Current Limiting And Ramp-Up Detection (3Rw40 Only) - Siemens SIRIUS 3RW30 Manual

Functions
5.1 Start modes
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 clockwise stop, the starting current is limited to
the maximum possible current (refer to chapter Setting the current limiting value (Page 114)).
Current limiting value
The current limiting value is set to the current required during starting as a factor of the rated
motor current (refer to chapter Setting the current limiting value (Page 114)). 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 28)).
As soon as the selected current 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 clockwise stop, the starting current is
limited to the maximum possible current (refer to chapter Setting the current limiting value
(Page 114)).
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SIRIUS 3RW30 / 3RW40
Manual, 10/2018, NEB535199502000/RS-AB/005