Abstract:
A system for managing shunt utilization among multiple power converters sharing a common DC bus is disclosed. Each power converter includes a shunt device, typically one or more power resistors, configured to dissipate power from the DC bus. The power converter is configured according to an initial set of configuration parameters to selectively connect the shunt device to the DC bus. Each power converter monitors the amount of power being dissipated from the DC bus via the shunt device connected to that power converter and determines a utilization rate for the shunt device. As the utilization rate increases, the configuration parameters are modified to less frequently connect the shunt device to the DC bus. As the utilization rate decreases, the configuration parameters are modified to more frequently connect the shunt device to the DC bus.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates generally to a system for dissipating regenerative energy in a power converter and, more specifically, to distributing dissipation of regenerative energy among multiple power converters sharing a common DC bus. 
         [0002]    Adjustable speed motor drives (ASD) are used to control the speed of AC motors and are a common type of power converter to share a DC bus. AC motors use three-phase electrical power connected to the stator windings of a motor to run the motor. Each stator winding is connected to a different conductor from a three-phase power source, in which each conductor delivers a different phase of the electrical power to the motor. The three-phase power source may be a direct connection to line power, but more commonly, the motor is connected to the ASD. The ASD allows for speed control of the motor not available by connecting the motor directly to line power. 
         [0003]    As is known in the art, there are many electrical topologies for ASDs used to convert the fixed voltage and frequency from the line input into a controlled voltage and frequency output for a three-phase motor. One common topology includes a rectifier section which converts the line power into a DC voltage used to charge a DC bus section of the ASD. An inverter section then controls a set of solid state switches, for example, via pulse width modulation (PWM), to convert the DC voltage from the DC bus into a variable voltage and frequency output to the motor. Controlling the variable voltage and frequency output to the motor controls the speed at which the motor rotates. 
         [0004]    As the ASD controls the speed of the motor, there are periods of operation when the motor may enter a regenerative condition such as decelerating a high inertial load or maintaining a constant speed in the presence of an overhauling load (i.e., a load that would tend to accelerate the speed of the motor). Under a regenerative operating condition, the motor operates as a generator sending power back through the inverter section and onto the DC bus section of the ASD, causing the voltage level on the DC bus to rise. Unless this power is removed from the DC bus in some manner, the voltage continues to rise until it becomes too great, causing an over voltage fault and shutting down the ASD. 
         [0005]    One way to avoid an over voltage fault is to provide the ASD with a conductive path connected to the DC bus on which to shunt the power generated by the motor. It is known to establish this alternate conductive path by selectively connecting an external resistor to the DC bus via an internal, solid-state switch such as a transistor. When the resistor is connected to the DC bus, current flows through the resistor and the power is dissipated from the resistor as heat. Control of the switch is performed as a function of the voltage level present on the DC bus. 
         [0006]    In systems in which multiple ASDs are present, it is also known to electrically connect the DC bus of each ASD, which is also referred to as providing a common DC bus for multiple ASDs. With a common or shared DC bus, when a first ASD is operating in a regenerative mode, a second ASD may be operating in a motoring mode. The second drive uses a portion or all of the power regenerated from the first drive to operate the second drive. However, in a shared DC bus system, operating conditions still exist in which more energy is regenerated onto the DC bus than is consumed by the ASDs in the system. As an example, both ASDs may simultaneously operate in a regenerative mode. Thus, an alternate conductive path is still required. 
