Vector control of AC induction motors provides superior dynamic performance compared to scalar control by decoupling torque and flux control. Vector control represents the stator currents as direct and quadrature components (Ids and Iqs) that independently control flux and torque. This allows precise and independent control of torque and flux to eliminate oscillations for applications requiring high performance like robotics. Vector control implementations determine the orientation of Ids and Iqs either directly by measuring airgap flux or indirectly by measuring slip speed to achieve independent control of the current components.

1. Industrial Drives Dated: 23/04/2015
Presentation on Vector Control of AC Induction Motors
By-
Vaibhav Goyal
677/MP/11
MPAE 2

2. Glimpse of InductionMotor
3 phase supply is given to the armature windings of the stator due to which rotating magnetic field
is developed. This flux cuts the rotor thereby inducing EMF and then current formation. Now a
current carrying conductor in a magnetic field experiences force due to which rotor starts rotating
and in the same direction as that of the rotating magnetic field.
Why does vector control provide superior dynamic performance of ac
motors compared to scalar control?
In scalar control there is an inherent coupling effect because both torque and flux are functions of
voltage or current and frequency. This results in sluggishresponseand is prone to instabilitybecause
of 5th order harmonics. Vector control decouples these effects.
Vector control (or fieldoriented control) offers more precisecontrol of ac motors compared to scalar
control. They are therefore used in high performance drives where oscillations inair gap flux linkages
are intolerable, e.g. robotic actuators, centrifuges, servos, etc. Moreover, Vector control technique
is used to vary the speed of Induction motor over a wide range.
Working
Circuit Diagram
Torque Speed Characteristics

3. Here we use two types of currents, namely Direct Current (Id) and Quadrature Current (Iq) responsible for
producing flux and torque respectively.
The stator current vector Isis the sum of the Ids and Iqs vectors. Thus, the stator current magnitude is
related to ids and iqs by:
2 2
s ds qsI i i 
Phasor Diagrams for Induction Motor
The steady state Phasor (or vector) diagrams for an induction motor in the d-q reference frame are
shown below:

4. The rotor flux vector is aligned with the daxis and the air gap voltage is aligned with the q axis.
The terminal voltage Vs slightly leads the air gap voltage because of the voltage drop across the
stator impedance. Iqs contributes real power across the air gap but Ids only contributes reactive
power across the air gap.
The first figure shows an increase in the torque component of current Iqs and the second figure
shows an increase in the flux component of current, Ids. Because of the orthogonal orientation of
these components, the torque and flux can be controlled independently. However, it is necessary
to maintain these vector orientations under all operating conditions.
How can we control the Iqs and Ids components of the stator current Is independently with the
desired orientation?
The basic conceptual implementation of vector control is illustrated in the below block diagram:
There are two approaches to vector control:
1) Direct field oriented current control
The rotation angle of the Iqs vector with respect to the stator flux is being directly determined (e.g.
by measuring air gap flux).
2) Indirect field oriented current control.
The rotor angle is being measured indirectly, such as by measuring slip speed.
Summary of Salient Features of Vector Control
1) Transient response will be fast because torque control by Iqs does not affect flux.
2) Vector control allows for speed control in all four quadrants (without additional control
elements) since negative torque is directly taken care of in vector control.