Abstract:
A system and method in accordance with the present technique for protecting motor drives from damage due to misconfiguration while coupled to a common DC bus, comprises sensing electrical operating parameters of the motor drives, automatically determining whether each of the drives is actually installed as a common bus leader or a common bus follower, and comparing the actual installation of the motor drives to a respective stored configuration of each of the motor drives as either a common bus leader or a common bus follower.

Description:
BACKGROUND 
       [0001]    The invention relates generally to motor drives. Particularly, this invention relates to a protection system implemented in a common bus connection between motor drives. 
         [0002]    In many modern automation settings, motors are generally coupled to motor drives configured to control the rotational speeds of the motors. In so doing, motor drives typically comprise components, such as converters, inverters and associated circuitry for controlling motors. Among electrical connections comprising the circuitry in motor drives is the electrical connection between the converter and the inverter of the motor drive, typically referred to as a “DC bus.” 
         [0003]    In certain instances it may be desirable to connect two or more motor drives via their respective DC buses. Such arrangements may be advantageous for many reasons, such as to minimize wiring and termination costs upon installation and servicing. Similarly, during operation, some motors may require input power while others may be available to regenerate power, the latter being applied to the common bus to aid in driving the then power-consuming motors. In such a configuration, the motor drives are said to share a common DC bus or to be in a common DC bus mode. A common DC bus mode may be established by connecting the DC bus of two or more motor drives, which could have identical hardware. In establishing a common DC bus connection between motor drives, it may be necessary to define functional modes of operation depending on how the motor drives are positioned in a circuit topology. That is, the configuration of each of the motor drives placed in a common DC connection may depend upon or may dictate how each motor drive in the circuit topology provides to or receives power from the common DC bus. 
         [0004]    Given a certain circuit topology, user configurations of the motor drives not in accordance with the circuit topology may damage the motor drives, leading to their malfunction. There is a need, therefore, for a technique that can avoid mis-configuration of motor drives when connected to a common DC bus. 
       BRIEF DESCRIPTION 
       [0005]    A system and method are provided for auto-detection of a common DC bus connection between motor drives. The auto-detection system and logic may implement a state machine or an algorithm adapted to ensure configurations of motor drives placed in a common DC bus mode comply with the manner the motor drives are installed in a given circuit topology. 
     
     
       DRAWINGS 
         [0006]    These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
           [0007]      FIG. 1  illustrates a circuit diagram of motor drives coupled via a common DC bus, in accordance with an exemplary embodiment of the present technique. 
           [0008]      FIG. 2  illustrates a user configurable computer screen, in accordance with an exemplary embodiment of the present technique for managing configuration of motor drives sharing a common DC bus. 
           [0009]      FIGS. 3A and 3B  illustrate a block diagram of an exemplary state machine in accordance with an exemplary embodiment of the present technique for automatically detecting and preventing mis-configuration of motor drives on a common DC bus. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Referring now to the figures, particularly,  FIG. 1  is a circuit diagram of two motor drives  10  and  12  coupled via a common DC bus connection  14 . The motor drive  10  is referred herein to as a common DC bus leader (CBL), while the motor drive  12  may be referred herein to as common a DC bus follower CBF. As will be discussed further below, the common bus connection  14  connects the CBL  10  and the CBF  12  such that a supply of DC power is provided from the CBL  10  to the CBF  12 . 
         [0011]    CBL  10  is provided with a three-phase alternating current AC input supply  16 , typically from the power grid, via a three-phase contactor  18 . The contactor  18  routes the AC supply, via sensors  20 , to converter  22  which converts the AC input into a DC bus. The sensors  20  are adapted to detect incoming phases of the AC input supplied to the converter  22 . That is, the sensors  20  detect whether a three-phase input is supplied to the converter  22  (i.e., present at the converter input terminals). 
         [0012]    The converter  22  provides DC input power to an inverter  24  which provides output power to motor  26 . As will be appreciated by those skilled in the art, the inverter is typically controlled by a control circuit that triggers solid state switches in the inverter to generate an AC output waveform of desired frequency for driving the motor. The converter  22  and the inverter  24  are connected by a DC bus. The DC bus includes sensors  28  for detecting DC bus voltage and a capacitor  30  for smoothing the DC power provided by the converter  22 . Contact points  32  and  34  disposed on the DC bus connect the CBL  10  to the common DC bus  14 . The DC bus further includes precharge circuitry  36 , in the form of a precharge bypass relay  38  and precharge resistor  40  in the illustrated embodiment used to limit inrush current to the DC bus. The DC bus also includes a resistor  42  coupled to a solid state switch  44  used to regulate the DC bus in accordance with voltages detected across the DC bus. 
