User Manual

Functions
5.1 Start modes
SIRIUS 3RW30 / 3RW40
30 Manual, 10/2010, 535199502000 DS 03
5.1.2 Current limiting and ramp-up detection (3RW40 only)
The SIRIUS 3RW40 soft starter measures the phase current (motor current) continuously
with the help of integrated current transformers.
The motor current that flows during the startup process can be actively limited by means of
the soft starter. The current limiting function takes priority over the voltage ramp function. As
soon as a parameterizable current limit is reached, in other words, the voltage ramp is
interrupted and the motor is started with the current limiting value until it has started up
successfully. The current limiting function is always active with SIRIUS 3RW40 soft starters.
If the current limiting potentiometer is set to the clockwise stop (maximum), the starting
current is limited to five times the set rated motor current.
Current limiting value
The current limiting value is set to the current required during starting as a factor of the rated
motor current. Since the starting current is asymmetrical, the set current corresponds to the
arithmetic mean value for the three phases.
Example
If the current limiting value is set to 100 A, the currents might be approx. 80 A in L1, 120 A in
L2, and 100 A in L3 (refer to chapter Starting current asymmetry (Page 21)).
As soon as the selected curre
nt limiting value is reached, the motor voltage is reduced or
controlled by the soft starter to prevent the current from exceeding the limit. The set current
limiting value must be high enough to ensure that the torque generated in the motor is
sufficient to accelerate the motor to nominal speed. Three to four times the value of the
motor's rated operational current (Ie) can be assumed as typical here.
The current limiting function is always active because it is required by the intrinsic device
protection. If the current limiting potentiometer is set to the clockwise stop (maximum), the
starting current is limited to five times the set rated motor current.