Non-Linear Loads - Danfoss VLT AHF005 Design Manual

Introduction to Harmonics a...

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.
3 3
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 ) = a h × sin
See following illustrations.
1
0.
0
-
-
0
1
0.
0
-
-
0
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 1
h =2
8
)
h ω 1 t
1 2
1 2
)
2
× 100 %
MG.80.C4.02 - VLT
AHF005/010 Design Guide
3 4
3 4
Using the THD, the relationship between the RMS current
I and the fundamental current I
RMS
I RMS = I 1 × 1 + THD 2
The same applies for voltage.
The true power factor PF (λ) is:
P
PF =
S
In a linear system the true power factor is equal to the
displacement power factor:
PF = DPF = cos ( ϕ )
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5 6
5 6
can be expressed as:
1
7
7