Abstract:
A method of controlling a power converter ( 20 ) of a motor drive system ( 10 )controls the power converter ( 20 ) during a first operating mode by applying a current control scheme, which sets power converter commands to control torque current flowing from the power converter ( 20 ) to the motor ( 30 ) to achieve desired motor speed; and initiates a second operating mode when power supply to the power converter ( 20 ) is interrupted. The second operating mode includes controlling negative torque current between the power converter ( 20 ) and the motor ( 30 ) so that mechanical energy from the motor ( 30 ) charges an element ( 58 ) on a power supply side of the power converter ( 20 ). The first operating mode is resumed when the input power recovers. Torque current between the power converter ( 20 ) and the motor ( 30 ) is also controlled to limit a maximum transient DC bus voltage.

Description:
RELATED APPLICATION 
   This application claims priority under 35 U.S.C. § 119(e) of Provisional Application No. 60/611,298 filed Sep. 21, 2004, the entire contents of which are herein incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention relates to electrical power conversion, and more particularly to a power converter controlling apparatus and method proving ride through capability during power interruption in a motor drive system. 
   BACKGROUND OF THE INVENTION 
   An exemplary motor drive system includes the following main components: a motor, such as a three-phase permanent magnet synchronous motor (PMSM); a multi-phase power inverter; a DC power supply; and a current controller, which generates gating signals for output to the power inverter. The power inverter converts DC power from power supply into multi-phase AC power, e.g., utilizing a configuration of insulated-gate bipolar transistors (IGBTs), as a function of rotor position/speed. Rotor position/speed can be monitored using sensors or derived using sensorless techniques. The current controller controls the power inverter, e.g., using pulse width modulation (PWM) control, so that the power converter outputs the desired multi-phase AC power to the motor. Thus, during operation of the motor, the power converter converts DC power from the DC power supply into multi-phase AC power and supplies such multi-phase AC power to the motor, to create motor torque. 
   Variable speed motor drive systems are increasingly used in aerospace applications. In those applications, the size of DC energy storage components is typically minimized to achieve high power density, and reliable operation during DC power interruptions is a key requirement. Most current implementations respond to power interruptions by disabling inverter gating and opening all contactors between the inverter and the motor to leave the motor in a free deceleration mode. In a speed sensorless system, system operation is typically necessary to derive motor position/speed information. Because such systems will lose rotor position information once gating is disabled and the contactors are opened, it is difficult to achieve resynchronization after a power interruption. After power is resumed, the system must go through soft start and resynchronization when speed sensorless techniques are used, before resuming normal operation. Even with a speed sensor, soft start is still required. Such soft start and resynchronization procedures cause delays and non-smoothness, which is particularly undesirable for aerospace applications. 
   SUMMARY OF THE INVENTION 
   In one respect, the present invention is a method of controlling a power converter of a motor drive system, the method comprising: controlling the power converter during a first operating mode by applying a current control scheme, which sets power converter commands to control torque current flowing from the power converter to the motor to achieve desired motor speed; and initiating a second operating mode when power supply to the power converter is interrupted, wherein the second operating mode includes controlling a negative torque current between the power converter and the motor so that mechanical energy from the motor charges an element on a power supply side of the power converter. The method further comprises controlling the torque current to limit a maximum transient DC bus voltage. 
   In another respect, the present invention is a power converter controlling apparatus for controlling a power converter of a motor drive system, the controlling apparatus comprising: a current controller, which outputs gating signals to the power converter as a function of a torque current reference and a flux current reference; a torque current reference generator, which generates the torque current reference used by the current controller; and a flux current reference generator, which generates flux current reference used by the current controller, wherein the current controller controls the power converter during a first operating mode to create torque current flowing from the power converter to the motor to achieve desired motor speed; and controls the power converter in a second operating mode, initiated when power to the power converter is interrupted, so that a negative torque current between the power converter and the motor draws mechanical energy from the motor to charge an element on a power supply side of the power converter. The torque current reference generator includes a DC bus voltage regulator to limit maximum transient DC bus voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of an exemplary motor drive system to which principles of the present invention may be applied to provide ride through capability during a power supply interruption in accordance with an embodiment of the present invention; 
       FIG. 2  is a functional block diagram illustrating functional components of a motor torque current reference generator in accordance with an embodiment of the present invention; 
       FIG. 3  illustrates a generator mode controller for regulating torque current during a power interruption in accordance with an embodiment of the present invention; 
       FIG. 4  illustrates a max DC link voltage regulator, which operates in conjunction with both the motor mode controller and the generator mode controller to limit the maximum transient DC bus voltage when transitioning between motor mode and generator mode control in accordance with an embodiment of the present invention; 
       FIG. 5  is a flow diagram illustrating a torque current control technique for providing ride through capability during power supply interruption in accordance with an embodiment of the present invention; and 
       FIGS. 6A–6C  are signal waveforms illustrating test results of toque current control during power supply interruption consistent with principles of the present invention. 
