AN2388 datasheet(10/54 Pages) STMICROELECTRONICS

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Vector control of AC induction machines

AN2388

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space vector. In order to estimate the rotor flux vector is possible to use two different

strategies:

●

DFOC (Direct Field Oriented Control): rotor flux vector is either measured by means of

a flux sensor mounted in the air-gap or measured using the voltage equations starting

from the electrical machine parameters.

●

IFOC (Indirect Field Oriented Control): rotor flux vector is estimated using the field

oriented control equations (current model) requiring a rotor speed measurement.

The usual terminology “Sensorless” specifies that no position/speed feedback devices are

used.

With these algorithms, the stator currents of the induction machine are separated into flux

and torque producing components by utilizing transformation to the d-q coordinate system.

On this reference frame the torque component is on the q axis and the flux component is on

the d axis. The vector control system requires the dynamic model equations of the induction

motor and returns to the instantaneous currents and voltages in order to calculate and

control the variables.

The technique described in this application note is IFOC. Indirect vector control of the rotor

currents can be implemented using the following data:

●

Instantaneous stator phase currents, ia, ib, and ic

●

Rotor mechanical position

●

Rotor electrical time constant

The motor must be equipped with sensors to monitor the three-phase stator currents and a

rotor position feedback device. An encoder is normally mounted on the shaft rotor for this

purpose but in order to have a cheaper solution is possible to use a speed feedback device

such as a tachometer.

The key for understanding how vector control works is to explain the coordinate reference

transformation process. From the perspective of the stator, a sinusoidal input current is

forced to the stator. This time variant signal causes the generation of a rotating magnetic

flux. The speed of the rotor is a function of the rotating flux vector. From a stationary

perspective, the stator currents and motor and the rotating flux vector look like AC

quantities.

Keep in mind that the rotor flux speed is not equal to the revolving magnetic field, produced

by the stator phase windings, during the transient conditions. Looking at the motor from this

perspective during steady state conditions, the stator currents become constant.

2.2.1

Space vector definition and projection

The three-phase voltages, currents and fluxes of AC-motors can be deeply studied in terms

of complex space vectors. Assuming that ia, ib, ic are the instantaneous currents in the stator

phases we can define the stator current vector is by:

where

and

represent the spatial operators.

This current space vector describes the three phase sinusoidal system.

α +

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