Parallel connection of motors
The frequency converter is able to control several mo-
tors connected in parallel. If the motors are to have
different rpm values, use motors with different rated
rpm values. Motor rpm is changed simultaneously,
which means that the ratio between the rated rpm val-
ues is maintained across the range. The total current
consumption of the motors is not to exceed the maxi-
mum rated output current I
INV
for the frequency con-
verter.
Problems may arise at the start and at low rpm values
if the motor sizes are widely different. This is because
the small motors' relatively high ohmic resistance in
the stator calls for a higher voltage at the start and at
low rpm values.
In systems with motors connected in parallel, the elec-
tronic thermal relay (ETR) of the frequency converter
cannot be used as motor protection for the individual
motor. For this reason further motor protection must
be used, e.g. thermistors in each motor or an individual
thermal relay. (Circuit breakers are not suitable as pro-
tection).
NB!
Parameter 107 Automatic motor adaption,
AMT cannot be used when motors are
connected in parallel. Parameter 101 Tor-
que characteristic must be set to Special
motor characteristics [8] when motors are
connected in parallel.
Motor cables
See Technical data for correct dimensioning of motor
cable cross-section and length. Always comply with
national and local regulations on cable cross-section.
NB!
If an unscreened/unarmoured cable is
used, some EMC requirements are not
complied with, see EMC test results in the
Design Guide.
If the EMC specifications regarding emission are to be
complied with, the motor cable must be screened/arm-
oured, 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 area (cable clamp). This
is enabled by different installation devices in different
frequency converters. Mounting with twisted screen
ends (pigtails) is to be avoided, since these spoil the
screening effect at high frequencies. If it is necessary
to break the screen to install a motor isolator or motor
relay, the screen must be continued at the lowest pos-
sible HF impedance.
VLT
®
2800 Series
MG.27.A2.02 - VLT
®
is a registered Danfoss trademark 69
Installation
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