   

   DETAILED DESCRIPTION 
   Embodiments of the present invention are more specifically set forth in the following description, with reference to the appended drawings. In the following description and accompanying drawings like elements are denoted with similar reference numbers. Further, well-known elements and related explanations are omitted so as not to obscure the inventive concepts presented herein. 
   In one general aspect of the present invention, a voltage source inverter (VSI) based motor drive system selectively initiates generator mode control, during power interruption, to transition the motor from a motor mode to a generator mode. In one embodiment, during generator mode, the mechanical energy on the motor shaft is used to boost up and maintain a DC link capacitor voltage at a certain level, which is slightly higher than the normal operation DC link voltage. This is done to limit the inrush current when the input power recovers. Only a small amount of energy is required to compensate for the inverter power losses. The motor will decelerate slowly until power supply is recovered. During this period, because the motor is still under control, speed information is still available and DC capacitor voltage is still high enough. After supply power recovery, the motor can be switched back to motor mode. This is achieved without the need for a soft start and re-synchronization process. This is particularly significant for typical speed sensorless motor drive systems, because the speed information will typically not be available if current control loop is disabled. During power interruption, if motor speed is lower than a certain speed limit, the system will shut down because there is not enough energy to support DC capacitor voltage at a certain level. Under this condition, the motor will need to be restarted after power recovery. 
     FIG. 1  is a block diagram of an exemplary motor drive system  10  to which principles of the present invention may be applied to provide ride through capability during a power supply interruption in accordance with an embodiment of the present invention. As shown in  FIG. 1 , the motor drive system includes: a power inverter  20 , a motor  30 ; a DC power supply  40 ; a DC bus  50 ; a blocking diode  56 ; a DC link capacitor  58 ; electrical contactors  60  providing electrical connection between the inverter  20  and the motor  30 ; a current controller  70 ; a torque current reference generator  100 ; and a flux current reference generator  200 . 
   The DC power supply  40  can be a DC generator, a diode rectifier, an active rectifier, etc., and may include soft start circuitry. The blocking diode  56  prevents power from feeding back to the DC power supply  40  or other systems supplied by the same DC power source. The DC link capacitor  58  is connected at the input side of the inverter  20 . The inverter  20  can be any type of voltage source inverter. The motor  30  can be any type of motor, such as a brushless synchronous motor. The current controller  70  can be any type of current controller that generates gating according to current reference. As is known in the art, a typical current controller controls torque current and flux current components flowing from the inverter to the motor based on direct axis (d-axis) and quadrature axis (q-axis) reference values (i.e., Id-ref for flux current control and Iq-ref for torque current control). Iq-ref is used to control motor torque, and also is referred to herein as Iq*. The flux current reference generator  200  can be any type of flux current generator. Id_ref is used to control motor flux, and also is referred to herein as Id*. 
     FIG. 2  is a functional block diagram illustrating functional  30  components of the torque current reference generator  100  in accordance with an embodiment of the present invention. As shown in  FIG. 2 , the torque current reference generator  100  includes: a power interruption detector  110 ; a generator mode controller  120 ; a speed estimator or detector  130 ; a motor mode controller  140 ; and a max DC bus voltage regulator  150 . The torque current reference generator  100  further includes weighting elements  165 ,  170  and combiner  160 . These illustrated functional elements combine to generate a torque current control reference (Iq*), which is output to the current controller  70  to control torque current between the inverter  20  and the motor  30  depending on the state of operation. The operation of these elements will be described in greater detail below. It should be recognized that the illustration of elements in  FIG. 2  is for ease of explanation, and that various physical configurations, e.g., using various combinations of hardware, software, logic circuitry, ASICs, etc., can be implemented to achieve these functions. The speed estimator or detector  130  can be a sensor or sensorless. 
