Patent Publication Number: US-2012038310-A1

Title: Inrush current control for a motor starter system

Description:
BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to motor starting systems and, more particularly, to an inrush current control for a motor starting system. 
     Electrical systems employ contacts to switch a flow of current on and off. Contacts are closed to allow passage of the flow of current and open to stop the flow of current. Generally, the contacts may be used in contactors, circuit-breakers, current interrupters, motor starters, or other electrical devices. A contactor is an electromechanical device designed to switch an electrical load ON and OFF on command. Traditionally, electromechanical contactors are employed to control operation of various electrical loads such as motors, lights and the like. Depending on their rating, electrical contactors are capable of handling various levels of switching currents. When faced with fault currents that greatly exceed the rating, electrical contactors may fail. 
     Conventional electromechanical contactors typically employ mechanical switches. However, as these mechanical switches tend to switch at a relatively slow speed, predictive techniques are employed in order to estimate occurrence of a zero crossing, often tens of milliseconds before the switching event is to occur, in order to facilitate opening/closing near the zero crossing for reduced arcing. Such zero crossing prediction is prone to error as many transients may occur in this prediction time interval. 
     As an alternative to slow mechanical and electromechanical switches, fast solid-state switches have been employed in high speed switching applications. As will be appreciated, solid-state switches change between a conducting state and a non-conducting state through controlled application of a voltage or bias. For example, by reverse biasing a solid-state switch, the switch may be transitioned into a non-conducting state. While conventional solid-state switches have the speed to react to zero crossings to mitigate against contact arcing, solid-state switches lack the desired low on-resistance of conventional electromechanical contactors. 
     Switching currents on or off during current flow may produce arcs, or flashes of electricity, which are generally undesirable. As described above, contactors may switch alternating current (AC) near or at a zero-crossing point where current flow is reduced compared to other points on an alternating current sinusoid. In contrast, direct current (DC) typically does not have a zero-crossing point. As such, arcs may occur at any instance of interruption. 
     Presently, micro-electrical mechanical system (MEMS) switches are being considered for use in switching systems. Presently, MEMS generally refer to micron-scale structures that for example can integrate a multiplicity of functionally distinct elements, for example, mechanical elements, electromechanical elements, sensors, actuators, and electronics, on a common substrate through micro-fabrication technology. MEMS switches provide a fast response time that is suitable for use in both AC and DC applications. However, MEMS switches are sensitive to arcing. In order to mitigate the arcing, MEMS switches are connected in parallel with a Hybrid Arcless Limiting Technology (HALT) circuit and a Pulse-Assisted Turn On (PATO) circuit. The HALT circuit facilitates arcless opening of the MEMS switches while the PATO circuit facilitates arcless closing of the MEMS switches. 
     This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention. 
     BRIEF DESCRIPTION OF THE INVENTION 
     According to one aspect of an exemplary embodiment, a motor starter system includes a plurality of switches, and a controller operatively connected to each of the plurality of switches. The controller is configured and disposed to selectively activate select ones of the plurality of switches upon detecting a particular phase angle of each of a plurality of phases of a multi-phase electrical source. 
     According to another aspect of the exemplary embodiment, a motor system includes a multi-phase load having a plurality of phase windings, and a motor starter system having a plurality of switches. Each of the plurality of switches is electrically connected to respective ones of the plurality of phase windings. A controller is operatively connected to each of the plurality of switches. The controller is configured and disposed to selectively activate select ones of the plurality of switches upon detecting a particular phase angle of each of a plurality of phases of a multi-phase electrical source. 
     According to another aspect of the invention, a method of operating a motor starter system having a plurality of switches connected between a multi-phase load having a plurality of phase windings and a multi-phase electrical supply including a plurality of phases includes sensing a phase angle of each of the plurality of phases, and selectively activating select ones of the plurality of switches based on a predetermined phase angle of each of the plurality of phases. 
     These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is block diagram illustrating a motor system including a motor starter system having a plurality of micro-electromechanical system (MEMS) switch systems in accordance with an exemplary embodiment; 
         FIG. 2  is a schematic diagram of a motor system including a motor starter system 
         FIG. 3  is a schematic diagram of the motor starter system of  FIG. 2  including a plurality of MEMS switch systems in accordance with an exemplary embodiment; and 
         FIG. 4  is a schematic diagram of a MEMS switch system in accordance with an exemplary embodiment. 
