Siemens SIRIUS 3RW30 Manual page 19

Figure 3-4
Soft start /soft stop
This means that, since the motor voltage is controlled by the electronic soft starter during the
startup process, the consumed starting current and the starting torque generated in the motor
are also controlled.
The same principle is applied during the stop process. This ensures that the torque generated
in the motor is gradually reduced, so that the application can stop smoothly (the soft stop
function is only supported by the 3RW40).
The frequency remains constant during this process and corresponds to the mains frequency,
in contrast to frequency controlled starting and stopping of a frequency converter.
Bypass mode
Once the motor has been started up correctly, the thyristors are subject to fully advanced
control, meaning that the whole mains voltage is applied to the motor terminals. As the motor
voltage does not have to be controlled during operation, the thyristors are bridged by integral
bypass contacts that are rated for AC1 current. This minimizes the waste heat generated
during uninterrupted duty (which is caused by the thyristor's power loss), and minimizes
heating up of the switching device's environment.
The bypass contacts are protected by an integrated, electronic arc quenching system during
operation. If they are opened in the event of a fault, e.g. if the control voltage is temporarily
interrupted, mechanical vibrations occur, or the coil operating mechanism or the main contact
spring has reached the end of its service life and is defective, the equipment is not damaged.
The diagram below shows the method of operation of the SIRIUS 3RW30 and 3RW40 soft
starters:
SIRIUS 3RW30 / 3RW40
Manual, 01/2010, 535 1995-02 DS01
3.3 Functional principle of the SIRIUS 3RW30 and 3RW40 soft
Reduced torque characteristic of a three-phase induction motor during starting with a
SIRIUS 3RW30 or 3RW40 soft starter
Product description
19