Moeller DF6-340 Series - Page 39

Installation
01/02 AWB8230-1413GB
36
Parallel connection of motors to a single frequency
inverter
DF6 frequency inverters can control several parallel-connected
motors. If the motors are to run at different speeds, this must be
implemented through the number of connected pole pairs and/or
the gear transmission ratio.
Connecting motors inparallel reduces the load resistance at the
frequency inverter output, reduces the total stator inductivity is
and increases the leakage capacitance. As a result, the current
distortion is larger than it is in a single-motor circuit. To reduce the
current distortion, chokes or sinusoidal filters can be connected at
the frequency inverter output.
If motors with widely differing ratings (for example 11 kW and
30 kW) are connected in parallel to the output of a frequency
inverter, problems may arise during starting and at low speeds.
Motors with a low rating may be unable to develop the required
torque. This is due to the relatively high ohmic resistances of their
stators. They require a higher voltage during the start phase and
at low speeds.
Figure 31: Parallel connection of multiple motors
K1M
F1
M1
U1 V1 W1
K2M
F2
M2
U1 V1 W1
K3M
F3
M3
U1 V1 W1
M
3
˜
M
3
˜
M
3
˜
Caution!
If a frequency inverter controls a number of motors in
parallel, the contactors for the individual motors must be
designed for AC
-3 operation. Do not use the mains
contactors listed in table in the appendix (section “Mains
contactors”, page 165). These mains contactors are
designed only for the mains (primary) currents of the
frequency inverter. If they are used in multiple-motor
circuits, their contacts may weld.
h
The current consumption of all connected motors must
not exceed the rated output current I
2N
of the frequency
inverter.
h
Electronic motor protection can not be used when
operating the frequency inverter with a several connected
motors. You must however, protect each motor with
thermistors and/or overload relays.
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