Motor Cables; Motor Thermal Protection; Brake Connection - Danfoss VLT FCD 311 Design Manual

The frequency converter is able to control several
motors connected in parallel. If the motors are to
have different rpm values, use motors with different
rated rpm values. Motor rpm is changed simultane-
ously, 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
quency converter.
Problems may arise at the start and at low rpm val-
ues if the motor sizes are widely different. This is
because the small motors' relatively high ohmic re-
sistance 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 con-
verter cannot be used as motor protection for the
individual motor. For this reason further motor pro-
tection must be used, e.g. thermistors in each motor
(or an individual thermal relay).
NB!:
Parameter 107 Automatic motor adap-
tion, AMT cannot be used when motors are
connected in parallel. Parameter 101 Torque charac-
teristic 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 ca-
ble 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 dif-
ferent 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 iso-
40
VLT
lator or motor relay, the screen must be continued at
the lowest possible HF impedance.

Motor thermal protection

The electronic thermal relay in UL-approved fre-
quency converters has received the UL-approval for
for the fre-
INV
single motor protection, when parameter 128 Motor
thermal protection has been set for ETR Trip and
parameter 105 Motor current, I
grammed to the rated motor current (see motor
nameplate).

Brake connection

No. 81 (optional)
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 ca-
ble clamps. Dimension the cross-section of the
brake cable to match the brake torque.
See chapterDynamic Braking for dimensionering of
brake resistors.
®
FCD Series
82 (optional)
R- R+
NB!:
Please note that volt-
ages up to 850 V DC occur on the terminals.
MG.04.A1.02 - VLT is a registered Danfoss trade mark
has been pro-
M, N
Brake resistor
terminals