     
    
    
     The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Micro-electromechanical system (MEMS) switches employed in motor starter are arranged include a number or MEMS switch systems connected in series (m) and a number of MEMS switch systems connected in parallel (n) to form an (m)×(n) array. The number of MEMS switch systems connected in series (m) is dependent upon voltage rating for a single MEMS switch and a worst possible voltage level possible for the array during a surge. The number of MEMS switches connected in parallel (n) is dependent upon, in general, the current rating of a single MEMS switch and a worst possible long duration current through the array of MEMS switches. The worst possible long duration current through the array of MEMS switches is roughly equivalent to a short circuit fault condition during motor start up. Staring a motor with an existing short circuit results in a high in-rush current and a short circuit current, the combination representing the worst possible long duration current through the array of MEMS switches. Thus, motor in-rush current plays a role in MEMS circuit design. That is, minimizing in-rush current will reduce the number of MEMS switches in a circuit thereby reducing costs and overall complexity of the MEMS circuit. 
     As used herein, the term “zero crossing” should be understood to represent a point when a sign of a function changes, e.g., from positive to negative, represented by a crossing of an axis of a graph of the function. The term “phase” should be understood to mean one of a plurality of alternating currents that reach a peak value at a different time. The term “phase angle” should be understood to mean an angular component of one of a plurality of phases. The term “phase winding” should be understood to mean one of a plurality of individual conductor windings on a stator of a polyphase motor or generator. 
     Presently, MEMS generally refer to micron-scale structures that for example can integrate a multiplicity of functionally distinct elements, for example, mechanical elements, electromechanical elements, sensors, actuators, and electronics, on a common substrate through micro-fabrication technology. It is contemplated, however, that many techniques and structures presently available in MEMS devices will in just a few years be available via nanotechnology-based devices, for example, structures that may be smaller than 100 nanometers in size. Accordingly, even though example embodiments described throughout this document may refer to MEMS-based switching devices, it is submitted that the embodiments should be broadly construed and should not be limited to micron-sized devices. 
       FIG. 1  illustrates a motor system  2  in accordance with an exemplary embodiment. Motor system  2  includes a multi-phase electrical load  4  operatively coupled to a multi-phase power source  6  through a motor starter system  7 . Motor starter system  7  includes a plurality of switches indicated generally at  8 . In accordance with an exemplary embodiment, motor starter system  7  includes a controller  12  is operatively connected to switches  8 . As will become more readily apparent below, controller  12  detects a phase angle associated with each of a plurality of phases of multi-phase power source  6 . Based on a particular phase angle for each of the plurality of phases, controller  12  selectively actives select ones of the plurality of switches  8  by providing gate drive pulses. 
     As best shown in  FIG. 2 , multi-phase load  4  includes a first phase winding  20 , a second phase winding  21 , and a third phase winding  22  thereby defining a three-phase load. In accordance with one aspect of the exemplary embodiment, multi-phase load  4  takes the form of a three-phase electric motor  25 . Electric motor  25  is electrically connected to multi-phase power source  6  through motor starter system  7 . As shown, multi-phase power source  6  includes a first phase  30 , a second phase  31  and a third phase  32 . More specifically, first phase winding  20  is electrically connected to first phase  30  through a first switch  40 , second phase winding  21  is electrically connected to second phase  31  through a second switch  41  and third phase winding  22  is electrically connected to third phase  32  through a third switch  42 . In addition, a first voltage sensor  44  is arranged between first switch  40  and first phase  30 , a second voltage sensor  45  is arranged between second switch  41  and second phase  31 , and a third voltage sensor  46  is arranged between third switch  42  and third phase  32 . 
     In accordance with an exemplary embodiment illustrated in  FIG. 3 , each switch  40 - 41  takes the form of a MEMS switch system. Each MEMS switch system  40 ,  41 , and  42  is connected to a corresponding Hybrid Arcless Limiting Technology/Pulse Activated Turn-On (HALT/PATO) circuit  50 ,  51 , and  52 . As used herein, the term “MEMS switch system” is used to represent a single MEMS switch or an array of MEMS switches arranged in a series configuration (m), a parallel configuration (n), or a series/parallel configuration (m×n). 
