Operating Principle - Siemens SIRIUS 3RW52 Manual

1.3

Operating principle

Soft starters are used to start three-phase induction motors with reduced torque and reduced
starting current.
The 3RW52 soft starter starts the motor as soon as the switch-on command is issued (t
During the ramp-up time (t
start the motor up smoothly.
The 3RW52 soft starter features internal run-up recognition. If it detects that the motor has
reached its rated operating speed before the ramp-up time expires, the motor voltage is
immediately increased to 100 % of the line voltage (t
and the power semiconductors are bypassed. The 3RW52 soft starter is then in bypass
operation.
Coast-down to zero speed is activated when the switch-on command is cancelled (t
the motor is shut down. The power semiconductors also ensure that the motor coasts down
smoothly to a stop. The power supply to the motor is maintained (t
ramp-down time is reached (t
to a standstill (t
①
②
t
1
t
2
t
3
t
4
SIRIUS 3RW52 Soft Starter
Manual, 01/2019, A5E35630451002A/RS-AD/004
1
).
6
Speed
Voltage
Motor start-up with set starting voltage U
Rated speed n achieved
e
End of the set ramp-up time
(t -t )
3 1
Switch-on command is cancelled, motor is
switched off
to t ), the current is conducted via power semiconductors which
3
to t ). It may take longer for the motor to actually coast down
4 5
t
5
t
6
U
S S
U
e
n
e
1.3 Operating principle
). The internal bypass contacts close
2
) until the end of the
5
End of the set ramp-down time
(t -t )
5 4
Motor at standstill
Set starting voltage
Rated operational voltage
Rated motor speed
Description
).
1
) and
4
9