Siemens SIRIUS 3RW40, Manual

Page: 29 / 212

Functions
5.1 Start modes
SIRIUS 3RW30 / 3RW40
Manual, 01/2010, 535 1995-02 DS01
29
Figure 5-3 Principle of the voltage ramp for the starting current characteristic
Typical applications of the voltage ramp
The voltage ramp principle is valid for all applications, e.g. pumps, compressors, conveyor
belts.
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]).
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