         [0007]    However, sharing a DC bus among multiple ASDs each having a corresponding shunt resistor is not without its drawbacks. If a first ASD is configured to connect its respective shunt resistor at a lower voltage level than a second ASD, the first shunt resistor will be more heavily utilized to dissipate excess energy on the DC bus than the second shunt resistor. Further, shunt resistors are sized, in part, according to the power rating of the ASD to which they are connected. If the second ASD has a higher power rating than the first, the shunt resistor of the first ASD is subject to excessive loading and premature failure. Even if the first and second ASDs are configured to connect their respective shunt resistors at the same voltage level, measurement noise and bias will result in one of the two ASDs connecting its shunt resistor first, resulting in the same potential for excessive loading and/or premature failure of the shunt resistor. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0008]    The subject matter disclosed herein describes a system for managing shunt utilization among multiple power converters sharing a common DC bus. Each power converter includes a shunt device, typically one or more power resistors, configured to dissipate power from the DC bus. The power converter is configured according to an initial set of configuration parameters to selectively connect the shunt device to the DC bus. Each power converter monitors the amount of power being dissipated from the DC bus via the shunt device connected to that power converter and determines a utilization rate for the shunt device. As the utilization rate increases, the configuration parameters are modified to less frequently connect the shunt device to the DC bus. As the utilization rate decreases, the configuration parameters are modified to more frequently connect the shunt device to the DC bus. 
         [0009]    According to one embodiment of the invention, a method of controlling shunt utilization in a power converter sharing a DC bus with at least one other power converter, wherein the DC bus includes a first rail and a second rail, includes the steps of monitoring a magnitude of a voltage present on the DC bus, connecting a shunt device between the first rail and the second rail to discharge a desired amount of power when the magnitude of the voltage present on the DC bus exceeds a threshold, determining a utilization rate of the shunt device, and decreasing the desired amount of power discharged by the shunt device as the utilization rate increases. 
         [0010]    According to another embodiment of the invention, a power converter for connection to a shared DC bus with variable shunt utilization includes a DC bus having a first rail, a second rail, and an output connection configured to connect the DC bus of the power converter to the shared DC bus. A pair of terminals is configured to connect to an external shunt device, and a switch is configured to selectively connect at least one of the terminals to one of the first or second rails of the DC bus. A sensor is configured to generate a signal corresponding to a magnitude of a voltage present on the DC bus. A non-transitory storage medium stores a program, and a processor is configured to read the program from the non-transitory storage medium. The processor is further configured to execute the series of instructions to read the signal corresponding to a magnitude of a voltage present on the DC bus, determine a utilization rate of the external shunt device, and control the switch as a function of the utilization rate and of the signal corresponding to a magnitude of a voltage present on the DC bus. 
         [0011]    These and other advantages and features of the invention will become apparent to those skilled in the art from the detailed description and the accompanying drawings. It should be understood, however, that the detailed description and accompanying drawings, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Various exemplary embodiments of the subject matter disclosed herein are illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which: 
           [0013]      FIG. 1  is a schematic representation of a first and a second power converter according to one embodiment of the present invention; 
           [0014]      FIG. 2  is a schematic representation of one of the power converters of  FIG. 1 ; 
           [0015]      FIG. 3  is a graphical representation of shunt operation according to one embodiment of the present invention; 
           [0016]      FIG. 4  is a graphical representation of modulation of the shunt device according to one embodiment of the present invention; 
           [0017]      FIG. 5  is a graphical representation of varying the voltage thresholds of the shunt device according to one embodiment of the present invention; 
           [0018]      FIG. 6  is a graphical representation of varying the duty cycle of modulation of the shunt device according to one embodiment of the present invention; 
           [0019]      FIG. 7  is a flowchart illustrating variation of the voltage thresholds of the shunt device according to one embodiment of the present invention; 
           [0020]      FIG. 8  is a flowchart illustrating variation of the duty cycle of modulation of the shunt device according to one embodiment of the present invention; and 
           [0021]      FIG. 9  is a flowchart illustrating determination of the utilization rate of a shunt device according to one embodiment of the present invention. 
       
