20 | Danfoss Drives · DKDD.PB.551.A1.02
The maximum distance between the soft starter and motor
depends on the voltage drop and the cable capacitance.
Cable must be selected so that when the motor is running fully
loaded, the voltage drop at the motor terminals does not exceed
the limit specied in local electrical regulations.
For distances greater than 500 metres or when using parallel
cabling, cable capacitance may be a factor. If in doubt, please
contact Danfoss with details of the soft starter model, mains
voltage and frequency. If you want to use standard products, we
recommend using Line Reactors.
Calculation
The main point is to limit the di/dt so the SCR’s are not damage
with the inrush currents due to increase cable capacitance. The
output inductors must be installed as close to the soft starter as
possible.
The output inductors must be smaller than the inductance of the
motor.
Minimum rated current of coil, is stated from start current.
(Soft starter selection and application, normally 3 – 4,5 x FLC)
V
p
V
p
= V
rms
× √2
V Motor supply voltage
Ilr Locked rotor current
ω 2 × π × frequency
L
coil
Coil Inductance
Cabling: What is the maximum allowable cable,
distance between a soft starter and the motor?
V
p
< L
coil
di
(
dt
)
V
(√3 I
LR
) ω
di
dt
100 A
.
1 μS
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