     In the exemplary embodiment shown, HALT/PATO circuit  50  includes a balanced diode bridge  58 . Balanced diode bridge  58  includes a first branch  60  and a second branch  61 . As used herein, the term “balanced diode bridge” is used to represent a diode bridge that is configured such that voltage drops across both the first and second branches  60 ,  61  are substantially equal. First branch  60  of balanced diode bridge  58  includes a first diode (D 1 )  63  and a second diode (D 2 )  64 . In a similar fashion, second branch  61  of balanced diode bridge  58  includes a third diode (D 3 )  67  and a fourth diode (D 4 )  68  operatively coupled together. When conducting, balanced diode bridge  58  establishes an equipotential point between a cathode (not separately labeled) of first diode (D 1 )  63  and a cathode (not separately labeled) of second diode (D 2 )  64 . Of course, the equipotential point could also be between an anode (not separately labeled) of third diode (D 3 )  67  and an anode (not separately labeled) of fourth diode (D 4 )  68 . The equipotential point ensures that, during opening and closing, voltage across MEMS switch system  40  remains low (e.g., less than 1 volt). 
     HALT/PATO circuit  50  is also shown to include a HALT circuit portion  73  connected in parallel to a PATO circuit portion  75 . HALT circuit portion  73  includes a HALT switch  76  shown in the form of a switching device  77 . Switching device  77  is connected in series with a HALT capacitor  78  and an inductor  81 . PATO circuit portion  75  includes a pulse switch  85  shown in the form of a switching device  86  connected in series with a pulse capacitor  87  and a diode (D 5 )  89 . HALT switch  76  and Pulse switch  85  are selectively activated by controller  12 . HALT/PATO circuit  50  is further shown to include a voltage snubber  93  that is connected in parallel with first MEMS switch system  40 , HALT circuit portion  73 , and PATO circuit portion  75 . Voltage snubber  93  limits voltage overshoot during fast contact separation of first MEMS switch system  40 . Voltage snubber  93  is shown in the form of a metal-oxide varistor (MOV)  94 . However, it should be appreciated by one of ordinary skill in the art that voltage snubber  93  can take on a variety of forms including circuits having a snubber capacitor connected in series with a snubber resistor and/or other devices or combinations of devices that constitute a snubber, 
     As best shown on  FIG. 4 , MEMS switch system  40  includes a MEMS switch  92 . In the illustrated embodiment, a MEMS switch  92  is depicted as having a first connection  93 , a second connection  94  and a third connection  95 . In one embodiment, first connection  93  may be configured as a drain connection, second connection  94  may be configured as a source connection and third connection  95  may be configured as a gate connection. Gate connection  95  is connected to a gate driver  96 . The gate driver  96  includes a power supply input (not shown) and control logic input  97  that are connected to receive signals from controller  12  and provide the means for changing the state of MEMS switch  92 . It should be appreciated that while the MEMS switch  92  is illustrated as a single switch, two or more switches may be combined in parallel, in series, or some combination thereof to provide the necessary voltage and current capacity needed for the application. It should also be appreciated that MEMS switch systems  41  and  42  include similar components. 
     In manner similar to that described above, HALT/PATO circuit  51  includes a balanced diode bridge  100 . In the illustrated embodiment, balanced diode bridge  100  includes a first branch  103  and a second branch  104 . First branch  103  of balanced diode bridge  100  includes a first diode (D 1 )  106  and a second diode (D 2 )  107  coupled together. In a similar fashion, second branch  104  of balanced diode bridge  100  includes a third diode (D 3 )  110  and a fourth diode (D 4 )  111  operatively coupled together. When conducting, balanced diode bridge  58  establishes an equipotential point between a cathode (not separately labeled) of first diode (D 1 )  63  and a cathode (not separately labeled) of second diode (D 2 )  64 . Of course, the equipotential point could also be between an anode (not separately labeled) of third diode (D 3 )  67  and an anode (not separately labeled) of fourth diode (D 4 )  68 . The equipotential point ensures that, during opening and closing, voltage across MEMS switch system  40  remains low (e.g., less than 1 volt). 