         [0013]    The motor drive  10  contains a control unit  46  configured to control and monitor the operation of the CBL  10 . The control unit  46  comprises a contactor enable relay  48 , coupled to the contactor  18  and to a control unit  80  of the CBF  12 . The control unit  46  further comprises an interface  50 , a memory  52  and a drive circuit  54 , all of which are connected to a processor  56 . The processor  56  is also connected to sensors  20  and  28 . The drive circuit  54  is connected to the solid state switch  44  of the CBL  10  for controlling the voltage regulation of the DC bus, precharge bypass relay  38  of the CBL  10  for controlling the precharge of the DC bus and inverter  24  of the CBL  10  for controlling the generated AC output waveform. 
         [0014]    The CBF  12  comprises components similar to those described above with reference to the motor drive CBL  10 . Thus, the CBF  12  includes a three-phase AC supply  58 , leading to converter  60 , via three-phase detecting sensor  62 . Although the CBF  12  may have appropriate hardware configured to receive an AC input supply, such hardware may not be utilized while the motor drive  12  is configured as a CBF in a common DC bus mode. 
         [0015]    The converter  60  of the CBF  12  provides DC input power to inverter  64  which provides power to motor  66 . Similar to the CBL  10 , the converter  60  and the inverter  64  are connected via a DC bus. The DC bus of the CBF  12  includes sensors  68  for detecting DC bus voltage, and a capacitor  70  for smoothing the DC power provided by the converter  60  or by the common DC bus connection  14 . Contact points  72  and  74  disposed on the DC bus of the CBF  12  connect the motor drive  12  to the common DC bus  14 . The motor drive  12  may also include precharge circuitry  95 , similar to that shown with reference to the motor drive  10 , however, such precharge circuitry may not be active for a motor drive configured as a common bus follower, such as the CBF  12 . The DC bus of the CBF  12  further includes a resistor  76  coupled to a switch  78  used to regulate the DC bus of the motor drive, in accordance with voltages detected across the DC bus of the CBF  12 , however, such circuitry may not be active for a motor drive configured as a common bus follower, such as the CBF  12 . 
         [0016]    Similar to the CBL  10  motor drive, the CBF  12  motor drive, too, contains a control unit  80  configured to control and monitor the operation of the CBF  12 . The control unit  80  comprises a contactor enable relay  82 , coupled to the contactor  18  and to the contactor enable relay  48  of the control unit  46  of the CBL  10 . The control unit  80  further comprises an interface  84 , a memory  86  and a drive circuit  88 , all of which are connected to a processor  90 . The processor  90  is further connected to sensors  62  and  68 . The drive circuit  88  is connected to the solid state switch  78  of the CBF  12  for controlling the voltage regulation of the DC bus, precharge bypass relay  96  of the CBF  12  for controlling the precharge of the DC bus, and inverter  64  of the CBF  12  for controlling the generated AC output waveform. 
         [0017]    The control units  46  and  80  are coupled via interfaces  50  and  84 , respectively, to an external monitoring and configuring unit comprising a processor  92  connected to a user interface  94 , which will typically include a keyboard  94   a  and a display  94   b . The external monitoring and configuring unit comprising elements  92 ,  94   a  and  94   b  may, for example, enable a user to configure the motor drives  10  and  12  as common bus leaders or followers in accordance with a certain circuit topology. Accordingly, the external unit may notify a user of system faults arising from any user misconfiguration of the motor drives and, thereby, protecting the system from being damaged in such events. 
         [0018]    As further illustrated in  FIG. 1 , the common bus DC connection  14  may establish a common bus mode such that the motor drive  10  is a common bus leader while the motor drive  12  is a common bus follower. In a common DC bus mode, the “common bus leader” has the ability to supply DC power to multiple common bus followers, while the “common bus follower” does not convert incoming AC power to DC power, but draws DC power from the common bus  14 . Further, while in a common bus mode the CBF  12  may be provided with functionalities from CBL  10 , such as DC bus precharge, DC bus regulation, phase loss detection, ground fault detection and discharging of the DC bus. 
         [0019]    In a common DC bus mode, the sensors  20 ,  28 ,  62  and  68  may indicate whether the motor drives  10  and  12  are correctly configured as a CBL and a CBF, respectively. For example, when the motor drive  12  is configured as a CBF the sensors  68  may sample DC bus voltage to determine whether its magnitude across the DC bus of the motor drive  12  is in accordance with a common DC bus mode. Similarly, the sensor  62  may sample the power at the three-phase input  58  for loss of any phase signals, again, in accordance with configurations and installments enabling the motor drive  12  as a CBF. For example, the auto-detection logic and system may detect that the CBF  12  is in a “common bus mode” when its DC bus voltage is greater than a “bus under-voltage” level while three-phase power is not present, as described below with reference to  FIGS. 3A and 3B . Similarly, the motor drives  10  and  12  may include a detection feature allowing incoming three-phase AC power only to be applied to a motor drive configured as a CBL, while allowing DC power without three-phase power present to be applied to a CBF. 
