Abstract:
A solid state motor starter, commonly referred to as a soft starter, is constructed in a manner to be easily manufacturable while at the same time combining all the required components in a relatively small package. A solid state power switch is clamped between a pair of bus bars in an offset manner to accommodate a discrete switching relay mounted in an inverted manner between the line input and the bus bar in communication with the load outputs. When in a motor run mode, current is shunted away from the solid state power switching device and through the switching relay, a substantially linear current path is achieved to reduce power loss and heat buildup. The arrangement allows for a heat sink mounted to one of the bus bars with adequate spacing between the heat sink and the discrete relay for insertion of a cooling fan. Current sensing is achieved with a Hall effect sensor mounted to the bus bar in a small current sensing region that is created by having a pair of slots in the bus bar to direct current. A pair of magnetic pins extend perpendicular from the bus bar to create the magnetic flux for the Hall effect sensor to sense current flow through the current sensing region. The Hall effect sensor circuit board also contains a thermistor which is mounted with adequate insulation yet in thermal communication with the bus bar.

Description:
This is a Divisional of Ser. No. 09/267,052, filed Mar. 12, 1999, now U.S. Pat. No. 6,087,800. 
    
    
     BACKGROUND OF THE INVENTION 
     Solid state motor starters, commonly referred to as “soft starters,” control the starting and stopping of electrical motors with gated semiconductor devices such as SCRs, thyristors, or generally, solid state power switches. The present invention relates generally motor starters, and more particularly, to a compact solid state motor starter designed to reduce space requirements and be integrally combined in one complete small package. 
     Industry standard soft starter structural arrangement typically consists of several separate discrete component groups. Such groups include controllers, bypass contactors, sensors, overload protection, snubbers, cooling fans, power semiconductors, power bus bars, insulators, assembly hardware and mounting plates. When assembled as a unit, these prior art motor starters are quite large and cumbersome. 
     The controllers are usually housed in Class II enclosures with discrete screw type terminal blocks and mounting feet. The size and power requirements of the controller may vary depending on the application and sophistication of the control. The controller package is often broken up into several separate printed circuit board assemblies requiring interconnects and mounting. 
     Discrete bypass contactors are used to shunt the power semiconductors after the motor has reached its running speed. The bypass contactors require mounting hardware, coil leads with terminations, and power conductors from the line and load side of the soft starter. 
     Soft starters may have several different types of sensors. The most basic sensor typically found in a soft starter, is the current sensor, which is typically a rather bulky configuration. Some of the most common methods for sensing current include a large current transformer with matching current meter and with rather cumbersome mounting brackets. Another method includes a ferrite toroid with a matching current meter or printed circuit board assembly, and also requiring a bulky and cumbersome mounting arrangement. Yet another method includes a Rogowski coil and matching circuit board assembly, also requiring bulky mounting brackets. 
     In further detail, the most common industrial practice is to measure current using the same principles as a transformer. A magnetic field is induced around a conductor as current is passed through the conductor. This magnetic field is induced into a magnetic core. The core material can range from being a very good magnetic material, for example ferrous magnetic iron or steel, or it can include a very weak magnetic material, such as air. A second coil is also required, and is looped around the magnetic coil material, or around the current carrying member. The amount of magnetically induced current into the second coil is dependent on the reluctance of the core material used, and the amount of signal current desired. The current signal therefore should be proportional to the actual current in the conductor of interest. A scale is developed to read the coupled current signal value in the conductor as an actual current signal. The meter used is typically a current meter. However, if the second coil circuit has many turns of small gauge wire, the coupled signal has a low current value, and therefore a volt meter can alternatively be used. The following description describes in further detail some of the most common methods presently used to accomplish such current sensing. The output of the second coil may alternatively be used to drive an overload relay. 
     Perhaps the most common method to measure current using the principles of a transformer, is to encircle the conductor with a wire forming a number of loops, and measuring the current inductively induced in that wire. This method is similar to an air core transformer and is commonly referred to as a current transformer. Another method is to encircle a conductor with a rigid piece of ferrite core material having good magnetic reluctance and then wind the ferrite material with wire loops and measure the inductively induced current. This method is similar to an air iron core transformer and is commonly referred to as simply a toroid. 