     HALT/PATO circuit  51  is also shown to include a HALT circuit portion  116  connected in parallel to a PATO circuit portion  118 . HALT circuit portion  116  includes a HALT switch  120  shown in the form of a switching device  121 . Switching device  121  is connected in series with a HALT capacitor  122  and an inductor  125 . PATO circuit portion  118  includes a pulse switch  130  shown in the form of a switching device  131  connected in series with a pulse capacitor  132  and a diode (D 5 )  134 . HALT/PATO circuit  51  is further shown to include a voltage snubber  139  that is connected in parallel with second MEMS switch system  41 , HALT circuit portion  116 , and PATO circuit portion  118 . Voltage snubber  139  limits voltage overshoot during fast contact separation of second MEMS switch system  41 . Voltage snubber  139  is shown in the form of a metal-oxide varistor (MOV)  140 . However, it should be appreciated by one of ordinary skill in the art that voltage snubber  139  can take on a variety of forms including circuits having a snubber capacitor connected in series with a snubber resistor. 
     In manner also similar to that described above, HALT/PATO circuit  52  includes a balanced diode bridge  144 . In the illustrated embodiment, balanced diode bridge  144  includes a first branch  146  and a second branch  147 . First branch  146  of balanced diode bridge  144  includes a first diode (D 1 )  149  and a second diode (D 2 )  150  coupled together. In a similar fashion, second branch  147  of balanced diode bridge  144  includes a third diode (D 3 )  153  and a fourth diode (D 4 )  154  operatively coupled together. When conducting, balanced diode bridge  58  establishes an equipotential point between a cathode (not separately labeled) of first diode (D 1 )  63  and a cathode (not separately labeled) of second diode (D 2 )  64 . Of course, the equipotential point could also be between an anode (not separately labeled) of third diode (D 3 )  67  and an anode (not separately labeled) of fourth diode (D 4 )  68 . The equipotential point ensures that, during opening and closing, voltage across MEMS switch system  40  remains low (e.g., less than 1 volt). 
     HALT/PATO circuit  52  is also shown to include a HALT circuit portion  160  connected in parallel to a PATO circuit portion  162 . HALT circuit portion  160  includes a HALT switch  166  shown in the form of a switching device  167 . Switching device  167  is connected in series with a HALT capacitor  168  and an inductor  170 . PATO circuit portion  162  includes a pulse switch  176  shown in the form of a switching device  177  connected in series with a pulse capacitor  178  and a diode (D 5 )  180 . HALT/PATO circuit  52  is further shown to include a voltage snubber  186  that is connected in parallel with second MEMS switch  42 , HALT circuit portion  160 , and PATO circuit portion  162 . Voltage snubber  186  limits voltage overshoot during fast contact separation of third MEMS switch system  42 . Voltage snubber  186  is shown in the form of a metal-oxide varistor (MOV)  187 . However, it should be appreciated by one of ordinary skill in the art that voltage snubber  186  can take on a variety of forms including circuits having a snubber capacitor connected in series with a snubber resistor. 
     In further accordance with the exemplary embodiment, controller  12  includes a central processing unit  191 , a memory  193 , and a phase angle detector  194 . Phase angle detector  194  senses a particular phase angle of each of the first, second, and third phases of multi-phase electrical source  6 . For example, phase angle detection, using input from each voltage sensor  44 ,  45 , and  46  detects a zero crossing for each phase  30 ,  31 , and  33 . Controller  12  then activates the associated one of the MEMS switch systems  40 ,  41 , and  42  after a predetermined delay following the zero crossing. The predetermined delay may be anywhere from zero seconds up to the required time to achieve the particular phase angle for the associated MEMS switch system  40 ,  41 , and/or  42 . When each phase reaches the predetermined phase angle, controller  12  selectively sends a gate signal to close a corresponding one of the plurality of MEMS switch systems  8 . By timing the activation of MEMS switch systems  8 , controller  12  reduces in-rush current to each of the first, second and third MEMS switch systems  40 - 42 , thereby reducing the in-rush current experienced by the motor starter. 
