ST7MC implementation of symmetrical SVPWM

AN2154

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Figure 7.

Center-aligned SVPWM patterns

3

ST7MC implementation of symmetrical SVPWM

A practical implementation of symmetrical SVPWM has been implemented using ST7MC,

the STMicroelectronics 8-bit microcontroller dedicated to Motor Control. Thanks to its

embedded Motor control peripheral, the space vector modulation can be easily implemented

without excessive computational load for the CPU. The firmware developed is strongly

based on the three-phase AC Induction Motor Control Software Library, downloadable from

the web site: www.st.com and widely described in application note AN1904 (downloadable

from the same web page). This library can handle AC asynchronous motor control in open

harmonic injection. Starting from the mentioned library, the symmetrical SVPWM has been

generated mainly by modifying the 'U' event interrupt service routine dedicated to the sinus

plus 3rd harmonic waveform generation.

3.1

Fixed settings

The aim of this paragraph is to illustrate which of the parameters defined in AC Induction

Motor control library have been fixed during the symmetrical SVPWM algorithm

development. All these settings are not mandatory, but a modification of one of them could

require some firmware modification in order to restore the full firmware functioning. Relating

to AC asynchronous motor control library, the main parameters which have been set are:

●

Speed regulation: open loop operation has been selected.

●

PWM pattern: center-aligned PWM must be chosen for symmetrical SVPWM.

●

PWM frequency: it has been set equal to 15.625 kHz. This has been achieved by fixing

allows both the elimination of the acoustical noise introduced by the switches and to

perform all the necessary computations for PWM patterns generation in 8-bit format.

●

Repetition counter: it has been fixed at 2 so that a U event is generated every 1.5 PWM

periods.

●

Accordingly with ST7MC Starter Kit schematic rev. C-6, the index modulation is

adjustable from potentiometer RV2 in the range between 0 to 0.866, and the output

waveform frequency is adjustable from potentiometer RV1 in the range between 3 and

225 Hz. Maximum and minimum frequency can be eventually varied by changing

HIGHEST_FREQ and LOWEST_FREQ parameters values in MTC_param.h header

file.