Abstract:
A method for controlling at least two series-cell based drives connected in parallel. The method comprises detecting a failed cell in one of the drives, inhibiting operation of the drive with the failed cell, determining a peak output voltage capability of the drive with the failed cell, communicating the peak output voltage capability to a master controller, and limiting a maximum voltage that each drive can apply to a motor. The method also comprises bypassing the failed cell and resuming operation of the drive with the failed cell after the motor voltage falls below the peak output voltage capability of the drive with the failed cell.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of U.S. Provisional Patent Application No. 60/708,066, filed on Aug. 12, 2005. 
    
    
     BACKGROUND 
     This application discloses an invention that is related, generally and in various embodiments, to a system and method for parallel control of series-cell based drives. 
     Variable frequency drives are known in the art. A variable frequency drive typically includes a control circuit and a power circuit. For synchronous motor applications, the variable frequency drive may also include a field supply. The control circuit controls the operation of the power circuit and, for synchronous motor applications, also enables/disables the associated field supply. The power circuit includes a rectifier and an inverter, and provides power to a winding of a motor connected to the variable frequency drive. For synchronous motor applications, the field supply provides power to an exciter for a motor field circuit. 
     The control circuit typically includes a speed regulator, a flux regulator, a magnetizing current regulator, a torque current regulator, a DQ-3Φ transform, a pulse width modulator, and a motor model. The speed regulator provides a magnetizing current reference, and the flux regulator provides a torque current reference. The control circuit compares the magnetizing current reference to a measured magnetizing current, and the magnetizing current regulator determines a Q-axis voltage reference. The control circuit also compares the torque current reference to a measured torque current, and the torque current regulator determines a D-axis voltage reference. Additional feed-forward signals may be added to the D-axis voltage reference and the Q-axis voltage reference to provide a higher dynamic response. The DQ-3Φ transform transforms the Q-axis voltage reference and the D-axis voltage reference from two-phase information into three-phase values. The pulse width modulator converts the three-phase values to switching commands that are sent to the power circuit. The motor model generally utilizes measured voltage and/or current signals to determine motor parameters such as the motor speed, the motor flux, the motor flux angle. For applications where low cost is a business requirement, the motor model may only utilize the variable frequency drive output current or the motor current to determine motor parameters. The motor model also converts measured currents into a magnetizing current component and a torque current component for use in the magnetizing current regulator and the torque current regulator, respectively. 
     Many of the functions performed by the control circuit are implemented in software. The software is written such that calculations are done at two or more different rates so as to save processor execution time. In general, the pulse width modulator operates at the fastest rate and is usually implemented in hardware. The magnetizing current regulator, the torque current regulator, and the DQ-3Φ transform blocks are typically executed at a data rate of 1-10 kilohertz so that a fast response of the control is achieved in limiting the output current of the variable frequency drive in case of sudden changes in the load or the output circuit. The speed regulator and the flux regulator typically operate at a slower rate of 100-1000 hertz because both motor speed and motor flux change at a much slower rate than the magnetizing current and the torque current. The motor model also is usually computed at this rate. Communications from the control circuit to the outside world, which includes communications to an external device (from the customer), is typically at a rate of 1-10 hertz. 
     When the power needed to be delivered to a load exceeds the power available from a single variable frequency drive, it is known to connect a number of variable frequency drives in parallel in order to meet the power requirement of the load. To control such parallel arrangements, a master-slave arrangement is often utilized. In a master-slave configuration, a master controller often operates as an interface between external controls (e.g., customer controls) and the various variable frequency drives. The respective control circuits communicate with the master controller and also communicate directly with each other. 
     In general, for the master-slave arrangement, one variable frequency drive is configured as the master drive, and the other variable frequency drives are configured as slave drives. The master drive typically performs the speed regulator function and the flux regulator function, and issues current commands (torque current reference and magnetizing current reference) to itself and all the slave drives. The slave drives operate on current commands from the master drive and do not utilize their respective speed regulators or flux regulators. Thus, the slave drives operate with fewer regulators than the master drive. The slave drives utilize the current commands from the master drive to regulate each of their output currents and produce the required motor voltage. In addition to the current commands, the master drive must also send either the motor flux angle or the motor frequency information to the slave drives to enable the slave drives to convert the respective Q-axis voltage references and D-axis reference voltages to the correct 3-phase reference frame. To provide good dynamic performance, the required communication rate between the master drive and the slave drives is in the range of 100-1000 hertz. The master controller is required to have knowledge of the master drive so that a slave drive can be configured as the new master drive in case the original master drive needs to be removed from the system. 
