Moeller DF51 - Page 26

Engineering
10/05 AWB8230-1541GB
22
Connecting motors in parallel reduces the load resistance at the
frequency inverter output and the total stator inductivity, 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, motor reactors or a section “Motor
reactors”page 213a section “Sine-wave filter”page 214 sine-
wave filters can be connected at the frequency inverter output.
Example:
Copper wire winder
• 16 motors
• P = 60 W
• I = 0.21 A
• U = 400 V
• p.f. = 0.7
• Direct starting current: 2 A
If a wire breaks during winding, the corresponding motor is
automatically switched off. When the wire is rejoined, the motor
can be started up again. Only one motor can be started up at a
time.
Frequency inverter selection:
DF51-340-2K2, rated current 5.5 A.
DEX-LM3-008 motor reactor
If motors with widely differing ratings (for example 0.37 kW and
2.2 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 motor rating may be unable to develop
the required torque due to the relatively high ohmic resistance of
their stators. They require a higher voltage during the start phase
and at low speeds.
Figure 13: Parallel connection of several motors to one frequency inverter
Q11
F1
M1
U1 V1 W1
Q12
F2
M2
U1 V1 W1
Q13
F3
M3
U1 V1 W1
M
3
˜
M
3
˜
M
3
˜
Caution!
If a frequency inverter controls several motors in parallel,
the contactors for the individual motors must be designed
for AC-3 operation. Do not use the mains contactors listed
in the table in the appendix (paragraph “Mains
contactors”, page 210). These mains contactors are
designed only for the frequency inverter’s mains (primary)
currents. If contactors of this size are used in the motor
circuit, the contacts could weld.
15 motors in operation: 15 x 0.21 A = 3.15 A
Connect one motor: 2.00 A
5.15 A
h
Constant Torque Curve control mode (a PNU A044, or
PUN 244, ) necessary.
h
The current consumption of all motors connected in
parallel must not exceed the frequency inverter’s rated
output current I
2N
.
h
Electronic motor protection can not be used when
operating the frequency inverter with several parallel
connected motors. You must, however, protect each
motor with thermistors and/or overload relays.
h
The use of motor-protective circuit-breaker at the
frequency inverter’s output can lead to nuisance tripping.