Danfoss AHF005 Design Manual page 8

Introduction to Harmonics a...
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The integer multiples of the fundamental frequency ω1 are
called harmonics. The RMS value of a non-sinusoidal
waveform (current or voltage) is expressed as:
h max
2
∑
I RMS = I
( h
h =1
The amount of harmonics in a waveform gives the distortion
factor, or total harmonic distortion (THD), represented by the
ratio of RMS of the harmonic content to the RMS value of the
fundamental quantity, expressed as a percentage of the
fundamental:
(
h max
I h
∑
THD
=
I
h
=2 1
Using the THD, the relationship between the RMS current I
and the fundamental current I
I RMS I THD
= 1 × 1 +
The same applies for voltage.
The true power factor PF (λ) is:
P
PF
=
S
8
1 2
1 2
)
)
2
× 100 %
can be expressed as:
1
2
MG.80.C3.02 - VLT
AHF005/010 Design Guide
3 4
3 4
In a linear system the true power factor is equal to the
displacement power factor:
PF DPF cos
= =
In non-linear systems the relationship between true power
factor and displacement power factor is:
DPF
PF
=
1 + THD
The power factor is decreased by reactive power and harmon-
ic loads. Low power factor results in a high RMS current that
produces higher losses in the supply cables and transformers.
In the power quality context, the total demand distortion
(TDD) term is often encountered. The TDD does not character-
RMS
ize the load, but it is a system parameter. TDD expresses the
current harmonic distortion in percentage of the maximum
demand current I
h max
∑
TDD =
h =2
Another term often encountered in literature is the partial
weighted harmonic distortion (PWHD). PWHD represents a
weighted harmonic distortion that contains only the harmon-
ics between the 14th and the 40th, as shown in the following
definition:
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is a registered Danfoss trademark
5 6
5 6
( ϕ )
2
.
L
( )
I h
2
× 100 %
I L
7
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