, and up to 1 second for IEC rated products. The main circuit breaker  30  typically provides the short circuit protection for the motor control center  10 . The clearing time for clearing the arcing event is about 50 ms (e.g., about three cycles) for a NEMA application. Motor control centers  10  are described in greater detail, for example, in commonly assigned U.S. Patent Application Publications 2009/0086414, 2008/0258667, 2008/0023211 and 2008/0022673, which are hereby incorporated herein by reference. 
     Safety is a primary concern in the electrical industry. One point of focus relates to electrical safety with respect to uncontrolled arc flash events within motor control centers. Arc flash is a dangerous condition associated with the explosive release of energy caused by an electrical arc (e.g., arcing event). This fault can result from many factors including, for example and without limitation, dropped tools, accidental contact with electrical systems, build up of conductive dust, corrosion, or improper work procedures. A relatively minor event initiated within a motor control center subunit, for example, caused by a flash over to ground, could quickly propagate into a phase-to-phase fault and then a full blown three phase arcing fault, with the arc potentially jumping to the line side of the subunit circuit breaker or fuse. Hence, arc flash energy can be significant and pose a serious safety hazard. 
     There is room for improvement in motor control centers, and in bus assemblies therefor. 
     SUMMARY 
     These needs and others are met by embodiments of the disclosed concept, which are directed to a bus assembly for a motor control center wherein, among other benefits, the bus assembly limits arc flash energy, thereby reducing short circuit and other electrical stresses on bus and motor control center components which, in turn, allows more compact motor control center bus assembly and subunit configurations to be implemented. 
     As one aspect of the disclosed concept, a bus assembly is provided for a motor control center. The motor control center includes an enclosure, an electrical switching apparatus removably coupled to the enclosure, and at least one subunit. The bus assembly comprises: a horizontal bus structured to be electrically connected to the electrical switching apparatus; a number of vertical buses each structured to electrically connect a corresponding one of the at least one subunit to the horizontal bus; and a number of short circuit protective devices each being disposed between the horizontal bus and a corresponding one of the vertical buses. 
     The motor control center may be structured to withstand an arcing event. The arcing event may have an associated peak current level and arc flash energy, wherein each of the short circuit protective devices is structured to reduce the peak current level and arc flash energy experienced by the subunit. The motor control center may be rated to withstand the arcing event for a predetermined duration, wherein the short circuit protective devices are structured to clear the arcing event from the corresponding one of the vertical buses faster than the predetermined duration. 
     The number of short circuit protective devices may be a number of circuit breakers or fuses. The short circuit protective devices may be plug-in modules removably connected to the horizontal bus. 
     A motor control center, which incorporates the aforementioned bus assembly is also disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full understanding of the disclosed concept can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
         FIG. 1  is an isometric view of a portion of a motor control center; 
         FIG. 2  is an isometric view of one of the buckets of the motor control center of  FIG. 1 , also showing a portion of the motor control center bus assembly in phantom line drawing; 
         FIG. 3  is a schematic diagram for the motor control center and bus assembly therefor of  FIG. 2 ; 
         FIG. 4  is a schematic diagram of a motor control center and bus assembly therefor, in accordance with an embodiment of the disclosed concept; and 
         FIG. 5  is a schematic diagram of a motor control center and bus assembly therefor, in accordance with another embodiment of the disclosed concept. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As employed herein, the term “motor control center” refers to any known or suitable low voltage control gear expressly including, but not limited to, switchboards. 
     As employed herein, the term “low voltage control gear” refers to any known or suitable electrical apparatus having a horizontal bus feeding a series of vertical buses and associated electrical switching apparatus or subunits and expressly includes, but is not limited to, motor control centers and switchboards. 
     As employed herein, the statement that two or more parts are “connected” or “coupled” together shall mean that the parts are joined together either directly or joined through one or more intermediate parts. Further, as employed herein, the statement that two or more parts are “attached” shall mean that the parts are joined together directly. 
     As employed herein, the term “number” shall mean one or an integer greater than one (i.e., a plurality). 
       FIGS. 4 and 5  show schematic diagrams of a bus assembly  100  for a motor control center  200  in accordance with the disclosed concept. The motor control center generally includes an enclosure  202  (partially shown in simplified form in  FIGS. 4 and 5 ; see also, for example and without limitation, enclosure  12  of  FIG. 1 ), an electrical switching apparatus such as, for example and without limitation, a circuit breaker  204 , which is removably coupled to the enclosure  202 , and at least subunit  300 , 302 , 304 , 306 . 