     For proper operation, each variable frequency drive is required to know its position in the system (either master or slave) and is required to operate differently based on its position. As the master drive performs more tasks than the slave drives, the control of the master drive is more complex than the control of the slave drives. Although the master-slave configuration works well for a number of different applications, the master-slave configuration tends to be relatively complex and relatively expensive to implement. In addition, configuring such arrangements for use with series-cell based drives generally requires a relatively large communication overhead. 
     SUMMARY 
     In one general respect, this application discloses a method for parallel control of series-cell based drives. According to various embodiments, the method comprises detecting a failed cell in one of the drives, inhibiting operation of the drive with the failed cell, determining a peak output voltage capability of the drive with the failed cell, communicating the peak output voltage capability to a master controller, and limiting a maximum voltage that each drive can apply to a motor. The method also comprises bypassing the failed cell and resuming operation of the drive with the failed cell after the motor voltage falls below the peak output voltage capability of the drive with the failed cell. 
     According to other embodiments, the method comprises receiving a peak output voltage capability from each of the drives, comparing the peak output voltage capabilities to determine which is lowest, and communicating the lowest peak output voltage capability to each of the drives. 
     Aspects of the invention may be implemented by a computing device and/or a computer program stored on a computer-readable medium. The computer-readable medium may comprise a disk, a device, and/or a propagated signal. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       Various embodiments of the invention are described herein by way of example in conjunction with the following figures. 
         FIG. 1  illustrates various embodiments of a system for parallel control of variable frequency drives; 
         FIG. 2  illustrates various embodiments of a power circuit of one of the variable frequency drives of the system of  FIG. 1 ; 
         FIG. 3  illustrates various embodiments of a controller of one of the variable frequency drives of the system of  FIG. 1 ; 
         FIG. 4  illustrates various embodiments of a speed droop module of the controller of  FIG. 3 ; 
         FIG. 5  illustrates various embodiments of a flux share module of the controller of  FIG. 3 ; and 
         FIG. 6  illustrates various embodiments of a method for parallel control of series-cell based drives. 
     
    
    
     DETAILED DESCRIPTION 
     It is to be understood that at least some of the figures and descriptions of the invention have been simplified to focus on elements that are relevant for a clear understanding of the invention, while eliminating, for purposes of clarity, other elements that those of ordinary skill in the art will appreciate may also comprise a portion of the invention. However, because such elements are well known in the art, and because they do not necessarily facilitate a better understanding of the invention, a description of such elements is not provided herein. 
       FIG. 1  illustrates various embodiments of a system  10  for parallel control of variable frequency drives. The system  10  comprises a master controller  12 , and independent variable frequency drives  14  in communication with the master controller  12 . Although only two variable frequency drives  14  are shown in  FIG. 1 , one skilled in the art will appreciate that the system  10  may comprise any number of variable frequency drives  14 . According to various embodiments, the variable frequency drives  14  may be series-cell based drives. An example of series-cell based drives is disclosed in U.S. Pat. No. 5,625,545, the contents of which are hereby incorporated by reference in their entirety. 
     The master controller  12  may be embodied as one or more programmable logic controllers, and may serve as the interface between other controls (e.g., customer controls) and the variable frequency drives  14 . The master controller  12  is structured and arranged to communicate with the variable frequency drives  14  at a data rate that is substantially less than any computation rate within the variable frequency drives  14 . As described in more detail hereinbelow, satisfactory current sharing between the variable frequency drives  14  may be realized with a data communication rate on the order of approximately 1-10 hertz, which is substantially less than any computation rate within the variable frequency drives  14 . The master controller  12  may send commands such as speed and flux demands and start or stop requests to each of the variable frequency drives  14 . Responsive thereto, each variable frequency drive  14  may send status information (e.g., healthy or faulted) and feedback information (e.g., output current, voltage and frequency) to the master controller  12 . 
