3.7.2. Parallel Connection of Motors
The adjustable frequency drive can control
several parallel-connected motors. The total
current consumption of the motors must not
exceed the rated output current I
M,N
for the
adjustable frequency drive.
NOTE
Installation with cables connec-
ted in a common joint, as in the
illustration below, is only recom-
mended for short cable lengths.
NOTE
When motors are connected in
parallel, par. 1-29
Automatic
Motor Adaptation (AMA)
cannot
be used.
NOTE
The electronic thermal relay
(ETR) of the adjustable frequen-
cy drive cannot be used for mo-
tor protection for the individual
motor of systems with parallel-
connected motors. Provide fur-
ther motor protection with, for
example, thermistors in each
motor or individual thermal re-
lays (circuit breakers are not
suitable for protection).
Problems may arise at start and at low RPM values if motor sizes are widely different because
small motors' relatively high ohmic resistance in the stator calls for a higher voltage at start and
at low RPM values.
3.7.3. Motor Thermal Protection
The electronic thermal relay in the adjustable frequency drive has received UL approval for single
motor protection when par. 1-90
Motor Thermal Protection
is set for
ETR Trip
and par. 1-24
Motor
current, I
M,N
is set to the rated motor current (see motor nameplate).
For thermal motor protection, it is also possible to use the MCB 112 PTC thermistor card option.
This card provides an ATEX certificate to protect motors in explosion hazard areas, Zone 1/21 and
Zone 2/22. Please refer to the
Design Guide
for further information.
3. How to Install
VLT
®
AutomationDrive FC 300 Instruction
Manual
44
MG.33.A9.22 - VLT is a registered Danfoss trademark
3
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