Siemens 3RW44 Manual page 16

Introduction
2.1 Basic physical principles of the three-phase asynchronous motor and operating principle of the soft starter
Figure 2-4
Startup
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 stopping process. This way the torque generated in
the motor is slowly reduced, thus enabling a smooth ramp-down of the application.
The frequency remains constant during this process and corresponds to the line frequency,
in contrast to frequency-controlled startup and stopping of a frequency converter.
Once the motor has started up correctly, the thyristors are fully utilized, meaning that the
complete line voltage is applied to the motor terminals. As the motor voltage does not have
to be controlled during operation, the thyristors are bridged by integrated bypass contacts.
This minimizes the waste heat generated during continuous operation (which is caused by
the thyristor's power loss). Therefore the area around the switching devices heats up less.
16
Reduced torque characteristic of a three-phase asynchronous motor during startup with
a SIRIUS 3RW44 soft starter
Manual, 03/2017, NEB535219502000/RS-AB/008
3RW44 soft starters