     Each variable frequency drive  14  comprises a controller  16  in communication with the master controller  12 , and a power circuit  18  in communication with the controller  16 . Each power circuit  18  is coupled to a winding of a motor  20 . According to various embodiments, each power circuit  18  may be coupled to a different winding of a multi-winding alternating current motor, to the same winding of a single winding alternating current motor, or any combination thereof. One skilled in the art will appreciate that the system  10  may be utilized with any type of alternating current motor such as a synchronous motor, an induction motor, or a permanent magnet motor. For synchronous motor applications, each variable frequency drive  14  may also include a field supply that is enabled/disabled by the controller  16 , and each field supply may be connected to an exciter for a motor field circuit as is known in the art. As shown in  FIG. 1 , for each variable frequency drive  14 , a voltage output by the associated power circuit  18  is fed back to the associated controller  16  and a current output by the associated power circuit  18  is also fed back to the associated controller  16 . 
     The variable frequency drives  14  do not communicate directly with each other. Each variable frequency drive  14  is structured and arranged to communicate with the master controller  12  at a data rate that is substantially less than any computation rate within the variable frequency drives  14  (e.g., approximately one to ten hertz). The relatively low data rate allows for the system  10  to be simpler and less expensive than systems utilizing master/slave arrangements. The system  10  may be utilized with any type of alternating current motor 
       FIG. 2  illustrates various embodiments of a power circuit  18  of the system  10 . The power circuit  18  is representative of the power circuits  18  of each of the variable frequency drives  14 . The power circuit  18  comprises a rectifier  22  and an inverter  24  connected to the rectifier  22 . The rectifier  22  is connected to the controller  16 , and the inverter  24  is connected to a winding of the motor  20 . According to various embodiments, the inverter  24  may comprise any number of switching devices such as, for example, insulated gate bipolar transistors, integrated gate commutating thyristors, MOSFETs, etc. 
       FIG. 3  illustrates various embodiments of a controller  16  of the system  10 . The controller  16  is representative of the controllers  16  of each of the variable frequency drives  14 . The controller  16  comprises a speed droop module  26  and a flux share module  28 . The controller  16  also comprises a DQ-3Φ transform  30 , a pulse width modulator  32 , and a motor model  34 . According to various embodiments, the motor model  34  is structured and arranged to determine a motor speed estimate and a motor flux estimate based on stator voltage and stator current measurements. The motor model  34  may also provide values for a magnetizing current component, a torque current component, and motor flux angle. 
       FIG. 4  illustrates various embodiments of a speed droop module  26  of the system  10 . The speed droop module  26  is representative of the speed droop modules  26  of each of the variable frequency drives  14 . The speed droop module  26  is structured and arranged to regulate a torque current component in the associated variable frequency drive  14 . The speed droop module  28  comprises a speed ramp  36 , a speed droop gain  38 , a speed regulator  40 , and a torque current regulator  42 . In operation, the speed ramp  36  receives a speed demand from the master controller  12 , and linearizes any change in the speed demand to a rate that is adjustable by a user. The speed droop gain  38  scales a torque current component value, and the scaled torque current component value is subtracted from the output of the speed ramp  36  to form the speed reference for the speed regulator  40 . Thus, the speed droop module  26  is also structured and arranged to determine a speed reference based on the torque current component value. The introduction of torque current into the speed reference promotes torque current sharing by the variable frequency drives  14 . 
     According to other embodiments, the speed droop module  26  may be structured and arranged such that the speed droop gain  38  scales the torque current reference instead of the torque current component value. For such embodiments, the speed reference for the speed regulator  40  is formed by subtracting the scaled torque current reference from the output of the speed ramp  36 . 
     When one of the variable frequency drives  14  is delivering more torque current than the other variable frequency drives  14 , the speed droop module  26  operates to subtract a larger torque current component value from the output of the speed ramp  36 . This action leads to the variable frequency drive  14  reducing its output frequency for a period of time until the phase shift between its output voltage and the back-emf of the motor  20  is reduced to a desired value. As there is a direct relationship between torque current and the phase angle between the output voltage of the variable frequency drive  14  and the back-emf of the motor  20 , the reduced phase angle results in less torque current being delivered by the variable frequency drive  14 . The other variable frequency drives  14 , which have a smaller torque current component value being subtracted from the respective outputs of their speed ramps  36 , are able to increase their output frequency, thereby allowing them to deliver more torque current such that all of the variable frequency drives  14  are delivering substantially the same amount of torque current. 
