ST AN1448 Application note
AN1448
APPLICATION NOTE
HOW TO REDUCE 3r d HARM ONI CS WI TH
ST6200C MOTOR CONTROL SOFTWARE
by Microcontroller Division Application Team
INTRODUCTION
Universal motors are widely used in home appl iances such as vacuum cleaner s, 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 wi th the 3rd harmoni cs li mits 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. T he efficiency of this method has been proven on a 1500W vacuum cleaner
under various load conditions. The m easureme nt of h armonic component s and m otor power
was done with a digital power meter (WT1030). The results show m uch better harmonic performance than symmetrical phase control methods.
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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 of fices. M any hou seho ld applianc es and similar el ectrical eq uipme nt 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 w ith the l imitati on of harmonic currents i njected i nto
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 distr ibution 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 i s to give th e general requirements and the m aximum perm issible valu es of the ha rmonic comp onen ts of th e inp ut curr ent wh ich ma y be produce d by a
piece of equipment tested individually under specified conditi ons. 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 Z
M (see Figure 1.) or current trans-
formers 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
ZS
S
Legend:
S = Supply source
M = Measurement equipment
A = Equipment under test
M = Input impedance of measurement equipment
Z
s = Internal impedance of the supply source
Z
n = Harmonic component of order n of the line current
I
G
~
In
A
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