Industrial Drives Dated: 23/04/2015

Presentation on Vector Control of AC Induction Motors

By-

Vaibhav Goyal

677/MP/11

MPAE 2

Glimpse of Induction Motor3 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 sluggish response and is prone to instability because of 5th order harmonics. Vector control decouples these effects.

Vector control (or field oriented control) offers more precise control of ac motors compared to scalar control. They are therefore used in high performance drives where oscillations in air 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.

WorkingCircuit Diagram

Torque Speed Characteristics

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 Is is the sum of the Ids and Iqs vectors. Thus, the stator current magnitude is

related to ids and iqs by:

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:

The rotor flux vector is aligned with the d axis 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.