         [0020]    In an exemplary common DC bus mode of operation implemented in a given circuit topology, where motor drive  10  is configured as a CBL and motor drive  12  is configured as a CBF, three-phase AC power is provided to the AC inputs  16  of the CBL  10  but not to the CBF  12 . However, if the motor drive  10  is inadvertently configured to be a CBF and then supplied with a three-phase AC power, the sensors  20  would indicate to the processor  56  the presence of the three-phase supply. In turn, the processor  56  would open the contactor enable relay  48  so as to open the AC contactor  18  and remove AC power  16  from the drive. At the same time, the processor  56  may send a signal, via interface  50 , to the monitoring unit  92  to issue a common DC bus drive fault. 
         [0021]    For a motor drive configured as a CBF, such as the motor drive  12 , DC bus voltage with no phase present is a valid operating condition for the common bus CBF  12 . That is, lack of phase detection by the sensors  62  maintains the CBF  12  functional. 
         [0022]    As previously mentioned, CBL  10  includes precharge circuitry  36  for powering its DC bus as well as for powering the DC bus of the CBF  12 . In some embodiments, the CBF  12  may also include the precharge circuitry  95 . For a CBF, such precharge circuitry may be used in cases where three-phase power is inadvertently applied to the CBF  12  so as to limit inrush current until a common bus fault is issued. 
         [0023]    In the CBF  12 , the solid state switch  78  remains open during the regulation and discharge of its DC bus by CBL  10 . Accordingly, the CBF  12  will either share its regenerative energy with the common bus CBL  10  and possibly other CBF&#39;s, which may be motoring while the CBF  12  is regenerating. In some embodiments, if the regenerative energy is too large to be regulated by the resistor  42  of the CBL  10 , a regenerative converter may be used. 
         [0024]    After three-phase power is applied to the CBL  10  and precharge is successfully achieved, the CBL  10  is said to be in a “bus ready” state. For the CBF  12  such a state is reached only after a predetermined amount of time elapses, such as 2.5 seconds, allowing its DC bus voltage to be greater than the “bus under voltage” level. Such a time delay prevents the common bus CBF  12  from drawing DC bus current until the CBL  10  completes it precharge cycle. 
         [0025]      FIG. 2  is an exemplary user configurable screen  110  for configuring a motor drive in accordance with an exemplary embodiment of the present technique. The screen  110  may be displayed on the user interface  94   a  so that a user may conveniently control and monitor the motor drives  10  and  12 . 
         [0026]    The screen  110  may be used by a user to configure, for example, the motor drives  10  and  12  as common bus leader or follower, respectively, in a common DC bus mode. Accordingly, the screen  110  may include selection tabs  112  from which a user can select and alter settings, such as connections of the drives, their associated axes and power. In an exemplary embodiment, choosing from tab selection  112  the choice labeled “power” may display a list of choices  114  from which the user can select options for configuring the motor drives  10  and  12  as a CBL and a CBF, respectively. 
         [0027]      FIGS. 3A and 3B  illustrate exemplary control logic  120  or a state machine, in accordance with an exemplary embodiment of the present technique. The logic  120  includes an auto-detection scheme implemented by a control unit or units, such as control units  46  and/or  80  ( FIG. 1 ), having a series of steps and conditions that should be met to ensure that the configurations and installments of the motor drives  10  and  12  are properly matched when both drives are placed in a common DC bus mode. The logic  120  begins at block  122  labeled “BUS INIT” during which control power is applied to a motor drive such as the CBL  10  and/or CBF  12  ( FIG. 1 ) and the control units  46  and/or  80  are initialized, as indicated by loop  124 . Thereafter, the logic  120  proceeds to block  126  ensuring precharge circuitry is not bypassed. Thereafter the logic  120  proceeds to block  128  where an AC contactor, such as the AC contactor  18   FIG. 1 , is enabled (i.e., closed) so that three-phase power may be connected to the motor drive  10  as shown in  FIG. 1 . The logic  120  then proceeds to block  130 , labeled “WAIT AC”, from which the logic  120  branches-off according to certain conditions implemented as part of the auto detection scheme, as indicated by text incorporated into lines extending from blocks in  FIG. 3 . 
         [0028]    Extending to the left of block  130  is line  132  labeled “phases=NONE.” That is, for a motor drive configured as a CBF no phases (i.e., incoming AC power) are expected to be detected by the sensors  62  of motor drive  12  when such a motor drive is configured as a CBF. At this stage of the auto detection scheme, the DC bus of the CBF  12  may be charging so that its voltage may rise to a desired voltage, referred to here as an “Under-Voltage” (V UV ). If the voltage of the DC bus, i.e., V bus , is below V UV , then block  134 , labeled “CBF AutoDetect=Off”, follows. This would indicate that the motor drive, whose DC bus is designated for charging, is not yet charged. In such a case, the logic  120  proceeds from block  134  to block  130  and continues to loop until a condition changes. 