     Similarly, a core, constructed of several laminations, can be positioned around a conductor with wire coiled around one portion of the lamination loop to measure the inductively induced current in the coil, which is also similar to an iron core transformer. In order to assist assembly, a variation of this scheme was developed in which a lamination core is split so that the conductor to be monitored does not have to be passed through the core before it can be properly positioned. The core is then opened about the lamination split, the conductor of interest is inserted into the core at the desired position, and the core can then be closed to maintain the low reluctance of the magnetic loop. Yet another method is to use thin steel laminations as a ferrite core material, and then wind the ferrite material with wire loops. Since the core area is small and the wire gauge is thin, the inductively induced voltage can then be measured. This method is similar to an iron core transformer and is referred to as a Rogowski coil. 
     All of the aforementioned current measuring techniques discussed and typically used in soft starters have one common physical limitation that is a major disadvantage in constructing a compact motor starter. That common disadvantage is that the second coil, or the ferrite core, used to develop the induced current or voltage signal must be positioned about the periphery of the conductor of interest. Since motor starters require relatively large conductors, any additional material about the conductors results in excessively large packaging of the motor starters. Further, in any three phase motor starter which has three separate conductors that must be monitored, the potential for cross-talk, or interference, between the current sensors becomes quite high. 
     Soft starters may also require thermal monitoring to protect the power semiconductors. One common method for thermal protection includes a bi-metal disk or “Popit” requiring mounting brackets, hardware, and electrical insulation depending where it is located with respect to the current carrying members of the soft starter. In operation, when the bi-metal disk reaches the trip temperature, the bi-metal disk snaps into the stressed position and changes the state of the electrical contacts, thereby signaling to the control circuit that a temperature limit has been reached. However, bi-metal disks respond very slowly to temperature changes because of their large inherent material mass and have a very narrow temperature range. If monitoring of several temperature ranges were required, a separate bi-metal disk would be required for each temperature range. Another type of thermal protection uses infrared heat sensors. Although these devices do not require placement on a current carrying member, they must be in close proximity to it. Therefore, mounting brackets and a matching circuit board assembly is required and the sensor must be “aimed” at the component to be monitored. Heat sensitive resistors, or thermistors, can also be used to measure the temperature of electrical components. Heat sensitive resistors change resistance with temperature change. The change in resistance is then calibrated to a voltage, which in turn is used as a temperature reference and indicates the temperature of the component. Thermistors respond very quickly to temperature changes because of their small inherent material mass. 
     Bi-metal disks and thermistors are usually located near or on current carrying members in electrical equipment. They both require discrete electrical leads or terminals that require routing and termination. Prior art use of these devices has also required separate mounting fasteners or brackets. Additional electrical insulation or barriers are then required to protect these devices from the line potential of the current carrying members. Since these devices are typically mounted individually, they then require additional space in the piece of electrical equipment to be monitored, which therefore increases the size of the equipment. 
     Soft starters also require a snubber assembly, which typically includes a resistor and a capacitor in series to protect the power semiconductor components from transient noise. The snubber assembly is connected across the line and load terminals of the motor starter, and have discrete leads. These devices also require mounting brackets and associated hardware. 
     Where natural convection is not sufficient to cool the motor starter, a cooling fan is necessary to provide forced air. The cooling fan normally increases the size of the enclosure, or is mounted externally and vents the starter through a vent in the package. In either case, the cooling fan oftentimes adds considerable size to the overall package. 
     Soft starters also include overload protection which is required on all power control equipment and can be accomplished by using overload relays or an overload circuit board assembly. Typically, when the current being measured reaches a preset limit, the overload changes state and disconnects the motor from the power source. The overload can be a discrete device or an integral function of the controller. Such devices usually have a limited range and are very application sensitive with respect to motor current. 
     Soft starters use discrete semiconductors or SCR “pucks.” Depending on power requirements, such devices can become rather large and add to packaging complexity and increase the size significantly. In multi-phase applications, where multiple power conductors are required, physical spacing between poles is dependent on the operating voltage. The size of the conductors is also proportional to the amount of in-rush current that must be carried and the amount of heat that must be removed from the power semiconductors. 
     All the aforementioned components of the soft starter are usually mounted to a single mounting panel that results in a quite large overall package. Such prior art soft starters assembled in this manner, require excessive production assembly time, have excessive volume and mass associated with it, and have an enclosure that is exceedingly too large. 
     SUMMARY OF THE INVENTION 
     The present invention offers a solid state motor starter that solves the aforementioned problems and provides a soft starter assembly that integrates the aforementioned components into a relatively small package resulting in reduced wall or floor space requirements, while simultaneously providing an easily manufacturable motor starter. 