     FIG. 3  illustrates a generator mode controller  120  in accordance with an embodiment of the present invention. As shown in  FIG. 3 , the generator mode controller  120  includes: a comparator  122 ; a proportional integrator (PI) or Led-Lag compensator  124 ; and a saturator  126 . In  FIG. 3 , V dc * is a reference voltage for the DC link capacitor  58 , which is slightly higher than the normal operation DC link voltage. The difference between the DC link voltage (V dc ) and V dc * is determined by comparator  122 , which outputs an error signal that is fed into the PI or Lead-Lag compensator  124 , which creates the current reference I 2 *. The saturator  126  is used to limit the I 2 * at range of negative torque current. In this way, under normal condition, when the DC link voltage is lower than V dc *, the output of the generator mode controller  120  will be negative. However, this negative value will not exert any impact on the control because the weighting element  170  is set to zero by the power interrupt detector  110 . 
     FIG. 4  illustrates an embodiment of the max DC bus voltage regulator  1   50 , which limits the maximum transient DC bus voltage when transitioning between motor mode and generator mode. The max DC bus voltage regulator  150  includes: a V ref  storage unit  154 ; a comparator  152  which compares V ref  with the V dc ; and a DC link voltage control element  156  (e.g., a PI or Lead lag Controller), which generates a current reference I 3 *. A saturator  158  is used to limit the I 3 * in the range of positive torque current. In this way, under normal condition when DC link voltage is lower than V ref , the output of the regulator  150  will be zero. As explained in greater detail below, I 3 * limits the maximum DC bus voltage during motor mode and generator mode transitions. 
   Operation of the torque current reference generator  100  will next be described, with reference to the flow diagram of  FIG. 5 . After the motor has started (e.g., using a soft start) and achieved synchronization (S 502 ), the torque current reference I 1 * generated by the motor mode controller  140  will be output as I q * so that current control is performed normally (S 504 ). When the power interruption detector  110  detects a power interruption (S 506 ), the output of generator mode controller  120  will be activated for I 2 *, which will cause a transition to generator mode current control (S 508 ). 
   When the power interruption detector  110  detects power interruption, K 1  in weighting element  165  will be set to 0 and K 2  in weighting element  170  will be set to 1. The current reference I* will come from generator mode controller  120  and max DC bus voltage regulator  150 , e.g., Iq*=I 2 *+I 3 . Negative torque current will be created to support DC capacitor voltage at V dc *. The drive system  10  operates at generator mode (S 510 ). When power supply is resumed (S 516 ), K 1  in weighting element  165  will be set to 1 and K 2  in weighting element  170  will be set to 0. The current reference I q * will come from motor mode controller  140  and max DC bus voltage regulator  150  (e.g., Iq*=I 1 *+I 3 *). Positive torque current will be created to spin the rotor. The drive system operates at motor mode. During power interruption, if it is determined that motor speed is lower than a certain limit (S 512 ), both K 1  and K 2  will be set to 0 and the system will be shut down (S 514 ). 
   The max DC bus voltage regulator  150  is added to limit the maximum DC bus voltage during the transition between two different operating modes. The DC link capacitor  58  with higher capacitance value will have better susceptibility in tolerating these transitions. But the high power density requirement for aerospace applications usually does not allow this luxury. The DC link voltage regulator  150  determines when the DC link voltage is detected higher than a predefined threshold, V ref  in element  154  (which is mainly application dependent with one condition, that is the Vref should be higher than Vdc* defined in the generator mode controller  120 ), and a positive torque current reference I 3 * will be commanded to release the extra energy in the DC link capacitor to the load. 
     FIGS. 6A–6C  are signal waveforms illustrating test results of torque current control, demonstrating ride through capability during power interruptions. 
   In  FIG. 6A , waveform (a) is the DC bus voltage at the power supply side of the blocking diode  56 ; waveform (b) is the DC bus voltage across the DC link capacitor  58 ; waveform (c) is the terminal voltage of motor  30 ; and waveform (d) is the output current of inverter  20 . As shown in waveform (b), the DC bus voltage across the DC link capacitor  58  is boosted and maintained at a certain level, which is slightly higher than the normal operation DC link voltage, during power interruption. 
     FIG. 6B  illustrates portions of waveforms (a)–(d) during a power interruption in greater detail (“zoomed-in”).  FIG. 6C  illustrates portions of waveforms (a)–(d) during power interruption in still greater detail. In  FIG. 6C , it can be seen that the motor current (waveform (d)) is in phase with motor voltage (waveform (c)) during motoring mode and out of phase with motor voltage during generator mode.