MG.60.C4.02 – VLT is a registered Danfoss trademark
11
VLT
®
6000 HVAC
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Parallel coupling of motors
VLT 6000 HVAC is able to control several
motors connected in parallel. If the motors
are to have different rpm values, the motors
must have different rated rpm values. Motor
rpm is changed simultaneously, which means
that the ratio between the rated rpm values
is maintained across the range.
The total current consumption of the motors
is not to exceed the maximum rated output
current I
VLT,N
for the VLT frequency converter.
Problems may arise at the start and at low
rpm values if the motor sizes are widely
different. This is because the relatively high
ohmic resistance in small motors calls for a
higher voltage at the start and at low rpm
values.
In systems with motors connected in parallel,
the electronic thermal relay (ETR) of the VLT
frequency converter cannot be used as motor
protection for the individual motor.
Consequently, additional motor protection is
required, such as thermistors in each motor
(or individual thermal relays).
NB!
Parameter 107
Automatic Motor
Adaptation, AMA
and
Automatic Energy
Optimization, AEO
in parameter 101
Torque characteristics
cannot be used if
motors are connected in parallel.
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Motor cables
See
Technical data
for correct sizing of motor
cable cross-section and length.
Always comply with national and local regulations
on cable cross-sections.
NB!
If an unscreened cable is used, some EMC
requirements are not complied with, see
EMC test results
.
If the EMC specifications regarding emission are
to be complied with, the motor cable must be
screened, unless otherwise stated for the RFI
filter in question. It is important to keep the motor
cable as short as possible so as to reduce the
noise level and leakage currents to a minimum.
The motor cable screen must be connected to
the metal cabinet of the frequency converter and
to the metal cabinet of the motor. The screen
connections are to be made with the biggest
possible surface (cable clamp). This is enabled by
different installation devices in the different VLT
frequency converters.
Mounting with twisted screen ends (pigtails) is to
be avoided, since these spoil the screening effect
at higher frequencies.
If it is necessary to break the screen to install a
motor isolator or motor contactor, the screen
must be continued at the lowest possible HF
impedance.