    
    
       [0022]    In describing the various embodiments of the invention which are illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended that the invention be limited to the specific terms so selected and it is understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose. For example, the word “connected,” “attached,” or terms similar thereto are often used. They are not limited to direct connection but include connection through other elements where such connection is recognized as being equivalent by those skilled in the art. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    According to one embodiment of the invention, a first motor drive  10  and a second motor drive  10  are illustrated in  FIG. 1 , each motor drive  10  connected to a shared DC bus. Each of the first and second motor drives  10  have a common construction and will, therefore, be discussed in more detail with respect to one motor drive  10 . Optionally, motor drives of varying construction may be connected at their respective DC buses. Each motor drive  10  is configured to receive a three-phase input power  15  at rectifier section  20 . The rectifier section  20  may include any electronic device suitable for passive or active rectification as is understood in the art. Referring also to  FIG. 2 , the illustrated rectifier section  20  includes a set of diodes  22  forming a diode bridge that rectifies the three-phase input power  15  to a DC voltage on the DC bus  25 . Optionally, the rectifier section  20  may include other solid state devices including, but not limited to, thyristors, silicon controlled rectifiers, or transistors to convert the input power  15  to a DC voltage for the DC bus  25 . 
         [0024]    The DC voltage potential is present between a positive rail  27  and a negative rail  29  of the DC bus  25 . A DC bus capacitor  24  is connected between the positive and negative rails,  27  and  29 , to reduce the magnitude of the ripple voltage resulting from converting the AC voltage to a DC voltage. It is understood that the DC bus capacitor  24  may be a single capacitor or multiple capacitors connected in parallel, in series, or a combination thereof. The magnitude of the voltage potential between the negative and positive rails,  29  and  27 , is generally equal to the magnitude of the peak of the AC input voltage. A first sensor  61  measures the voltage on the DC bus  25  and provides a signal corresponding to the measured voltage to a processor  50  on the motor drive  10 . 
         [0025]    The DC bus  25  is connected in series between the rectifier section  20  and the inverter section  30 . The inverter section  30  consists of switching elements, such as transistors or thyristors as is known in the art. The illustrated inverter section  30  includes a transistor  32  and a flyback diode  34  connected in pairs between the positive rail  27  and each phase of the output voltage  35  as well as between the negative rail  29  and each phase of the output voltage  35 . The inverter section  30  receives gating signals  31  from the processor  50  to control the transistors  32  and to convert the DC voltage from the DC bus  25  into a controlled three phase output voltage  35  to the motor  40 . 
         [0026]    One or more modules are used to control operation of the motor drive  10 . Referring now to  FIG. 2 , the motor drive  10  includes a non-transitory storage device, or memory  45 , configured to store data and a series of instructions executable by the processor  50 . It is contemplated that the memory  45  may be a single device, multiple devices, or incorporated, for example, as a portion of another device such as an application specific integrated circuit (ASIC). The processor  50  is in communication with the memory  45  to read the instructions and data as required to control operation of the motor drive  10 . According to one embodiment of the invention, the processor  50  receives a reference signal identifying desired operation of the motor  40  connected to the motor drive  10 . The reference signal may be, for example, a speed reference or a torque reference. The program executes a control routine responsive to the reference signal and feedback signals from one or more sensors  36  providing voltage and/or current signals corresponding to one or more phases of the output  35  to the motor  40 . The control routine generates a voltage reference signal which, in turn, is provided to a modulation routine  55 . The modulation routine, such as pulse width modulation (PWM), generates gating signals  31  to control the transistors  32  responsive to the desired voltage reference signal. 