       FIG. 5  illustrates various embodiments of a flux share module  28  of the system  10 . The flux share module  28  is representative of the flux share modules  28  of each of the variable frequency drives  14 . The flux share module  28  is structured and arranged to regulate a magnetizing current component in the associated variable frequency drive  14 . The flux share module  28  comprises a flux ramp  44 , a flux droop gain  46 , a flux regulator  48 , and a magnetizing current regulator  50 . In operation, the flux ramp  44  receives a flux demand from the master controller  12 , and linearizes any change in the flux demand to a rate that is adjustable by a user. The flux droop gain  46  scales an error between an average magnetizing current component value provided by the master controller  12  and an output of the flux regulator  48 . The average magnetizing current component value is determined by the master controller  12  based on the magnetizing current component value reported by each of the variable frequency drives  14 . The output of the flux regulator  48  is the magnetizing current reference. The scaled error is added to the output of the flux ramp  44  to form the flux reference for the flux regulator  48 . Thus, the flux share module  28  is also structured and arranged to determine a flux reference based on the magnetizing current component value. The introduction of magnetizing current error into the flux reference serves to have the variable frequency drives  14  operate with a minimum error on the magnetizing current components, thereby promoting magnetizing current sharing by the variable frequency drives  14 . 
     When one of the variable frequency drives  14  is delivering a magnetizing current that is greater than the average magnetizing current component value provided by the master controller  12 , the flux share module  28  operates to add the scaled error to the output of the flux ramp  44 , effectively reducing the value of the flux reference. The reduction in the flux reference causes the flux regulator  48  to lower the magnetizing current reference, which in turn decreases the magnetizing current due to the action of the magnetizing current regulator  50 . When one of the variable frequency drives  14  is delivering a magnetizing current that is less than the average magnetizing current component value provided by the master controller  12 , the opposite occurs. Thus, all of the variable frequency drives  14  may be controlled to deliver substantially the same amount of magnetizing current. 
     According to other embodiments, the flux share module  28  may be structured and arranged such that the flux droop gain  46  scales an error between an average magnetizing current reference provided by the master controller  12  and an output of the flux regulator  48 . The average magnetizing current reference may be determined by the master controller  12  based on the respective magnetizing current reference reported by each of the variable frequency drives  14 . For such embodiments, the flux reference for the flux regulator  48  is formed by adding the scaled error to the output of the flux ramp  44 . 
     According to yet other embodiments, the flux share module  28  may be structured and arranged such that the output of the flux regulator  48  is utilized as the reference for the field supply current when the motor  20  is a synchronous motor. An error between the average field supply current and the actual field supply current may be utilized to effect a change in the flux reference. For such embodiments, the magnetizing current reference becomes a fixed value that is used in all of the variable frequency drives  14 . The fixed value may be, for example, zero, or a value which is equal to the output of a proportional regulator portion of the flux regulator  48 . 
     According to various embodiments, each of the variable frequency drives  14  may communicate certain information to the master controller  12 . For example, each variable frequency drive  14  may communicate a magnetizing current component value, a peak output voltage capability, a flux reference, and a status bit. The status bit may indicate whether or not a certain operation mode (e.g., an energy saver mode) of the variable frequency drive  14  is enabled or disabled. Responsive to the information communicated by the variable frequency drives  14 , the master controller  12  may determine an average magnetizing current component value based on the magnetizing current component values communicated by the respective variable frequency drives  14 , and may communicate the average magnetizing current component value to each of the variable speed drives  14 . The master controller  12  may also compare the peak output voltage capabilities communicated by the respective variable frequency drives  14 , determine which peak output voltage capability is the lowest, and communicate the lowest peak output voltage capability to each of the variable frequency drives  14 . The master controller  12  may further identify which of the flux references communicated by the respective variable frequency drives  14  is associated with the variable frequency drive  14  having the lowest peak output voltage capability, and may determine whether the status bit of the variable frequency drive  14  having the lowest peak output voltage capability indicates that the operation mode (e.g., energy saver mode) of the variable frequency drive  14  is enabled or disabled. According to various embodiments, the master controller  12  may communicate the flux reference associated with the variable frequency drive  14  having the lowest peak output voltage capability to each of the variable frequency drives  14  when the status bit of the variable frequency drive  14  having the lowest peak output voltage capability indicates that the operation mode (e.g., energy saver mode) is enabled. The above-described communications between the master controller  12  and the variable frequency drives  14  may occur at a data rate that is substantially less than any computation rate within the variable frequency drives  14  (e.g., approximately one to ten hertz). 