     In accordance with one aspect of the invention, a solid state motor starter includes a first electrically conducting bus bar adapted to receive an external current carrying conductor from a power source at a line input end, and a second electrically conducting bus bar adapted to receive an external current carrying conductor connectable to a motor at a load output end. There is at least one solid state power switching device clamped between the first and second electrically conducting bus bars, and a discrete electromagnetic power switching relay having an electrical input and an electrical output forming a bypass current path around the solid state power switch&#39;s device. The electrical input is connectable to the external current carrying conductor from the power source, and the electrical output is connected to the second electrically conducting bus bar in shunt of the solid state power switching device. The discrete electromagnetic switching relay is mounted such that the relay current path is in linear relation (i.e., in a straight line) with the second electrically conducting bus bar, thereby providing a linear current path through the solid state motor starter when the discrete electromagnetic switching relay is switched to relay power from the power source to the motor, which reduces heat build-up in the soft starter. 
     Additionally, in accordance with another aspect of the invention, the discrete electromagnetic switching relay of the motor starter is mounted rearwardly of the second electrically conducting bus bar and is optimally fitted in an inverted arrangement such that its internal contacts are in close relation to the first electrically conducting bus bar and its internal magnet is spaced furthest from the first electrically conducting bus bar. The motor starter also includes a heat sink mounted to the second electrically conducting bus bar in a spaced relation to the discrete electromagnetic switching relay so as to provide for a cooling fan mounted between the heat sink and the discrete electromagnetic switching relay to force air flow across the heat sink for additional cooling. Additionally, the large mass of the second electrically conducting bus bar serves as a heat sink when solid state power switching device is conducting. 
     A cover assembly is molded to fit over the solid state motor starter and has a heat sink tunnel to accommodate the cooling fan and the heat sink. A thermistor is mounted in the cover assembly to sense air flow temperature across the heat sink. A current sensor and thermistor assembly is attached directly to one of the electrically conducting bus bars which is modified to provide a relatively small current sensing region by cutting a pair of slots from the outer edges toward a central area of the bus bar. Current sensing can then be accomplished using a very small Hall effect sensor, as opposed to the prior art methods for current sensing for such large bus bars. Additionally, a common circuit board is used for the Hall effect sensor and a thermistor which is mounted to monitor heat buildup across the current sensing region. 
     In accordance with another aspect of the invention, a solid state motor starter having two distinct current paths therein and constructed in a relatively compact small package includes a first current path structure defined by a power supply input connected to a first bus bar which is in electrical communication with a pair of solid state power switches for completing electrical connection with a second bus bar when at least one of the solid state power switches is switched to an ON state to ramp-up power to a motor connectable to the second bus bar during motor startup and to ramp-down power to the motor during motor shutdown. A second current path structure is operable during a motor run mode and defined by the power supply input connected to an input of an inverted electromagnetic relay switchable between a current conducting mode and a current non-conducting mode. When the electromagnetic relay is in a current conducting mode, and the solid state motor starter is therefore in the motor run mode, the second current path is further defined by an electrical connection between an output of the electromagnetic relay and the second bus bar connectable to the motor. The second current path is advantageously a substantially linear current path across the motor starter which reduces not only power loss, but also minimizes heat buildup while in the motor run mode. 
     In accordance with yet another aspect of the invention, a current sensor assembly for use in a large surface electrically conducting bus bar includes a bus bar having therein a relatively narrow current path formed by a pair of slots, each slot extending from an outer edge of the bus bar inwardly to the relatively narrow current path. The pair of slots creates the relatively narrow current path in the direction of current flow. A pair of magnetic pins extending through the bus bar transversely to an electrical current path at an outer periphery of the relatively narrow current path. The magnetic pins are spaced apart to create a magnetic flux path between the pair of magnetic pins. The magnetic pins do not create magnetic flux per se, but concentrates the magnetic flux between the magnetic pins. Therefore, a magnetic flux path is created between the magnetic pins. Preferably, the relatively narrow current path is at or near the center of the bus bar to avoid interference from magnetic flux from neighboring bus bars. In some applications, it may be preferable to offset the narrow current path from center to further distance the magnetic flux path created between the magnetic pins. A Hall effect sensor is located between the magnetic pins and above the relatively narrow current path and within the magnetic flux path created by the pair of magnetic pins. Additionally, the Hall effect sensor is mounted on a circuit board together with a thermistor for monitoring the temperature of the bus bar in the current sensing region. 
