Vector control is a more advanced and precise method of controlling AC induction motors compared to scalar control. It involves transforming the motor currents and voltages into a rotating reference frame to obtain decoupled control similar to a DC motor. This allows for independent control of flux and torque for faster dynamic response and better performance than scalar control. The basic implementation of vector control uses Clarke and Park transformations to convert between stationary and rotating reference frames in the controller. It provides DC motor-like precision in speed and torque control of 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
 Poor dynamic response
 Low performance drives
 Higher power
dissipation
Vector
 Complex mathematical
model
 Precise 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 (de-qe) where the sinusoidal
variables appear as dc quantities in steady
state.
With vector control:
ids (induction motor)  If (dc motor)
iqs (induction motor)  Ia (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, ia, ib and ic are
converted to ids
s and iqs
s in the stationary
reference frame.
These are then converted to the synchronously
rotating reference frame d-q currents, ids and iqs.
 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. Inverse 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 iqs
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 iqs does not affect flux.
Vector control allows for speed control in all
four quadrants (without additional control
elements
Automatically 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
Minimizes 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
1. “Field Orientated Control of 3-Phase AC-Motors”;
Literature Number: BPRA073,Texas Instruments
Europe, February 1998
2. “Comparison of scalar and vector control strategies
of Induction Motors”; G.Kohlrusz, D.Fodor
3. “Scalar (V/f) Control of 3-Phase Induction Motors”;
Application Report SPRABQ8–July 2013

19. Thank You