       FIG. 6  illustrates various embodiments of a method  60  for parallel control of series-cell based drives  14 . The system  10  may be utilized to implement the method  60 . With series-cell based drives  14 , each drive  14  comprises a plurality of power cells that are connected in series in each phase output line of the drive  14 . For a variety of reasons, a failure may occur in one of the individual cells. It is known for a drive  14  with a failed cell to bypass the failed cell and continue to operate at a reduced peak output voltage capability. An example of cell bypass is disclosed in U.S. Pat. No. 5,986,909, the contents of which are hereby incorporated by reference in their entirety. The method  60  allows one or more of the series-cell based drives  14  to operate with failed cells in bypass. The method  60  also allows one or more of the series-cell based drives  14  to be removed from or introduced into the system  10  without any power interruption to the motor  20 . 
     The process flow of the method  60  begins at block  62 , where a failed cell is detected in one of the series-cell based drives  14 . The failure may be detected in any number of ways. For example, the failure may be detected based on a comparison between a cell output voltage and a command output, on a comparison between a cell internal voltage and a fixed value, by checking or verifying various cell components, through the use of diagnostics, etc. 
     From block  62 , the process advances to block  64 , where the controller  16  inhibits the normal operation of the drive  14  with the failed cell. As used herein, normal operation refers to how the drive  14  operates when no cells have failed. The controller  16  may inhibit the normal operation by inhibiting gating signals to the switching devices in the inverter  24  of the drive  14  with the failed cell. By doing so, the switching devices of the drive  14  stop switching and no voltage is applied to the motor  20  by the drive  14 . 
     From block  64 , the process advances to block  66 , where the peak output voltage capability of the drive  14  with the failed cell is determined. The peak output voltage capability of the drive  14  after the cell failure will be lower than the peak output voltage capability of the drive  14  prior to the cell failure. From block  66 , the process advances to block  68 , where the peak output voltage capability of the drive  14 , which is reduced after the cell failure, is communicated to the master controller  12 . 
     From block  68 , the process advances to block  70 , where the peak output voltage capability of the drive  14  with the failed cell is utilized to limit the maximum voltage that each series-cell based drive  14  can apply to the motor  20 . As the current regulators in the controller  16  regulate the D-axis voltage reference and the Q-axis voltage reference, the peak output voltage capability of the drive  14  with the failed cell may be utilized to limit the value of the voltage commands in the other drives  14 . As a result, the voltage applied to the motor  20  by each of the other drives  14  is reduced, and the motor voltage begins to decrease. 
     After the motor voltage decreases to a value that is less than or equal to the peak output voltage capability of the drive  14  with the failed cell, the process advances from block  70  to block  72 , where the drive  14  with the failed cell bypasses the failed cell and resumes its normal operation. The drives  14  without failed cells operate at a reduced voltage, without having to bypass any functional cells, to effectively match the voltage output by the drive  14  with the bypassed cell. As long as the status bit indicates that the operation mode is enabled, each drive  14  may utilize the flux reference of the drive  14  with the lowest peak output voltage capability. Such operation prevents the drive  14  with the bypassed cell from accepting power from the motor  20  because of the neutral-voltage shift that is applied to create a balanced three-phase voltage as described in U.S. Pat. No. 5,986,909. When time permits, the drive  14  with the bypassed cell may be shut down, and the failed cell may be replaced. The drive  14  will then communicate an increased peak output voltage capability, and the system  10  can operate as it did prior to the cell failure. For some applications, the failed cell may be replaced while the other drives  14  are still applying voltage to the motor  20 . According to various embodiments, an output contactor may be utilized between each drive output and the motor winding to disconnect a drive  14  from the motor  20  for such a purpose. 
     While several embodiments of the invention have been described herein by way of example, those skilled in the art will appreciate that various modifications, alterations, and adaptions to the described embodiments may be realized without departing from the spirit and scope of the invention defined by the appended claims. For example, although the method  60  is described with reference to a particular sequence, those skilled in the art will appreciate that some steps may occur in a different sequence, some steps may occur concurrently with other steps, and some steps may occur on a periodic or continuous basis.