2.6 Internal Equipment Functions
2.6.1 Working Principle
The VLT Low Harmonic Drive is a high-power adjustable frequency drive 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
Relay 12
Control & AUX
Feedback
Soft-Charge
Converter
Side Filter
Power Stage
AF Current
Sensors
Capacitor
Current Sensors
Frequency converter
Main’s
3
3
3
CTs
NC
Relay
L
c
L
c
L
c
C
ef
C
ef
C
ef
R
ef
R
ef
R
ef
I
r
I
s
I
t
130bb406.10
Figure 2.9 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 this via the
parallel filter path, lowering the stress on the supply grid. The Low Harmonic Drive meets the highest harmonic standards
with a THiD less than 5% at full load for <3% pre-distortion on a 3% unbalanced 3-phase grid.
2.6.2 IEEE519 Compliance
The Low Harmonic Drive is designed to meet IEEE519 recommendation for Isc/Il >20 for even individual harmonic levels.
The filter has a progressive switching frequency that creates a wide frequency spread, giving lower individual harmonic
levels above the 50th.
Introduction
Instruction Manual
MG37A222
Danfoss A/S © Rev. 2014-02-07 All rights reserved.
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2
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Summary of Contents for VLT AutomationDrive FC 302
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