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Problems may arise at the start and at low rpm
values if the motor sizes are widely different. This
is because the small motors' relati v ely 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
electronic thermal relay (ETR) of the frequency
converter cannot be used as motor protection
for the indi v idual motor. For this reason further
motor protection must be used, e.g. thermistors in
each motor or an indi v idual thermal relay. (Circuit
breakers are not suitable as protection).
NB!:
Parameter 107 Automatic motor adaption, AMT
cannot be used when motors are connected
in parallel. Parameter 101 Torque 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/armoured, 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 possible HF impedance.
Motor thermal protection
The electronic thermal relay in UL-approved frequency
converters has recei v ed the UL-approval for single
motor protection, when parameter 128 Motor thermal
52
®
VLT
2800 Series
protection has been set for ETR Trip and parameter
105 Motor current, I
M, N
the rated motor current (see motor nameplate).
Brake connection
No. 81 82 Brake resistor
R- R+ terminals
The connection cable to the brake resistor must
be screened/armoured. Connect the screen to the
metal cabinet of the frequency converter and to the
metal cabinet of the brake resistor by means of
cable clamps. Dimension the cross-section of the
brake cable to match the brake torque.
See the Design Guide for dimensioning
of brake resistors.
NB!:
Please note that voltages up to 850 V
DC occur on the terminals.
Earth connection
As leak current to the earth is higher than 3.5 mA, the
frequency converter must always be earthed according
to prevailing national and local regulations. To
guarantee that the earth cable has a good mechanical
connection to terminal 95, the cable cross section
must be at least 10 mm
that are terminated separately. To increase safety
you can install an RCD (Residual Current Device),
which guarantees that the frequency converter trips
when leak current becomes too high. See also
RCD Application Note MN.90.GX.02.
MG.28.E9.02 - VLT is a registered Danfoss trademark
has been programmed to
2
or 2 rated earth wires
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