         [0027]    Referring again to  FIG. 1 , the motor drives  10  are connected in a shared DC bus configuration. Each motor drive  10  includes terminals  28  connected to the positive rail  27  and the negative rail  29  of the DC bus  25 . The terminals  28  of each motor drive  10  are electrically connected to establish the shared DC bus. 
         [0028]    A shunt device  64  is connected to each motor drive  10  to dissipate excess power from the DC bus  25 . Terminals  66  are configured to provide an electrical connection between the motor drive  10  and the shunt device  64 . A switch  62  selectively connects either the negative rail  29  or the positive rail  27  to one of the terminals  66 . The other of the negative rail  29  or the positive rail  27  is connected to the other terminal  66 . Closing the switch  62  electrically connects the shunt device  64  across the DC bus  25  while opening the switch  62  disconnects the shunt device  64  from the DC bus  25 . Referring also to  FIG. 2 , the switch  62  may be a transistor  70 , for example, an insulated gate bipolar transistor (IGBT). The shunt device  64  may be a resistor  72  and, more specifically, a power resistor, configured to dissipate power across the resistor  72  as heat. A shunt control module  60  in the processor  50  receives the signal corresponding to the magnitude of the voltage present on the DC bus  25  and generates a control signal  71  for the switch  62 . 
         [0029]    In operation, the shunt control module  60  selectively connects the shunt device  64  to the DC bus  25  to reduce the amount of power present on the DC bus  25 . When a motor drive  10  enters a regenerative operating condition, the voltage level on the DC bus  25  increases. If the voltage level increases too much, components of the motor drive  10  may be damaged. Consequently, the voltage level on the DC bus  25  must be kept below a certain maximum value, illustrated as the over voltage level, VDC_OV,  119  in  FIG. 3  or the motor drive  10  will shut down to prevent the voltage level from increasing any further. According to one embodiment of the invention, the shunt control module  60  may use hysteretic control of the DC shunt device  64  to control the level of the DC voltage present on the DC bus  25 . When the voltage level on the DC bus  25  reaches a first threshold, the shunt control module  60  generates the control signal  71  closing the switch  62  and connecting the shunt device  64  across the DC bus  25 . As illustrated in  FIG. 2 , the shunt device  64  is a resistor  72  and, as is known in the art, the power dissipated in the resistor  72  is equal to the square of the current conducted through the resistor  72  times the resistance value. As the power is dissipated from the DC bus  25 , the voltage level on the DC bus  25  decreases. When the voltage level on the DC bus  25  drops below a second threshold, the shunt control module  60  generates the control signal  71  opening the switch  62  and disconnecting the shunt device  64  from across the DC bus  25 . Optionally, the first and second thresholds may be the same value; however, to prevent rapid transitions between a connected and disconnected state, the first and second thresholds are offset from each other defining a hysteretic control of the DC shunt device  64 . 
         [0030]    Referring next to  FIG. 3 , exemplary operation of the shunt control module  60  is illustrated. The level of voltage on the DC bus  105  is illustrated for one exemplary regenerative operation. The level of voltage on the DC bus  105  is normally held at a nominal DC voltage level  110 , VDC_NOM, which may be a function of the peak value voltage of the AC input power  15  resulting from rectifying the voltage. For example, a 230 VAC input results in a nominal DC voltage level  110  of 325 VDC and a 460 VAC input results in a nominal DC voltage level  110  650 V DC voltage. Optionally, an active rectifier may boost the nominal DC voltage level  110  by a desired offset value of 5-10%. At time t 1 , the motor drive  10  enters regenerative operation and the voltage level  105  on the DC bus  25  begins to rise. At time t 2 , the voltage level  105  reaches the turn on threshold  115 , VDC_ON and the shunt control signal  120  turns on. The shunt control signal  120  may be used to directly control shunt switch  62  or, as discussed in more detail below, may be used as an enable signal used in cooperation with a shunt switch signal  130 . Having connected the shunt device  64  across the DC bus  25 , the voltage level  105  begins to drop. At time t 3 , the voltage level  105  reaches the turn off threshold  112 , VDC_OFF and the shunt control signal  120  turns off. However, because the motor drive  10  is still in regenerative operation, the voltage level  105  begins to rise again. At time t 4 , the voltage level  105  again reaches the turn on threshold  115 , VDC_ON and the shunt control signal  120  turns on. This cycle repeats until the motor drive exits regenerative operation (e.g., between times t 4  and t 5 ). At time t 5 , the voltage level  105  again reaches the turn off threshold  112 , VDC_OFF and the shunt control signal  120  turns off. Because the motor drive  10  is no longer in regenerative operation, the voltage level  105  returns to the nominal DC voltage level  110 . 