         [0029]    Alternatively, if V bus &gt;V UV  in an amount of time greater than, for example 600 milliseconds (as labeled “time in state” (TIS) on logic  120 ), this is considered to indicate that the DC bus voltage is rising. Consequently, the logic  120  proceeds to block  136  labeled “CBF AutoDetect=On”, wherein the motor drive sets an internal common bus follower flag indicating that the motor drive may be installed as a CBF. The logic then proceeds from block  136  to decision junction  138 . If the motor drive is installed as a CBF but not configured as a CBF, the logic returns to block  130  via line  140 . In so doing, the logic  120  prevents the motor drive from being enabled without configuration as a CBF. This also prevents the drive from being enabled when the DC bus is energized by a regenerative load. However, in a case where the motor drive is installed and configured as a CBF, the logic proceeds from block  138  to block  142  via block  144  where the precharge bypass relay is turned on. 
         [0030]    If the motor drive is configured as a CBL, input signal phases are expected to be detected by sensors  20  ( FIG. 1 ) of the motor drive  10 . Accordingly, the logic proceeds from block  130  to decision junction  146 . If the motor drive  10  is mistakenly configured as a CBF, then the logic  120  proceeds to block  148  where the three-phase contactor  18  is disabled (i.e., opened). Thereafter, the logic  120  proceeds from block  148  to block  150  and a common bus fault is issued. In so doing, the logic  120  prevents AC power from being applied to a motor drive configured as a CBF. 
         [0031]    For a CBF, the logic remains at block  142 , as indicated by lines  152  and  154  leading to and from decision junction  156 , until a time of 2500 ms is reached. The period of time of 2500 ms is chosen so that the precharge of the CBL  10  is completed before CBF  12  is enabled, thus, avoiding damaging the precharge circuit of the CBL  10 . If condition V bus &gt;V UV  in decision block  156  is not satisfied during the 2500 ms delay, this indicates that the motor drive  12  has lost DC bus power from CBL  10  and the logic proceeds to block  158  to turn off the precharge bypass relay and then to block  134  to turn off the CBF AutoDetect flag. The logic then returns to block  130  to wait for DC bus power to be re-applied. 
         [0032]    For a motor drive configured as a CBF and for TIS&gt;2500 ms, the logic  120  proceeds from block  142  to block  160 , labeled “Bus_Ready.” In the “Bus Ready” state  160  ( FIG. 3B ), the logic  120  branches off according to various conditions corresponding to various configurations and installments of the motor drives  10  and  12 . Accordingly, if single-phase or 3-phase AC power is inadvertently applied to a motor drive configured as a CBF, such as the CBF  12 , while in the “Bus Ready” state  160 , the logic  120  proceeds from decision junction  162  to block  164  where the three-phase contactor  18  ( FIG. 1 ) is disabled (i.e., opened) and then to block  166  where the drive reports a common bus fault. The logic then proceeds to block  168  via block  170  to turn off the precharge bypass relay and then returns to block  130  to wait for DC bus power to be re-applied. In so doing, the common bus configuration prevents input AC power from being applied to a motor drive configured as a CBF. 
         [0033]    Referring again to block  160 , if no incoming AC power is applied to a motor drive configured as a CBF, the logic  120  proceeds, via line  172 , to decision junction  174  to determine whether the motor drive is configured as a CBF. If not, this would indicate that the drive is installed as a CBL. If the motor drive is configured as CBF, then the logic  120  proceeds from decision junction  174  to decision junction  176  to determine whether the condition V bus &lt;V UV  is satisfied. If this is not true, the logic  120  returns to the “Bus Ready” state of block  160  via line  178  as required in a normal mode of operation. If the condition V bus &lt;V UV  is satisfied, this may indicate the CBF lost DC bus power from the CBL  10 . As a consequence, the logic  120  proceeds to block  180  where the precharge bypass relay is set to an “off” state. From block  180  the logic  120  proceeds to decision junction  182  to determine whether the motor drive  12  may be enabled. If so, block  184  follows and the contactor  18  ( FIG. 1 ) is disabled (i.e., opened) and an undervoltage fault is reported in block  186  due to removal of DC bus power during enablement of an axis. Thereafter, the logic  120  returns to the “Wait AC” state of block  130 . If in decision junction  182  the motor drive  12  is determined to be not enabled, then the logic proceeds back to block  130  via line  188  without posting any fault as this is normal operation of the system when DC bus power is removed from a CBF. 
         [0034]    While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.