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  • Three phase current measurements using a single line resistor

    on the TMS320F240

    Literature Number: BPRA077 Texas Instruments Europe

    May 1998

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  • Contents

    Three phase current measurements using a single line resistor on the TMS320F240 iii

    Contents

    1. Introduction.............................................................................................................. 1

    2. Measurement Process............................................................................................. 1 2.1 Measurement process .................................................................................. 2 2.2 Hardware considerations .............................................................................. 4 2.3 Hardware limitations ..................................................................................... 5 2.4 Solution to circumvent the hardware limitations............................................ 5 2.5 Enhanced algorithm...................................................................................... 8

    3. Hardware sensor ................................................................................................... 10 3.1 Schematic .............................................................................................. 10 3.2 Cost comparison......................................................................................... 11

    3.2.1 Three shunts on each inverter leg ............................................. 11 3.2.2 Isolated phase sensors.............................................................. 12

    4. Software Implementation ....................................................................................... 12 4.1 Implementation of the first solution ............................................................. 12 4.2 Implementation of the advanced method.................................................... 16

    5. Other solutions found in the literature .................................................................... 18 5.1 Principle ...................................................................................................... 18 5.2 Performance comparison............................................................................ 19

    6. Results................................................................................................................... 20 6.1 Hardware configuration............................................................................... 20 6.2 First method................................................................................................ 20 6.3 Second method tested................................................................................ 22 6.4 Closed loop control with the second method .............................................. 23 6.5 Speed limitation .......................................................................................... 24

    7. Conclusion............................................................................................................. 26

    References ................................................................................................................ 27

    Appendix A: Linker command file .............................................................................. 28

    Appendix B: User interface Quick Basic program...................................................... 29

    Appendix C: Software program describing the first method....................................... 32

    Appendix D: Software program describing the second method ................................. 49

  • Contents

    Three phase current measurements using a single line resistor on the TMS320F240 iv

    List of Figures

    Figure 1: Schematic of a system including power, control and load.................................. 1

    Figure 2: Schematic diagram of the inverter module......................................................... 2

    Figure 3: Inverter supplying a net of three star windings................................................... 3

    Figure 4: Signals controlling the upper transistors ............................................................ 4

    Figure 5: Plot1,2,3: PWM signals - Plot4: Idc current........................................................ 5

    Figure 6: 3 Symmetrical PWMs with & without addition of weight..................................... 6

    Figure 7: Line current, only one current is detectable ....................................................... 7

    Figure 8: 2 currents detectable on the DC line.................................................................. 8

    Figure 9: Phase current detectable when needed............................................................. 9

    Figure 10: Three bridges inverter with Idc measurement ................................................ 10

    Figure 11: Basic schematic for Idc current measurement ............................................... 11

    Figure 12: Current measurement using three shunts...................................................... 12

    Figure 13: PWM generation related to the time in the case of a symmetric PWM .......... 13

    Figure 14: V/f control and Idc current measurement block diagram................................ 16

    Figure 15: Synchronisation diagram of the control.......................................................... 16

    Figure 16: Flowchart of the Period interrupt: PR_int ....................................................... 17

    Figure 17: Meas_pattern function flowchart .................................................................... 18

    Figure 18: 11 % sensed an built current comparison ...................................................... 21

    Figure 19: 15 % sensed an built current comparison ...................................................... 22

    Figure 20: The perturbation of the measurement process decreased by five ................. 23

    Figure 21: FOC with shunt measurement ....................................................................... 24

    Figure 22: FOC with shunt measurement at high speed................................................. 25

    Figure 23: FOC with shunt measurement at very low speed........................................... 26

  • BPRA077

    Three phase current measurements using a single line resistor on the TMS320F240 1

    Three phase current measurements using a single line resistor

    on the TMS320F240 ABSTRACT

    Most inverter control systems require a knowledge of the phase currents. The simplest method of obtaining these currents is to measure them directly. Depending on the motor winding connections, this requires at least two sensors to be applied directly to the motor phases. Usually, these types of sensors are expensive due to their need to be isolated. There is a second method of measuring these phase currents using a simple, cheap resistor. However, under certain conditions, the measurement becomes difficult and even impossible due to hardware limitations. In this paper, a solution is described for circumventing this problem and results are given following its implementation on a Digital Signal Processor, the TMS320F240 from Texas Instruments.

    1. Introduction Most three phase motor control algorithms require that the motor phase currents be known in order to deliver high motor performance. The following system recombines the three phase currents of the motor using only one simpl

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