UM0324 datasheet(16/105 Pages) STMICROELECTRONICS

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Getting started with the library

UM0324

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Getting started with the library

2.1

Introduction to AC induction motor FOC drive

The AC IM IFOC software library is designed to achieve the high dynamic performance in

AC motor control offered by the field oriented control (FOC) strategy.

Through complex machine electrical quantity transformations, this well-established drive

system optimizes the control of the motor, to the extent that it is able to offer decoupled

torque (Te) and magnetic flux (λ) regulation. That is, it offers the same optimum and

favorable conditions as DC motors but, in this case, carried out with rugged and powerful AC

induction motors.

With this approach, it can be stated that the two currents iqsλ

r and i

λr, derived from stator

currents, have in AC Induction Motor (IM) the same role that armature and field currents

have in DC motors: the first is proportional to mechanical torque the second to the rotor flux.

In more detail, in the context of FOC, rotor flux position is indirectly calculated, starting from

transformed equations of the machine, by means of known motor parameters and stator

current measurements: this is why the controller is an indirect controller and, hence the

phrase IFOC drive.

In other words, it can be stated that IFOC drive is halfway between dynamic controllers

(speed, position …) and machine core. So, the system may well be depicted as in Figure 6,

if introduced in a loop for speed control.

Figure 6.

FOC drive placed in a speed loop

Basic information on field oriented structure and library functions is represented in Figure 7.

●

The

θλr calculation block estimates rotor flux angle, which is essential to transformation

blocks (Park, Reverse Park) for performing field orientation, so that the currents

supplied to the machine can be oriented in phase and in quadrature to the rotor flux

vector. More in depth information about reference frame theory and FOC structure is

available in [1][2] and Section 4.4.3 on page 61.

●

The space vector PWM block (SVPWM) implements an advanced modulation method

that reduces current harmonics, thus optimizing DC bus exploitation.

●

The current reading block allows the system to measure stator currents correctly, using

either cheap shunt resistors or market-available isolated current Hall sensors (ICS).

●

The speed-reading block handles tachogenerator or incremental encoder signals in

order to acquire properly rotor angular velocity or position.

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