Functions; Start Modes; Voltage Ramp - Siemens SIRIUS 3RW30 Manual

Functions

5.1

Start modes

You can choose between different startup functions reflecting the wide range of applications
and functionality of the SIRIUS 3RW30 and 3RW40 soft starters. The motor start can be
optimally adapted to each particular application.
5.1.1

Voltage ramp

The SIRIUS 3RW30 and 3RW40 soft starters achieve soft starting by means of a voltage
ramp. The motor terminal voltage is increased from a parameterizable starting voltage up to
the mains voltage within a definable ramp-up time.
Starting voltage
The starting voltage determines the starting torque of the motor. A lower starting voltage
results in a lower starting torque and a lower starting current. The starting voltage selected
must be sufficiently high to ensure that motor starts up smoothly as soon as the start
command is received by the soft starter.
Ramp time
The length of the set ramp time determines the time taken to increase the motor voltage from
the parameterized starting voltage to the mains voltage. This influences the motor's
acceleration torque, which drives the load during the ramp-up process. A longer ramp time
results in a lower acceleration torque as the motor is started up. The startup is slower and
smoother as a result. The ramp time should be long enough for the motor to reach its nominal
speed. If the time selected is too short, in other words if the ramp time ends before the motor
has started up successfully, a very high starting current that can even equal the direct starting
current at the same speed occurs at this instant.
The SIRIUS 3RW40 soft starter limits the current to the value set with the current limiting
potentiometer (refer to chapter Current limiting and ramp-up detection (3RW40 only) [Page
29]). As soon as the current limiting value is also reached, the voltage ramp or the ramp time
is interrupted and the motor is started with the current limiting value until it has started up
successfully. In this case, the motor ramp-up time may be longer than the maximum
parameterizable 20 seconds ramp time (for further information about the maximum ramp-up
times and switching frequencies, refer to chapter 3RW40 2. to 7. power electronics [Page
148] ff).
The SIRIUS 3RW40 soft starter has intrinsic device protection, current limiting, and ramp-up
detection functions. These functions do not form part of the SIRIUS 3RW30 soft starter.
SIRIUS 3RW30 / 3RW40
Manual 01/2010, 535 1995-02 DS01
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