     Various other features, objects 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 the best mode presently contemplated for carrying out the invention. 
     In the drawings: 
     FIG. 1 is a perspective view of a motor starter according to the present invention. 
     FIG. 2 is a perspective view of a portion of FIG.  1 . 
     FIG. 3 is a sectional side view taken generally about line  3 — 3  of FIG.  1 . 
     FIG. 4 is an enlarged detailed sectional view of a portion of FIG. 3 taken along line  4 — 4 . 
     FIG. 5 is an enlarged cross-section of a portion of FIG.  4 . 
     FIG. 6 is an exploded perspective view of the structure of FIG.  2 . 
     FIG. 7 is an enlarged plan view of a portion of FIG.  6 . 
     FIG. 8 is a top plan view of a portion of FIG. 3 taken along line  8 — 8 . 
     FIG. 9 is an exploded perspective view of the structure of FIG.  8 . 
     FIG. 10 is a cross-sectional view taken generally along line  10 — 10  of FIG.  8 . 
     FIG. 11 is a side cross-sectional view of the structure of FIG. 10 taken along line  11 — 11 . 
     FIG. 12 is a perspective view of a portion of FIG.  1 . 
     FIG. 13 is a detailed view of a portion of FIG. 12 taken along line  13 — 13 . 
     FIG. 14 is a perspective view of an alternate embodiment of a motor starter incorporating the present invention. 
     FIG. 15 is a perspective view of a bus bar used in the motor starter of FIG.  14 . 
     FIG. 16 is a graph of Hall effect output versus Hall. effect current rating in percentage. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to the drawings, FIG. 1 shows a three phase, three-pole solid state motor starter  10 , also known as a soft motor starter. The motor starter  10  includes a cover assembly  12  having air inlets  14  on a motor connection end, or load end  16 . Similar air outlets  18  are located on a power source end, or line end  20  of the motor starter  10 . The cover assembly  12  also houses an electronic controller circuit  22  protected by a circuit cover  22 A. The controller circuit  22  is not an element of this invention and will not be further described. The motor starter  10  also includes a base assembly  24  to house each of three power pole assemblies  26 ,  28 , and  30 . Each of the power pole assemblies  26 ,  28 , and  30  are identical in construction for a given motor starter  10 , and as such, this disclosure will describe only one of the power pole assemblies  28  in further detail hereinafter. 
     FIG. 2 is a perspective view of a single power pole assembly, for example, that of power pole assembly  28  of the motor starter  10 . Power pole assembly  28  includes a first electrically conducting bus bar  32  that is adapted to receive a wiring lug connector (not shown) which in turn receives an external current carrying conductor from a power source (not shown) at an input end  36 . An L-shaped conductor  34  has a flange  38  having a pair of bolt holes  40  for mounting the power pole assembly  28  to the base assembly  24 , FIG. 1, at the load end  16 . Referring back to FIG. 2, the power pole assembly  28  of the motor starter  10  has a second electrically conducting bus bar  42  mechanically and electrically connected to the L-shaped conductor  34  to receive a wiring lug connector (not shown) to connect the pole assembly  28  to an external current carrying conductor (not shown) connectable to a motor (not shown) at a power output end  44 . In a preferred embodiment, a pair of solid state power switching devices  46  and  48 , such as SCRs, are clamped between the first and second electrically conducting bus bars  32  and  42 . Depending upon the power requirements of the motor to be driven by the motor starter  10 , the bus bars can be larger or smaller. Further, an alternate embodiment may use solid state switching devices, other than SCR&#39;s, depending upon switching characteristics required and overall power requirements. 
     A discrete electromagnetic switching relay  50  is mounted in the power pole assembly  28  in an inverted manner such that the internal contacts are facing downwardly at a lower end  52 , and the internal magnet and stater are at an upper end  54 . The switching relay  50  has a pair of stationary contacts  56  and  58 . The output stationary contact  56  is connected to the L-shaped conductor  34  with at least one mounting bolt  60 . The input stationary contact  58  is attached to the first bus bar  32  by two of six clamping bolts  62 . By mounting the switching relay in an inverted manner as shown, in the structure of the present invention, a substantially linear current path through the power pole assembly  28  is achieved for operation in a motor run mode, as will later be described. 
