1.3.2 Working Principle
The low harmonic drive is a high-power frequency converter with an integrated active filter. An active filter is a device that
actively monitors harmonic distortion levels and injects compensative harmonic current onto the line to cancel the
harmonics.
Mains
380 to
500 VAC
Optional
RFI
Optional
Fuses
Optional
Manual
Disconnect
HI Reactor
L
m
L
m
L
m
L
ac
L
ac
L
ac
AC
Contactor
Relay 12
Control & AUX
Feedback
Soft-Charge
Resistor
Converter Side
Filter
Power Stage
AF Current
Sensors
Capacitor
Current Sensors
VLT Drive
Main’s
3
3
3
CTs
L
c
L
c
L
c
C
ef
C
ef
C
ef
R
ef
R
ef
R
ef
I
r
I
s
I
t
130BB406.11
Figure 1.1 Basic Layout for the Low Harmonic Drive
Low harmonic drives are designed to draw an ideal sinusoidal current waveform from the supply grid with a power factor of
1. Where traditional non-linear load draws pulse-shaped currents, the low harmonic drive compensates that via the parallel
filter path, lowering the stress on the supply grid. The low harmonic drive meets the highest harmonic standards with a
THDi less than 5% at full load for <3% pre-distortion on a 3% unbalanced 3-phase grid.
Introduction
VLT
®
AutomationDrive FC 302 Low Harmonic Drive
132–630 kW
6
Danfoss A/S © Rev. 04/2015 All rights reserved.
MG37A322
1
1
