Danfoss VLT AHF005, Design Manual

Page: 9 / 59

3.1.2 Non-linear Loads
Non-linear loads (such as diode rectifiers) draw a non-sinusoidal current. The figure below shows the current drawn by a 6-pulse
rectifier on a three phase supply.
A non-sinusoidal waveform can be decomposed in a sum of sinusoidal waveforms with periods equal to integer multiples of the
fundamental waveform.
f (t) =
∑
ah × sin
(
h ω1t
)
See following illustrations.
1
1 2 3 4 5 6 7
0.
0
0
-
-
1
1 2 3 4 5 6 7
0.
0
0
-
-
130BB539.10
The integer multiples of the fundamental frequency
ω
1 are
called harmonics. The RMS value of a non-sinusoidal
waveform (current or voltage) is expressed as:
IRMS =
∑
h=1
hmax
I(h)
2
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:
THD =
∑
h=2
hmax
(
Ih
I1
)
2
× 100 %
Using the THD, the relationship between the RMS current
I
RMS
and the fundamental current I
1
can be expressed as:
IRMS = I1 × 1 + THD
2
The same applies for voltage.
The true power factor PF (
λ
) is:
PF = PS
In a linear system the true power factor is equal to the
displacement power factor:
PF = DPF = cos(ϕ)
Introduction to Harmonics a... AHF005/010 Design Guide
8 MG.80.C4.02 - VLT
®
is a registered Danfoss trademark
3 3