     The internal construction of the discrete electromagnetic switching relay  50  can be of standard construction. An example of such a relay is disclosed in U.S. Pat. No. 5,337,214 issued to Lindsey et al. on Aug. 9, 1994 and assigned to the Assignee of this invention. However, as one skilled in the art will readily recognize, the contacts  56  and  58  of the present invention, extend outwardly from opposite sides, whereas the contacts of the relay disclosed in U.S. Pat. No. 5,337,214 extend outwardly from the same side. One skilled in the art will readily recognize that such contact location is achievable with minor housing and structure modifications. 
     The power pole assembly  28  of FIG. 2 also has a heat sink  64  mounted on the first electrically conductive bus bar  32 . The heat sink  64  is spaced away from the discrete electromagnetic switching relay  50  to allow insertion and mounting of a cooling fan  66  therebetween. The cooling fan  66  is supported by the cover assembly  12 , FIG.  1 . In a preferred embodiment, each of the power pole assemblies  26 ,  28 , and  30 , each has its own cooling fan  66  mounted within cover  12  and engageable in space  68  of each of the power pole assemblies  26 ,  28  and  30 . 
     In operation, each power pole assembly  26 ,  28 , and  30  of the motor starter  10  have two distinct current paths. A first current path structure, operable during a motor start-up mode and a motor shut-down mode is defined by a power supply (not shown) connected to provide power to the first bus bar  32 . The first current path structure is further defined to include the solid state power switches  46  and  48  for completing electrical connection with the second bus bar  42  when at least one of the solid state power switches  46 ,  48  is switched to an ON state to ramp-up power to a motor (not shown) connectable through the L-shaped conductor  34  to the second bus bar  42  during motor start-up, and to rampdown power to the motor during motor shut-down. A second current path structure is operable during a motor run mode, which is initiated only after the motor has been ramped up to speed. The second current path structure is defined by the power supply connected to the input stationary contact  58  of the inverted electromagnetic relay  50  through the first bus bar  32 . The relay  50  is switchable between a current conducting mode and a current non-conducting mode. When the motor is being ramped-up or ramped-down, the switchable relay  50  is in a non-conducting mode, and therefore, the second current path is interrupted by the switching relay  50 . However, when the electromagnetic switching relay  50  is in a current conducting mode and the motor starter  10  is therefore in the motor run mode, the second current path is completed, and power is supplied to the output stationary contact  56  which is connectable to the motor. Once a motor is ramped-up to speed using the SCRs  46  and  48  in the first current path, the switchable contactor relay  50  is energized to bypass, or shunt, current from the SCRs, at which time the SCRs can be turned OFF. In this manner, the contact elements in contactor relay  50  are preserved by not experiencing the normal arcing which would occur otherwise. Similarly, to shut OFF a motor, the SCRs  46  and  48  are turned back ON nearly simultaneously with de-energizing relay  50  so that there is virtually no arcing within the relay  50 . The SCRs can then ramp-down the motor. 
     As can be seen from FIG. 2, the second current path structure provides a substantially linear current path from the first bus bar  32 , through the input stationary contact  58 , through relay  50 , and through the output stationary contact  56 . Such a linear current path not only reduces power loss during the motor run mode, it also minimizes heat build-up in the motor starter. Additionally, the unique configuration provides a compact structure saving valuable floor or wall space in application. 
     FIG. 3, shows a cross-section of the solid state motor starter  10  of FIG. 1, taken generally along line  3 — 3  of FIG.  1 . The power pole assembly  28  is mounted in the base assembly  24  with a set of mounting bolts  70 . Two of the mounting bolts  70  are located in the flange  38  of the L-shaped conductor  34  on the load output end  16 . Another set of mounting bolts (not shown) fasten the power pole assembly  28  to the base  24  at the line inlet end  36  through the second bus bar  42  into bores  72 , for example. The cooling fan  66  mounted to the cover assembly  12  is positioned between the inverted relay  50  and the heat sink  64  and directs air flow along lines  75  and out air outlets  18 . Cover assembly  12  also includes a circuit board sub-housing  74  for mounting of the electronic controller circuit  22 . The pair of solid state power switching devices  46  and  48  have input leads  47  and  49 , respectively, which are connectable to the electronic controller circuit  22 . Each of the switching devices  46  and  48  are held in place between the first bus bar  32  and the second bus bar  42 , with a pair of roll pins  76  and  78 , respectively, and clamped between the bus bars  32  and  42  with a set of clamping bolts  62 . 