         [0031]    A portion of the cycle of exemplary operation discussed with respect to  FIG. 3 , is illustrated in more detail in  FIG. 4 . According to the embodiment illustrated in  FIG. 4 , the shunt control module  60  utilizes the shunt control signal  120  as an enable signal and generates a periodic shunt switch signal  130 , which selectively enables/disables the shunt switch  62 . As shown, the shunt control module  60  is configured to connect the shunt device  64  in a modulated manner. The shunt control module  60  generates a pulsed shunt switch signal  130  at a repeated interval having a period, T, and a duty cycle, D. When both the shunt control signal  120  and the switch signal  130  are on, the shunt switch  62  is closed, connecting the shunt device  64  across the DC bus  25 . If either signal,  120  or  130 , is off, the shunt switch  62  is opened. Thus, during the period between t 2  and t 3  shown in  FIG. 3 , the shunt device  64  is repeatedly connected to and disconnected from the DC bus  25 . The modulation occurs, for example, at 500 Hz, resulting in a 2 msec period, T. The shunt control module  60  may also vary the duty cycle, for example, as a function of the level of DC voltage  105  present on the DC bus  25  to discharge more power across the shunt device  64  as the level of DC voltage  105  continues to increase. 
         [0032]    Although each motor drive  10  is connected to a corresponding shunt device  64 , connecting the motor drives  10  with a shared DC bus  25  presents additional challenges in regulating the voltage level  105  on the DC bus  25 . According to the embodiment illustrated in  FIG. 1 , each of the motor drives  10  is connected only at the respective DC bus terminals  28 . No additional communication is provided between the motor drives  10 . Although illustrated as identical motor drives  10 , each motor drive  10  may have different power ratings, different switching frequencies, or different shunt regulation methods. In addition, even identical motor drives  10  include variations in measurements of the voltage level  105  on the DC bus  25  due to component manufacturing tolerances that may manifest in varying offsets in sensors or noise susceptibility. Thus, even if motor drives  10  are configured to turn on and off their respective shunt devices  64  at the same threshold values  112 ,  115 , one of the motor drives  10  will command the shunt device  64  to connect prior to the other, resulting in a higher utilization rate for that drive  10 . In addition, if the motor drives  10  have different power ratings, the potential exists that the motor drive  10  with a lower power rating will connect its shunt device  64  first, resulting in the lower rating drive dissipating at least a portion of the illustrated energy supplied to the DC bus  25  from the motor drive  10  with a higher power rating. 
         [0033]    To overcome the above-described challenges, each shunt control module  60  maintains a utilization rate for the shunt device  64  connected to the respective motor drive  10 . Referring next to  FIG. 9 , the utilization rate is determined as a function of the power dissipated by the shunt device  64 . If the shunt device  64  is a power resistor  72  and a hysteretic control is utilized, the shunt control module  60  attempts to return the voltage level  105  on the DC bus  25  back to the shut off level  112  by dissipating power from the DC bus  25  in the power resistor  72 . To determine the utilization rate, the processor  50  first checks whether the shunt device  64  is connected across the DC bus  25 , as shown at step  300 . If the shunt device  64  is not connected, the power is set to zero, as shown at step  305 . However, if the shunt device  64  is connected to the DC bus  25 , the processor  50  determines the amount of power dissipated in the shunt device  64 , as shown in step  310 . For the power resistor  72  illustrated, the amount of power dissipated may be determined according to Equation 1. 
         [0000]    
       