     The bus assembly  100  preferably includes a horizontal bus  102  electrically connected to the aforementioned electrical switching apparatus (e.g., without limitation, a circuit breaker  204 ). A number of vertical buses  104 , 106 , 108 , 110  (four are shown in the examples shown and described herein) electrically connect corresponding subunits  300 , 302 , 304 , 306  to the horizontal bus  102 . A number of short circuit protective devices (SCPDs)  120 , 122 , 124 , 126  (four are shown in the examples shown and described herein) are employed between the horizontal bus  102  and corresponding vertical buses  104 , 106 , 108 , 110 . In the non-limiting example of  FIG. 4 , the bus assembly  100  includes four vertical buses  104 , 106 , 108 , 110  having subunits  300 , 302 , 304 , 306  and being electrically connected by way of SCPDs  120 , 122 , 124 , 126 , respectively, to the horizontal bus  102 . It will, however, be appreciated that any known or suitable alternative number and/or configuration of vertical buses, subunits and/or SCPDs could be employed, without departing from the scope of the disclosed concept. For example and without limitation, in  FIGS. 4 and 5  the subunits are starter units  300 , 302 , 304 , 306 , although other types and/or configurations of subunits (e.g., without limitation, feeder units (not shown)) could be employed. 
     Among other benefits, each of the SCPDs  120 , 122 , 124 , 126  is structured to protect the corresponding vertical bus  104 , 106 , 108 , 110  and associated starter unit  300 , 302 , 304 , 306 , respectively. More specifically, the SCPDs  120 , 122 , 124 , 126  limit arc flash energy, thereby providing the motor control center  200  with enhanced arc flash protection. That is, motor control centers  200  are typically structured to withstand certain arcing events having associated electrical current levels, including a peak current level and associated arc flash energy. Furthermore, the motor control center  200  is rated to withstand such arcing event for a predetermined duration. In accordance with the disclosed concept, each of the SCPDs  120 , 122 , 124 , 126  reduces the peak current levels and arc flash energy experienced by the associated subunits  300 , 302 , 304 , 306  (e.g., without limitation, starter units) and rapidly clears the arcing event from the corresponding vertical bus  104 , 106 , 108 , 110 . 
     In one non-limiting example, which is provided solely for purposes of illustration and is not meant to limit the scope of the disclosed concept, the motor control center  200  is a three-phase relatively low voltage (e.g., up to about 600 V) system having a three-phase horizontal bus  102  rated up to about 3200 A, and being fed by a primary or main circuit breaker  204  rated at about 3200 A. Typical vertical bus ratings are 300 A, 600 A, 800 A and 1200 A. The motor control center  200  is rated to withstand a short circuit current of a level typically in the range of about 42 kA, about 65 kA and about 100 kA for a time duration of about 50 ms for NEMA products, and up to 1.0 second for IEC products. The actual theoretical peak current levels for such motor control centers  200  could approach up to 231 kA, or more. The clearing time for such a system to clear the arcing condition is about 50 ms (e.g., about three cycles) for a NEMA application. The SCPD (see, for example, SCPD  120 ) limits the peak current level to about 50 kA, and a clearing time of about one-half of a cycle (e.g., without limitation, about 8 ms). Consequently, the arc flash energy associated with the arcing event can be advantageously predicted as being limited to a level of about 1.2 Cal/cm 2 . It will, therefore, be appreciated that the disclosed bus assembly  100  and, in particular, the short circuit protective devices (e.g., without limitation, SCPDs  120 , 122 , 124 , 126 ) therefor, substantially reduce prior art arc flash energy levels and thereby significantly improve arc flash protection for the motor control center  200 . 
     Preferably, each of the short circuit protective devices (see, for example, SCPD  120 ) is in the form of a plug-in module removably connected to the horizontal bus  102  of the bus assembly  100 . In the example of  FIG. 4 , the plug-in modules are fuses  120 , 122 , 124 , 126 , which are removably electrically connected to the horizontal bus  102  and/or corresponding vertical bus (see, for example, vertical bus  104 ) by any known or suitable electrical connector(s)  130 , 140  (e.g., without limitation, male/female electrical connectors (not shown)). 
