Vector Control of AC Induction Motors

1. Vector Control of Induction Motors Pranjal Barman Research Scholar Department of Electronics and Communication Engineering Tezpur University

2. Two control approaches of AC drives Scalar Control : Scalar control is the term used to describe a simpler form of AC motor control. Controlled by the adjustable magnitude of stator voltages and frequency in such a way that the air gap flux is always maintained at the desired value at the steady-state Vector Control : The machine current and voltage space vectors, the transformation of a 3 phase speed and time dependent system into a two co-ordinate time invariant system and effective PWM pattern generation

3. Scalar Control vs Vector Control Scalar Simpler form of motor control Good steady state performance P oor dynamic response Low performance drives Higher power dissipation Vector Complex mathematical model P recise control of ac motors Excellent dynamic response High performance drives Low power dissipation

4. Analogy with DC motor control There is a close parallel between torque control of a DC motor and vector control of an AC motor. The DC motor field flux produced by field current is orthogonal to the armature flux produced by the armature current . Because the vectors are orthogonal, they are decoupled, i.e. the field current only controls the field flux and the armature current only controls the armature flux.

5. Analogy with DC motor control DC motor-like performance can be achieved with an induction motor if the motor control is considered in the synchronously rotating reference frame (d e - q e ) where the sinusoidal variables appear as dc quantities in steady state. With vector control: i ds (induction motor)  I f (dc motor) i qs (induction motor)  I a (dc motor)

6. Principles of Vector Control The basic conceptual implementation of vector control is illustrated in the below block diagram:

7. Principles of Vector Control The motor phase currents, i a , i b and i c are converted to i ds s and i qs s in the stationary reference frame. These are then converted to the synchronously rotating reference frame d-q currents, i ds and i qs . In the controller two inverse transforms are performed: 1) From the synchronous d-q to the stationary d-q reference frame; 2) From d * -q * to a * , b * , c * .

8. Time invariant coordinate transform ( a,b,c )⇒( α,β ) (the Clarke transformation) which outputs a two co-ordinate time variant system ( α,β )⇒( d,q ) (the Park transformation) which outputs a two co-ordinate time invariant system

9. Clarke transformation

10. Park transformation

11. I nverse Park transformation Voltage transformation that modifies the voltages in d,q rotating reference frame in a two phase orthogonal system

12. Vector control types There are two approaches to vector control: 1) Direct field oriented current control - here the rotation angle of the i qs e vector with respect to the stator flux is being directly determined (e.g. by measuring air gap flux) 2) Indirect field oriented current control - here the rotor angle is being measured indirectly, such as by measuring slip speed

13. Field Orientation Control In direct FOC the field angle is calculated by using terminal voltages and current or Hall sensors or flux sense windings .

14. Salient Features of Vector Control Transient response will be fast because torque control by i qs does not affect flux. Vector control allows for speed control in all four quadrants (without additional control elements A utomatically limits operation to the stable region.

15. Space Vector PWM Inverter switches are driven with two complementary pulsed signals, providing care is taken to ensure that there is no overlap in the power switch transitions. SVPWM is a technique for generating such pulsed signals M inimizes the harmonic contents

16. These eight switch combinations determine eight phase voltage configurations The vectors divide the plan into six sectors The binary representations of two adjacent basic vectors differ in only one bit SVPWM, vectors and sectors

17. Conclusion Vector control/FOC used in high performance drives where oscillations in air gap flux linkages are intolerable, e.g. robotic actuators, centrifuges, servos, etc. Field Orientated Controlled AC machines thus obtain every DC machine advantage Since high computational power silicon devices, came to market it has been possible to realize far more precise digital vector control algorithms More computational effort and high speed processors are suitable

18. References “Field Orientated Control of 3-Phase AC-Motors”; Literature Number: BPRA073,Texas Instruments Europe, February 1998 “Comparison of scalar and vector control strategies of Induction Motors”; G.Kohlrusz , D.Fodor “Scalar (V/f) Control of 3-Phase Induction Motors”; Application Report SPRABQ8–July 2013

19. Thank You