ST AN1448 Application note

AN1448

APPLICATION NOTE

HOW TO REDUCE 3rd HARMONICS WITH

ST6200C MOTOR CONTROL SOFTWARE

by Microcontroller Division Application Team

INTRODUCTION

Universal motors are widely used in home appliances such as vacuum cleaners, washing machines, power tools and food processors. However, universal motors produce a strong 3rd harmonic current. When the TRIAC conduction is not in full wave mode, the motor current contains high amplitude 3rd harmonics which may not comply with the 3rd harmonics limits set by the IEC61000-3-2 standard. In practice, it is very difficult to comply with the harmonics standard when driving universal motors with a power of more than 1200W .

This application note presents an innovative, cost-saving solution for suppressing 3rd harmonic current in the power line. The control principle is based on modulation of the phase angle delay times and is easily implemented in software using a low cost ST6200C microcontroller. The output motor current waveform is modulated so as to suppress specified harmonic components. The efficiency of this method has been proven on a 1500W vacuum cleaner under various load conditions. The measurement of harmonic components and motor power was done with a digital power meter (WT1030). The results show much better harmonic performance than symmetrical phase control methods.

AN1448/0702

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HOW TO REDUCE 3rd HARMONICS WITH ST6200C MOTOR CONTROL SOFTWARE

1 IEC61000-3-2 STANDARD

Harmonic disturbances in the power line have increased with spread of electronic equipment into homes and offices. Many household appliances and similar electrical equipment with or without electronic control devices, are liable to introduce disturbances, especially harmonics of the supply frequency, into the systems to which they are connected. The harmonics do not represent any real or active power, but nevertheless they lead to substantial copper or iron losses. The IEC61000-3-2 standard deals with the limitation of harmonic currents injected into the public supply system.

1.1 SCOPE

The IEC61000-3-2 standard is applicable to all electrical and electronic equipment with an input current up to 16A and intended to be connected to public low-voltage distribution systems with a nominal 50Hz or 60Hz frequency and of the following types:

-nominal voltages up to 240V, single-phase, two or three wire;

-nominal voltages up to 415V, three-phase, three or four wire;

1.2 OBJECTIVE

The objective of this standard is to give the general requirements and the maximum permissible values of the harmonic components of the input current which may be produced by a piece of equipment tested individually under specified conditions. It also gives a practical method for testing a prototype.

1.3 MEASUREMENT CIRCUIT FOR SINGLE PHASE EQUIPMENT

The equipment under test is supplied in series with shunts ZM (see Figure 1.) or current transformers from a source with the same nominal voltage and frequency as the rated voltage and frequency of the equipment under test.

The measurement circuit for single phase equipment is show in Figure 1..

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HOW TO REDUCE 3rd HARMONICS WITH ST6200C MOTOR CONTROL SOFTWARE

Figure 1. Measurement circuit for single phase equipment

M
zM	In

ZS

S			A

G

~

Legend:

S = Supply source

M = Measurement equipment

A = Equipment under test

ZM = Input impedance of measurement equipment

Zs = Internal impedance of the supply source

In = Harmonic component of order n of the line current

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