     In addition to setting a predetermined delay, controller  12  can be employed to reactively signal MEMS switching systems  40 - 41  to close at a particular phase angle. For example, phase angle detection, using input from each voltage sensor  44 ,  45 ,  46  detects a predetermined phase angle for each phase  31 ,  32 , and  33 . When the predetermined phase angle is detected, a MEMS switch gate signal is sent to close the corresponding one of MEMS switch systems  40 - 42 . Such a reactive system is made possible by a microsecond or faster reaction time of each MEMS switch system  40 - 42 . Such fast reaction times render turn-on delay insignificant for a 60 Hz waveform. 
     In accordance with one aspect of the exemplary embodiment, controller  12  activates first MEMS switch system  40  when first phase  30  reaches a first phase angle, second MEMS switch system  41  is closed when second phase  31  reaches a second phase angle and third MEMS switch system  42  closes when third phase  32  reaches a third phase angle. In accordance with one aspect of the exemplary embodiment, after first MEMS switch system  40  closes, second MEMS system  41  is closed at a voltage peak between first and second phases  30  and  31 . Similarly, once second MEMS switch system  41  is closed, third MEMS switch system  42  is closed at a voltage peak between second phase  31  and third phase  32 . 
     In accordance with an exemplary embodiment, first MEMS switch system  40  is closed at a phase angle of 0°. Second MEMS switch system  41  is closed when second phase  31  reaches a phase angle of 30°, and third MEMS switch system  42  is closed when third phase  32  reaches a phase angle of 30°. In accordance with another aspect of the exemplary embodiment, controller  12  closed first MEMS switch system  40  when first phase  30  reaches a phase angle of 0°. Second MEMS switch system  41  is closed when second phase  31  reaches a phase angle of 60°, and third MEMS switch system  42  is closed when third phase  32  reaches a phase angle of 60°. In accordance with yet another aspect of the exemplary embodiment, controller  12  closes first MEMS switch system  40  when first phase  30  reaches a phase angle of 0°. Second MEMS switch system  41  is closed when second phase  31  reaches a phase angle of 90°, and third MEMS switch system  42  is closed when third phase  32  reaches a phase angle of 90°. In accordance with still another aspect of the exemplary embodiment, controller  12  closes first MEMS switch system  40  when first phase  30  reaches a phase angle of 0°. Second MEMS switch system  41  is closed when second phase  31  reaches a phase angle of 120°, and third MEMS switch system  42  is closed when third phase  32  reaches a phase angle of 120°. In accordance with a further aspect of the exemplary embodiment, controller  12  closes first MEMS switch system  40  when first phase  30  reaches a phase angle of 0°. Second MEMS switch system  41  is closed when second phase  31  reaches a phase angle of 120°, and third MEMS switch system  42  is closed when third phase  32  reaches a phase angle of 202°. 
     In further accordance with an exemplary embodiment, controller  12  is preprogrammed with the phase angles of a given load. The phase angles may be selected through simulation or based on calculations from is power factor. Since motor loads are highly inductive it is desirable to close MEMS switch systems  40 - 42  at or near the voltage peak. The above described phase angles are relative to closing a one of the MEMS switch systems and do not require closing MEMS switch systems  40 - 42  in a particular order. In accordance with one example, the first phase closed would be chosen by controller  12  upon receiving a signal to close when, for example, a user presses the start button. The next phase to cross the zero point would be the first phase closed and thus remaining phases would then close at the predetermined angles of the remaining phases. 
     At this point it should be understood that the exemplary aspects provide a circuit that lowers long duration current that may be passed through a MEMS switch. While described in terms of MEMS switches, it should also be apparent that the exemplary embodiments can be employed to control any solid state and/or mechanical switches. Activating switches at different phase angles reduces in-rush current. The lower long duration current allows for the use of lower rated switches, or fewer switches in a switch array. More specifically, while each phase winding  20 - 22  of electrical motor  25  is described as being connected to corresponding phase windings  30 - 31  by a MEMS switch, it should be understood that the number of and type of switch could vary. That is, depending upon the voltage/current rating of the multi-phase electrical load, each phase winding could be coupled to a corresponding phase of a multi-phase electrical source by one or more switches connected in series, parallel or a series/parallel array. The particular type of switches, e.g. mechanical, solid state or MEMS is dependent upon desired design parameters. In addition, the particular phase angles at the controller activates the switches are exemplary. The controller can be programmed to activate the switches at a variety of angles depending upon voltage/current requirements for the particular switch system. 
     While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.