     In the available space provided in area A, a current sensor and thermistor assembly  126  is attached to the lower side of the first electrically conducting bus bar  32  for measuring current through the bus bar and sensing temperature of the bus bar, as will be further described with reference to FIGS. 8-11. Space A is also utilized by the placement of a snubber circuit  77  for electrical noise reduction and transient protection purposes. The snubber leads are connected to the line and load connectors as is commonly known. 
     FIG. 4 shows an enlarged detailed view of a clamping bolt  62   a  taken along line  4 — 4  of FIG.  3 . The clamping bolt arrangement shown in FIG. 4 is exemplary of each of the six clamping bolts, three of which are shown in FIG. 2 assembled, and all of which are shown in FIG. 7 unassembled. Referring back to FIG. 4, the clamping bolt  62   a  clamps the solid state power switching device  46  between the first bus bar  32  and the second bus bar  42 . A series of Belleville washers  79  are used with the clamping bolts and are compressed with a fastening nut  83 . Clamping bolt  62   a  has an Allen head  80  engaged in an insulator  82  for insulating the second bus bar  42  from the first bus bar  32 . 
     FIG. 5 shows an enlarged cross-section of a portion of the clamping bolt  62   a,  the Belleville washers  79  and the fastening nut  83  of FIG.  4 . As indicated, the series of Belleville washers  79  include opposing concave and convex biased washers having small gaps  84  therebetween to allow contraction and expansion of the bus bars  32  and  42  during various operating modes, thereby maintaining a pressure on the devices  46 ,  48 . 
     Referring to FIG. 6, an exploded perspective view of the power pole assembly  28  of FIG. 2 is shown. As indicated, in the preferred embodiment, six clamping bolts  62  are used, along with six washers  81 , and six insulators  82 . The clamping bolts  62  extend through clamping holes  86  of the second bus bar  42 , through the clamping holes  88  of the first bus bar  32 , and through four clamping holes  90  of the heat sink  64 . The relay  50  is also mounted with Belleville washers  79  and clamping bolts  62  through holes  92  in the input stationary contact  58  to allow slight movement between the relay and the bus bar due to expansion and contraction. Each clamping bolt  62  is equipped with a corresponding set of Belleville washers  79  and a fastening nut  83  to clamp the power switching devices  46  and  48  between the first and second bus bars  32  and  42 . 
     The power switching devices  46  and  48  are centered on roll pins  76  and  78 , respectively, in the small diameter roll pin holes  94  of the first and second bus bars  32  and  42 . The heat sink  64  is additionally mounted to the second bus bar  42  with bolts  96  and nuts  98  through mounting holes  100  in the heat sink  64 , and through mounting holes  102 , in the first bus bar  32 , to provide additional support to the bus bar  42  to compensate for slots  104  and  105 , which will be further explained with reference to FIG.  7 . As earlier explained, relay  50  is attached at the input end  58  to the first bus bar  32  with the foremost clamping bolts  62  and fastening nuts  78 . The other end of the relay  50 , having the input stationary contact  56 , is mounted to the L-shaped conductor  34  via bolt  60  and nut  61 , which is counter-bored into the bottom side of the L-shaped conductor  34  to receive nut  61  therein. When installed, a connecting lug assembly bolts to the three holes shown. The L-shaped conductor  34  is bolted to the second bus bar  42  via bolts  106  and nuts  108 . Mounting holes  110  are counter-bored to accommodate a flush mounting of bolts  106  therein. 
     FIG. 7 shows a top detailed view of the first electrically conducting bus bar  32 . The SCRs  46  and  48  are shown in phantom mounted from underneath and centered about roll pins  76  and  78 . The six left-most clamping holes  88  are for clamping the SCRs to the first bus bar as previously explained. Mounting holes  112  are counter-bored to mount the entire power pole assembly  28 , FIG. 3, to the base assembly  24 . The remaining holes  114  are used for attaching a wiring connector lug (not shown). Slots  104  and  105  are cut into the bus bar  42  to converge current passing through the first bus bar  32  within a relatively small, and preferably centered, current sensing region  116 . Slots  104  and  105  extend inwardly from outer lateral edges  118  and  120 , and converge inwardly toward a pair of pin bores  122 . A pair of magnetic pins  124  are mounted in bores  122  and extend perpendicularly from the top surface of the bus bar  32 . The pins  124 , preferably of steel construction, are designed to concentrate and direct the magnetic flux created by the flow of current and the presence of the pins in the current path through the current sensing region  116 . The pins  124  extend outwardly from the bus bar  32  at a desired height, as shown in FIGS. 3 and 11, wherein a current sensor and thermistor assembly  126  is attached thereover. Threaded hole  128 , FIG. 7, is for mounting the current sensor and thermistor assembly to the first bus bar  32 . 