         
           
             
               
                 
                   P 
                   = 
                   
                     
                       V 
                       
                         DC 
                          
                         
                             
                         
                          
                         Bus 
                       
                       2 
                     
                     R 
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where:
 
V DC Bus  is the measured voltage level on the DC bus
 
R is the resistance value of the shunt resistor
 
The amount of power calculated in Eq. 1 is converted to a normalized value by dividing it by the rated amount of power the shunt device  64  may dissipate. At step  315 , the power dissipated by the shunt device  64  is provided as an input to a filter, such as a digital low-pass filter. Because the power was normalized, the input and consequently the output of the filter will typically be in a range of 0.0-1.0 where 0.0 indicates no power dissipated and 1.0 indicates rated power dissipated by the shunt device  64 . The output of the filter is provided to the shunt control module  60  as the utilization rate of the shunt device  64 . The shunt control module  60  regulates the amount of power dissipated by the shunt device  64  to the DC bus  25  as a function of the utilization rate.
 
         [0034]    According to one embodiment of the invention, the shunt control module  60  may vary the on and off thresholds,  115  and  112 , respectively, at which the shunt control signal  120  is turned on and off to vary the power dissipated by the shunt device  64 . Referring to  FIG. 7 , a flowchart  200  illustrates the steps executed by the processor  50  to vary the voltage thresholds  115 ,  112  at which the shunt device  64  turns on and off, respectively. At step  205 , the processor  50  obtains the voltage level  105  present on the DC bus  25 . The voltage level  105  is compared against the initial value of the turn on threshold  115  at step  210 . If the voltage level  105  is greater than the turn on threshold  115 , the shunt device  64  is connected as shown in step  215 . If the voltage level  105  is less than the turn on threshold  115 , the voltage level  105  is then compared against the initial value of the turn off threshold  112  at step  211 . If the voltage level  105  is less than the turn off threshold  112 , the shunt device  64  is disconnected as shown in step  213 . If, however, the voltage level  105  is less than the turn on threshold  115  but greater than the turn off threshold  112 , than the shunt device  64  remains in its current state. At step  220 , the utilization rate of the shunt device  64  is determined as discussed above with respect to  FIG. 9 . In step  225 , the turn on and turn off voltage thresholds  115 ,  112  are adjusted as a function of the utilization rate. As the utilization rate increases, at least one of the turn on and turn off voltage thresholds  115 ,  112  are increased such that the shunt device  64  is used less frequently. As the utilization rate decreases, the turn on and turn off voltage thresholds  115 ,  112  are decreased such that the shunt device  64  is used more frequently. According to one embodiment of the invention, a look up table is stored in memory  45  that defines new threshold values corresponding to varying utilization rates. Optionally, the threshold values may be varied, for example, as a linear function of the utilization rate. 
         [0035]    Referring also to  FIG. 5 , exemplary operation of the shunt control module  60  during three consecutive regenerative operations, during which the voltage thresholds  112 ,  115  are varied, is illustrated. According to the illustrated operation, the voltage level  105  on the DC bus  25  rises from the nominal level  110  to a peak value and then returns to the nominal level  110  over three consecutive regenerative operations. During the initial regenerative operation, the shunt control signal  120  turns on when the voltage level  105  reaches the initial turn on threshold  115  at t 1  and remains on until the voltage level  105  drops below the initial turn off threshold  112  at t 2 . After a short duration at the nominal DC voltage level  110 , the voltage level  105  on the DC bus  25  again begins to rise. During operation, the shunt control module  60  monitors utilization of the shunt device  64 . As a result, the value of the thresholds are increased from the initial turn on threshold  115  to a second turn on threshold  116  and from the initial turn off threshold  112  to a second turn off threshold  113 . During the second regenerative operation, the shunt control signal  120  turns on when the voltage level  105  reaches the second turn on threshold  116  at t 3  and remains on until the voltage level  105  drops below the second turn off threshold  113  at t 4 . After a short duration at the nominal DC voltage level  110 , the voltage level  105  on the DC bus  25  again begins to rise. Again, the shunt control module  60  monitors utilization of the shunt device  64 . As a result, the value of the thresholds are increased from the second turn on threshold  116  to a third turn on threshold  117  and from the second turn off threshold  113  to a third turn off threshold  114 . During the third regenerative operation, the shunt control signal  120  turns on when the voltage level  105  reaches the third turn on threshold  117  at t 5  and remains on until the voltage level  105  drops below the third turn off threshold  114  at t 6 . As a result of the increasing threshold values, the shunt control signal  120  is turned on for a shorter duration during each subsequent regenerative operation. If the voltage level  105  on the DC bus  25  remains at the nominal value  110 , or at least below the present values thresholds, for an extended duration, the shunt control module  60  will lower the threshold values until they return to the initial values. If the input power  115  to the motor drive  10  is 460 VAC, the initial turn on threshold  115  may be selected, for example, at 775 VDC and the initial turn off threshold  112  may be selected, for example, at 765 VDC. This provides a 10V hysteresis band to prevent toggling of the shunt control signal  120  at the turn on value. The voltage thresholds may be increased, for example, up to 30 VDC above the initial values, providing a range of 775-805 VDC for the turn on threshold and 765-795 VDC for the turn off threshold. 
