ECO REFERENCE MANUAL
8-5
SC
This means that the input current waveform
consists of a series of low frequency harmonics,
and this may in turn cause voltage harmonic
distortion, depending on the supply impedance.
Sometimes these harmonics need to be
assessed in order to ensure that certain levels
are not exceeded. Excessive harmonic levels
can cause high losses in transformers, and may
interfere with other equipment. In any case, the
rating and selection of cabling and protection
equipment must take into account these high
RMS levels. Some typical measured harmonic
levels are shown below.
In order to calculate the harmonics in a particular
supply system it is essential that the supply
impedance is known. This is usually stated in
terms of fault current levels, transformer size, and
installed impedance such as line inductors etc.
The addition of input line chokes reduces the
level of harmonic currents and also reduces the
overall RMS current as well as improving the
overall power factor.
Where supplies have very low impedance (such
as below 1%) an input inductor is recommended
in any case to limit peak currents in the drive.
0
20%
40%
60%
80%
50
150 250 350 450 550 650
Harmonic Frequency
Three Phase Supplies
Current (total RMS=100%)
Typical Harmonic content - Measured Results (50Hz supply)
Current
100%
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