     FIG. 8 shows the current sensor and thermistor assembly  126  as taken along line  8 — 8  of FIG.  3 . The steel pins  124  not only direct and concentrate the magnetic flux created by the current flow through the bus bar, but are also used for positioning the current sensor and thermistor assembly  126  to provide proper positioning of Hall effect sensor  130 , as will be further described with reference to FIGS. 9-11. 
     Referring now to FIG. 9, the current sensor and thermistor assembly  126  is shown in exploded view about magnetic pins  124 . The current sensor and thermistor assembly  126  includes a circuit board  132  having a Hall effect sensor  130  extending outwardly therefrom to sense current flow through the current sensing region  116 , and thus through the entire bus bar  32 . The Hall effect sensor  130  extends out perpendicular to the circuit board  132  such that a designated “sweet spot”  131  will ultimately be situated in the maximum flux path between the steel pins  124 . The so-called “sweet spot”  131  is typically marked on a Hall effect sensor  130  to designate the most active region in a Hall effect sensor. Circuit board  132  also has a thermistor  134  to measure temperature on the bus bar. A lead connector  136  is soldered to the circuit board and a wiring harness  138  extends therefrom for connection to the controller circuit  22 . The Hall effect sensor  130  and the thermistor  134  are connected as is customary. The current sensor and thermistor assembly  126  also has a positioning block  140  for receiving the circuit board  132  therein and properly positioning the Hall effect sensor  130  about pin holes  142  engageable with the pins  124  and into a Hall effect sensor slot  144  on the underside of the positioning block  140 , as best viewed in FIGS. 10 and 11. 
     The positioning block  140  also has a frustoconical thermistor tunnel  146  to receive the thermistor  134  in the most narrow part of the tunnel, as best viewed in FIG.  10 . The frustoconical thermistor tunnel  146  has its largest area in close proximity to the first bus bar  32  to sense the temperature of the bus bar in the current sensing region. The frustoconical thermistor tunnel has therein dispersed a thermally conductive paste  148  to ensure the conduction of heat from the surface of the bus bar  32  to the thermistor  134 . 
     Referring back to FIG. 9, the current sensor and thermistor assembly  126  also includes an insulator, or isolator  150 , which is designed to provide electrical isolation for the circuit board  132  and its components from the bus bar  32 . The current sensor and thermistor assembly  126  is mounted to the bus bar with a non-magnetic screw  152  through an insulator  154 , as also shown in FIG.  10 . 
     Referring to FIG. 11, the circuit board  132  is shown with the Hall effect sensor  130  situated in the Hall effect sensor slot  144  of the positioning block  140 . The positioning block has one pin hole  142   a  having a diameter of close proximity to that of the pins  124 , and the other pin hole  142   b,  having a slightly oval shape to allow for any slight pin misalignment. 
     Since the motor starter  10 , of the present invention has three power pole assemblies  26 ,  28 , and  30 , it is important to minimize any cross-talk among, or interference between, adjacent conductors that could effect the Hall device  130 . In order to do so, the narrow current sensing region  116 , FIG. 7, is centrally located from the outer lateral edges  118  and  120  of the first bus bar  32  which minimizes magnetic flux effects from adjacent bus bars. Further, by extending the magnetic pins  124  out from the surface  152  of the first bus bar  32  SO that the Hall effect sensor  144  intersects a maximum magnetic flux path caused by current flow perpendicular to pins  124 , cross-talk between adjacent conductors is further minimized. 
     Referring back to FIG. 7, slots  104  and  105  are each angled rearwardly from the current sensing region  116  to the outer lateral edges  118  and  120  SO that the slots  104 ,  105  extend between a pair of mounting holes  88  and  102  of the first bus bar  32 . Because the heat sink  64 , FIG. 2 is mounted over the slots  104  and  105 , FIG. 7, any structural weakening of the bus bar  32  is minimized by the strength of the heat sink  64 , FIG.  2 . It is noted that the slots  104  and  105  do not need to be angled to create the current sensing region  116 , nor do they need to be the same length. However, the slots are angled and the same length in the preferred embodiment to position the current sensing region  116  centrally on the bus bar to minimize cross-talk and minimize any structural weakening caused by slots  104  and  105 , as early described. 
