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Vector Control of Induction Machines

Vector Control of Induction Machines. Introduction. The traditional way to control the speed of induction motors is the V/Hz-control Low dynamic performance In applications like servo drives and rolling mills quick torque response is required. Desire to replace dc drives led to vector control

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Vector Control of Induction Machines

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  1. Vector Control of Induction Machines

  2. Introduction • The traditional way to control the speed of induction motors is the V/Hz-control • Low dynamic performance • In applications like servo drives and rolling mills quick torque response is required. • Desire to replace dc drives led to vector control • Braunschweig, Leonhard, Blaschke, Hasse,late 70-ies

  3. What is vector control? • Vector control implies that an ac motor is forced to behave dynamically as a dc motor by the use of feedback control. • Always consider the stator frequency to be a variable quantity. • Think in synchronous coordinates.

  4. Basic blocks of a vector controlled drive

  5. Addition of a block for calculation of the transformation angle

  6. The current is controlled in the d- and q-directions magnetization torque production

  7. Vector controller

  8. Stator and rotor of an induction machine

  9. Magnetization current from the stator

  10. The flux

  11. The rotation

  12. View from the rotor

  13. Induced voltage and current

  14. Torque production

  15. Ampere-turn balance

  16. Rotor flux orientation • Difficult to find the transformation angle since the direction of the flux must be known • Flux measurement is required • Flux sensors (and fitting) are expensive and unreliable • Rotor position measurement does not tell the flux position • The solution is flux estimation

  17. Rotor flux orientation using measured flux • Original method suggested by Blaschke • Requires flux sensors • Flux coordinates: aligned with the rotor flux linkage

  18. Rotor flux orientation

  19. From Chapter 4

  20. Transformation to flux coordinates

  21. The flux coordinate system is ”synchronous” only at steady-state. During transients the speed of the rotor flux and the stator voltage may differ considerably.

  22. The rotor equation (5.9)

  23. Split into real and imaginary parts

  24. Rotor flux dynamics are slow

  25. Torque control

  26. Rotor flux orientation using estimated flux • The rotor flux vector cannot be measured, only the airgap flux. • Flux sensors reduce the reliability • Flux sensors increase the cost • Therefore, it is better to estimate the rotor flux.

  27. The "current model" in the stator reference frame(Direct Field Orientation)

  28. The current model

  29. The "current model" in synchronous coordinates (Indirect Field Orientation)

  30. Transformation angle

  31. Remarks on indirect field orientation • Does not directly involve flux estimation (superscript f dropped) • Not ”flux coordinates” but ”synchronous coordinates” • Since the slip relation is used instead of flux estimation, the method is called indirect field orientation

  32. Indirect field orientation based on the current model

  33. Feedforward rotor flux orientation • Significantly reduced noise in the transformation angle • Fast current control is assumed (ref.value=measured value) • No state feedback => completely linear

  34. The voltage model • The current model needs accurate values of the rotor time constant and rotor speed • The trend is to remove sensors for cost and reliability reasons • Simulate the stator voltage equation instead of the rotor voltage equation

  35. Solve for the rotor current and insert in

  36. Multiplication by yields Solve for

  37. Direct field orientation using the voltage model

  38. Stator flux orientation "Direct self-control" (DSC) schemes first suggested by Depenbrock, Takahashi, and Noguchi in the 1980s. At low frequencies the current model can be used together with:

  39. Field weakening

  40. Current control

  41. Transfer function and block diagram of a three-phase load

  42. Review of methods for current control • Hysteresis control • Stator frame PI control • Synchronous frame PI control

  43. Hysteresis control(Tolerance band control) • Measure each line current and subtract from the reference. The result is fed to a comparator with hysteresis. • Pulse width modulation is achieved directly by the current control • The switching frequency is chosen by means of the width of the tolerance band. • No tuning is required. • Very quick response

  44. Drawbacks of hysteresis control • The switching frequency is not constant. • The actual tolerance band is twice the chosen one. • Sometimes a series of fast switchings occur. • Suitable for analog implementation. Digital implementation requires a very high sampling frequency.

  45. Stator frame PI control • Two controllers: one for the real axis and one for the imaginary axis • Cannot achieve zero steady-state error • Tracking a sinusoid means that steady-state is never reached in a true sense • Integral action is useless except at zero frequency

  46. Synchronous frame PI control • In a synchronous reference frame the current is a dc quantity at steady-state. • Zero steay state error is possible. • Coordinate transformations necessary • Easily implemented on a DSP • Usually the best choice!

  47. Design of synchronous frame PI controllers Remove cross-coupling

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