Siemens SIRIUS 3RW44 System Manual page 18

Introduction
Starting
U
L1-
L3
α
Figure 1-5:Phase angle control and schematic layout of a soft starter with internal bypass contacts
1-6
This means that because the electronic soft starter controls the motor voltage
during motor start-up, it simultaneously controls the ingoing starting current and
the starting torque generated in the motor.
The same principle is also used during the stopping process. The effect is that
the torque generated in the motor is slowly reduced, thus enabling soft stopping
of the application.
During this process, the frequency remains constant and corresponds to the
network frequency, contrary to the frequency-controlled starting and stopping of
a frequency converter.
Upon completion of motor start-up, the thyristors are fully utilized, resulting in the
complete network voltage being applied to the motor terminals. Since no motor
voltage control is required during operation, the thyristors are bridged by
integrated bypass contacts. This reduces the waste heat developing during
continuous operation which is caused by power loss of the thyristor. Therefore,
the area around the switching devices heats up less.
The following graph illustrates the mode of operation of the 3RW44 soft starter:
α
α
L2
L1
U
L1 -L3
ϕ t
M
3~
SIRIUS 3RW44 System Manual
GWA 4NEB 535 2195-02 DS 04
L3
G1