     Referring now to FIG. 12, a sub-housing  154  of the cover assembly  12  is shown with impressions  156 ,  158 , and  160  to accommodate the heat sinks  64  of each of the power pole assemblies  26 ,  28  and  30 , FIG.  1 . The direction of air flow is indicated by arrows  162 , FIG.  12 . In order to monitor the air flow temperature inside the cover assembly  12 , a circuit board  164  having a thermistor  166  is mounted between two of the impressions  158  and  160  on a lateral support  168 , as best viewed in FIG.  13 . The circuit board  164  fits within a friction-fit channel  170  within the lateral support  168 . An air channel  172  provides direct air contact to thermistor  166  when the circuit board  164  is fully engaged into channel  170  of lateral support  168 . 
     Referring to FIG. 14, an alternate embodiment of the invention is shown. A solid state motor starter  174  is shown with its outer enclosure removed. Motor starter  174  has a relatively short bypass power current path, which is substantially linear along line  176  commencing at the load lugs  178  and concluding at the line lugs  180 . The motor starter  174 , being of lower power handling requirements, uses internal phase-controlled thyristors as opposed to the larger hockey-puck SCRs shown in FIG. 1. A heat sink  182  is mounted directly to the motor starter  174 . 
     FIG. 15 shows an alternate embodiment of a bus bar configuration  184  as used in the motor starter of FIG.  14 . The bus bar  184  has a lug end  186  and contactor ends  188 . In order to create a relatively small current sensing region  190 , slots  192  and  194  are cut therein to force current flow through the current sensing region  190 , similarly to that described with reference to bus bar  42  of FIG.  7 . As indicated by a comparison of the slots  104  and  105  of FIG. 7, and the slots  192  and  194  of the bus bar  184 FIG. 15, the specific orientation of the slots is not critical, however, the size of the current sensing region is a function of current carrying requirements. The bus bar  184  of FIG. 15 can be similarly equipped with the current sensor and thermistor assembly  126  of FIG.  9 . The length of the pins  124  would be adjusted to accommodate the thickness of the bus bar  184  so that the pins would be flush on one side and extend only enough to intersect the Hall effect sensor  130 , as in FIG.  11 . 
     Referring to FIG. 16, a desired characteristic curve  200  for the Hall effect current sensor is shown as a function of Hall effect output versus the soft starter current rating in percent. The figure shows that the present invention combines the linear and non-linear characteristics of using a Hall effect current sensor in measuring current. It is noted that the desired characteristics can be obtained by varying a number of factors. For example, by varying the width and shape of the restricted current sensing region  116 , FIG. 7, and  190 , FIG. 15, the characteristic curve can be modified as desired. Further, varying the type and amount of material of the magnetic pins  124  can also modify the permeance to obtain the desired characteristic curve. As an example, a hollow roll pin could be used in place of a solid steel pin of the same external diameter, but the hollow roll pin will have much less permeance, resulting in a completely different current characteristic curve. 
     The desired characteristics for the Hall effect sensor voltage output curve is to have an essentially linear portion  202  during the most critical portion, or active region, of the soft starter operating range. While within this range, current readings are obtained during the device&#39;s steady state operation and can be accurately compared to external measuring devices, the Hall effect output voltage should begin to drop off noticeably in an initial round-off stage  204  which is beyond the device&#39;s normal operating range. The initial round-off stage  204  is preferably at approximately 120%-130% of the device&#39;s rating, as the magnetic pins start to go into saturation. As the pins continue to saturate, a midrange round-off  206  occurs at approximately between 130%-150% of the linear current measurement range. The Hall effect device continues to respond to current increases by moving into the extended portion of the saturated pin round-off stage  208  to extend the current measurement range to approximately 300% of the device&#39;s linear current range. The final usable current measuring stage  210  occurs when the pins are saturated, and the curve has a very small slope. This stage extends the current range to approximately 600%, or six times the active linear portion of the current measurement range. Magnitude of the overload current is restricted by the Hall effect power supply. In the final stage  212 , the Hall device current signal is hitting the power supply rail, therefore no further usable current measurement information is obtainable. 
     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.