     As previously noted, it will be appreciated that any known or suitable alternative number, type and/or configuration of short circuit protective devices  120 , 122 , 124 , 126  could be employed, without departing from the disclosed concept. For example and without limitation, in the example of  FIG. 5 , the bus assembly  100 ′ employs SCPDs which are circuit breakers  120 ′, 122 ′, 124 ′, 126 ′ that provide the aforementioned short circuit and arc flash protection to a corresponding vertical bus  104 , 106 , 108 , 110  and associated subunit(s)  300 , 302 , 304 , 306  (e.g., without limitation, feeder units; starter units (shown)), respectively. Preferably, although not necessarily, the circuit breakers  120 ′, 122 ′, 124 ′, 126 ′, like the fuses  120 , 122 , 124 , 126  discussed hereinabove with respect to the example of  FIG. 4 , form plug-in modules removably electrically connected between the horizontal bus  102  and corresponding vertical bus (see, for example, vertical bus  104 ) by suitable electrical connectors  130 ′, 140 ′. 
     The SCPDs  120 , 122 , 124 , 126  (FIG.  4 ), 120 ′, 122 ′, 124 ′, 126 ′ ( FIG. 5 ) also provide an advantageous safety feature in that the units can be racked out (e.g., withdrawn) and racked in while the operator is safely positioned behind a dead front door of the motor control center  200 . Specifically, the door acts as a barrier protecting the operator from arc flash and associated blast effects. In other words, the disclosed SCPD concept can be implemented with the various safety features of the racking system(s) described, for example and without limitation, in U.S. Patent Application Publications 2009/0086414, 2008/0258667, 2008/0023211 and 2008/0022673, which are incorporated herein by reference. 
     In addition to the aforementioned benefits, it will be appreciated that, by reducing the short circuit mechanical and electrical stresses on the vertical buses  104 , 106 , 108 , 110  and other components of the motor control center  200 , the disclosed concept affords the ability to develop relatively compact bus and subunit configurations. By way of example and without limitation, electrically insulated structural supports (e.g., braces) (not shown, but generally well known) are typically required for the vertical buses  104 , 106 , 108 , 110  of the motor control center  200 . Traditionally, relatively expensive materials such as, for example high-strength glass reinforced polyester or known or suitable thermoset plastic material has been employed to provide the requisite electrical insulation while being capable of withstanding arc flash energy and associated blast forces. With the reduced forces (e.g., without limitation, up to 25 percent reduction, or more) afforded by the disclosed bus assembly  100 , more readily available and cost-effective materials such as, for example and without limitation thermal plastic materials, can be employed to provide the necessary electrical insulation. Additionally, the positioning (e.g., spacing) of the structural support (e.g., without limitation, steel bracing brackets (not shown, but generally well known)) is dictated by the different bus ratings (e.g., without limitation 42 kA; 65 kA; 100 kA). In other words, to withstand the higher associated forces more braces are typically added as the bus rating increases. For example and without limitation, for a 42 kA rating the braces are generally disposed (e.g., spaced apart) about 18 inches, for a 65 kA rating the braces are disposed about 12 inches apart, and for a 100 kA the braces are disposed about every 6 inches. However, the reduced forces associated with the disclosed motor control center  200  and bus assembly  100  therefor enable the 42 kA bracing configuration (e.g., without limitation, braces every 18 inches) to become standard. 
     It will, therefore, be appreciated that the motor control center subunits  300 , 302 , 304 , 306  (e.g., without limitation, feeder units; starter units (shown)), SCPDs  120 , 122 , 124 , 126  ( FIG. 4 ), and vertical bus structures  104 , 106 , 108 , 110  can be more closely configured. For example and without limitation, that the UL recognized component series rating category DKSY2 can be employed for establishing the proper compact configuration. Thus, by way of example, for 100 kA system requirements at 480 V and the more demanding 600 V systems, motor control center starter units  300 , 302 , 304 , 306  and feeder units (not shown) can use a lower rated circuit breaker, and for 600 V systems, the need for an additional current limiter accessory can be eliminated. 
     While specific embodiments of the disclosed concept have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the disclosed concept which is to be given the full breadth of the claims appended and any and all equivalents thereof.