User Manual

Direct start
Current at direct
start
T/T
e
Current limit
Current at soft start
with voltage ramp
Current at soft start
with current limit
Voltage ramp
Load
n/n
s
Fig. 3.9-1
Current and torque characteristics for starting
In the following a more detailed discussion of the characteristics of various available soft starter
functions is presented.
3.9.2 Voltage ramp
The voltage across the motor is linearly increased during a settable time, starting from an
adjustable initial value (Fig. 3.9-2). The starting current and the starting torque, and hence the
accelerati
on, adjust themselves in accordance with the voltage ramp and the torque characteris-
tic of the load. This method is especially suitable for load-free start-ups and for working ma-
chines with increasing torque requirement at increasing speed (drives with larger inertial
masses, fans etc.).
100%
Percent
Voltage
Initial
Torque
Start
Run
Time (seconds)
Fig. 3.9-2
Soft start with voltage ramp
For drives with variable loading at the start – for example processing machines that normally
start up in a load-free condition, but which can be under load due to a fault – soft starters with
two voltage ramps are available (Fig. 3.9-3). The initial voltages and starting times of ramps are
separately adjustable an
d hence can be adapted to both operating states. It is possible to
switch between both ramps as required.
Ramp #2
Ramp #1
Start #1
Start #2
Run #1
Run #2
Time (seconds)
Percent
Voltage
100%
Initial Torque
#2
Initial Torque
#1
Fig. 3.9-3
Soft starter with changeable voltage ramp for various loading states at start.
3.9.3 Kickstart
Many drives have a high breakaway torque at rest, because for example bearings surfaces may
generate high initial friction. This requires a short period of increased starting voltage at the
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