         [0036]    When multiple motor drives  10  are connected to a common DC bus, variation of the voltage thresholds  115 ,  112  prevents the shunt device  64  of a single motor drive  10  from becoming overloaded if that motor drive  10  would otherwise be the first to connect its shunt device  64  to the DC bus  25 . If one motor drive  10  is continually connecting its corresponding shunt device  64  to the DC bus  25 , its voltage thresholds will increase such that a second motor drive  10  will begin to connect its corresponding shunt device  64  to the DC bus  25  before the first motor drive  10 . Further, as the second motor drive  10  is now the first to connect its shunt device  64  to the DC bus  25 , its voltage thresholds begin to increase while the voltage thresholds on the first motor drive  10  begin to decrease. Consequently, each of the motor drives  10  connected to the shared DC bus will share in dissipating regenerative energy from the DC bus  25  and their corresponding shunt devices  64  will be more evenly utilized. 
         [0037]    According to another embodiment of the invention, the shunt control module  60  may vary the duty cycle, D, of the modulated shunt switch signal  130  to vary the power dissipated by the shunt device  64 . Referring to  FIG. 8 , a flowchart  250  illustrates the steps executed by the processor  50  to vary the duty cycle, D, of the modulated shunt switch signal  130 . 
         [0038]    At step  255 , the processor  50  obtains the voltage level  105  present on the DC bus  25 . The voltage level  105  is compared against the initial value of the turn on threshold  115  at step  210 . If the voltage level  105  is greater than the turn on threshold  115 , the shunt control module  60  obtains the present value of the duty cycle, D, as shown in step  265 . The duty cycle, D, may be stored in and read from memory  45 . The shunt control module  60  next modulates, periodically turning on and off, the shunt switch signal  130  according to the duty cycle, D, as shown in step  270 . If the voltage level  105  is less than the turn on threshold  115 , the voltage level  105  is compared against the initial value of the turn off threshold  112  at step  261 . If the voltage level  105  is less than the turn off threshold  112 , the shunt control module  60  stops modulation of the shunt device  64  at step  263 . If the voltage level  105  is less than the turn on threshold  115  and greater than the turn off threshold  112 , the shunt control module continues execution in the present state at step  275 . At step  275 , the utilization rate of the shunt device  64  is determined as discussed above with respect to  FIG. 9 . In step  280 , the duty cycle, D, is adjusted as a function of the utilization rate. As the utilization rate increases, the duty cycle, D, is decreased such that the shunt device  64  is used less frequently. As the utilization rate decreases, the duty cycle, D, is increased such that the shunt device  64  is used more frequently. According to one embodiment of the invention, a look up table is stored in memory  45  that defines new values of the duty cycle, D, corresponding to varying utilization rates. Optionally, the duty cycle, D, may be varied, for example, as a linear function of the utilization rate. 
         [0039]    Referring also to  FIG. 6 , exemplary operation of the shunt control module  60  during two consecutive regenerative operations during which the duty cycle, D, of the shunt switch signal  130  is varied is illustrated. During the initial regenerative operation, an initial value of the duty cycle, D1, is used to modulate the shunt switch signal  130 . Regenerative operation continues for four cycles  135  of the modulation and is off for one cycle  135 . During the first regenerative operation, the shunt control module  60  monitors utilization of the shunt device  64 . As a result, the duration of the duty cycle, D, is decreased from the first duty cycle, D1, to a second duty cycle, D2. Regenerative operation is shown for four additional cycles  135  using the second duty cycle, D2. As a result of the decreasing value of the duty cycle, D, the shunt switch signal  130  is turned on for a shorter percentage of each subsequent regenerative operation. If the voltage level  105  on the DC bus  25  remains at the nominal value  110 , or at least below the turn on threshold  115 , for an extended duration, the shunt control module  60  will increase the duty cycle, D, until it returns to the initial value. 
         [0040]    When multiple motor drives  10  are connected to a common DC bus, variation of the duty cycle, D, prevents the shunt device  64  of a single motor drive  10  from becoming overloaded if that motor drive  10  would otherwise be the primary motor drive  10  to connect its shunt device  64  to the DC bus  25 . If one motor drive  10  is continually connecting its corresponding shunt device  64  to the DC bus  25 , its duty cycle decreases such that it dissipates less energy from the DC bus  25 , requiring another motor drive  10  to begin connecting its corresponding shunt device  64  to the DC bus  25 . Further, as the second motor drive  10  is now dissipating more energy, from the DC bus  25 , its duty cycle begins to decrease while the duty cycle on the first motor drive  10  begins to increase. Consequently, each of the motor drives  10  connected to the shared DC bus will share in dissipating regenerative energy from the DC bus  25  and their corresponding shunt devices  64  will be more evenly utilized. 
         [0041]    It should be understood that the invention is not limited in its application to the details of construction and arrangements of the components set forth herein. The invention is capable of other embodiments and of being practiced or carried out in various ways. Variations and modifications of the foregoing are within the scope of the present invention. It also being understood that the invention disclosed and defined herein extends to all alternative combinations of two or more of the individual features mentioned or evident from the text and/or drawings. All of these different combinations constitute various alternative aspects of the present invention. The embodiments described herein explain the best modes known for practicing the invention and will enable others skilled in the art to utilize the invention