VLT
®
6000 HVAC
MD.60.A2.02 - VLT is a registered Danfoss trademark
10
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■ LC filters for VLT 6000 HVAC
When a motor is controlled by a frequency
converter, resonance noise will be heard from the
motor. This noise, which is caused by the design of
the motor, occurs each time one of the inverter
switches in the frequency converter is activated.
Consequently, the resonance noise frequency
corresponds to the switching frequency of the
frequency converter.
For the VLT 6000 HVAC, Danfoss offers a LC filter to
dampen the acoustic motor noise.
This filter reduces the voltage rise time, the peak
voltage U
PEAK
and the ripple current ∆I to the motor,
thereby making current and voltage almost
sinusoidal. The acoustic motor noise is therefore
reduced to a minimum.
Because of the ripple current in the coils, there will
be some noise from the coils. This problem can be
solved entirely by integrating the filter in a cabinet or
similar.
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■ Examples of the use of LC filters
Submersible pumps
For small motors with up to and including 5.5 kW
rated motor power, use a LC filter, unless the motor
is equipped with phase separation paper. This
applies e.g. to all wet running motors. If these
motors are used without LC filter in connection with
a frequency converter, the motor windings will short-
circuit. If in doubt, ask the motor manufacturer
whether the motor in question is equipped with
phase separation paper.
Long motor cables
If screened/armoured motor cables longer than 150
m or unscreened/unarmoured motor cables longer
than 300 m are used, a LC filter should be applied.
The LC filter reduces the capacitive earth leakage
currents and the voltage peak loads.
NB!:
If a VLT frequency converter controls several
motors in parallel, the motor cables must be
added up to give the total cable length.
Well pumps
If immersion pumps are used, e.g. submerged
pumps or well pumps, the supplier should be
contacted for clarification of requirements. It is
recommended to use a LC filter if a VLT frequency
converter is used for immersion operations.
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