Rockwell Automation Low-Voltage Switchgear and Controlgear - Manual (Page 101)

beginning of the start-up. As soon as the drive begins turning, the torque requirement decreases
strongly and the start can be continued with a lower voltage. Soft starters with kickstart function
offer the required functionality (
Fig. 3.9-4 ).
Percent
Voltage
100%
Initial
Torque
Kickstart
Start Run
Time (seconds)
Fig. 3.9-4
The kickstart function briefly increases the voltage at the beginning of a start-up to overcome the
breakaway torque of the drive.
3.9.4 Current limitation
For starts with current limitation, the starting current is adjustable. The supply load can hence be
adjusted to local requirements – for example, of the electricity utility. The motor current is
measured in the soft starter and the motor voltage adjusted in accordance with the set current
limitation. The available motor torque falls with the square of the voltage or current. With strong
current limitation, a small starting torque is thus available (
Fig. 3.9-5 and Fig. 3.9-1 ).
A simplified version just sets a fixed reduced voltage during starting.
600%
Percent Full
Load Current
Start
Time (seconds)
Fig. 3.9-5
During starting with current limitation the starting current can be adjusted to the supply conditions
3.9.5 Soft stop
With some drives it is desirable to also control the stoppage of the motor with a soft stop instead
of a possible abrupt stop when the motor voltage is disconnected. This may for example be the
case with conveyor equipment, in which sudden stopping could result in the transported goods
being displaced or falling over. Soft starters with an adjustable voltage ramp upon stopping are
one suitable solution. For pump controllers, see Section
3.9.6 .
LVSAM-WP001A-EN-P - April 2009 3-25