Siemens SIRIUS 3RW52 - Operating principle

Description
1.3 Operating principle
SIRIUS 3RW52 Soft Starter
Manual, 01/2019, A5E35630451002A/RS-AD/004
9
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
1
).
During the ramp-up time (t
1
to t
3
), the current is conducted via power semiconductors which
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
2
). The internal bypass contacts close
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
4
) and
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
5
) until the end of the
ramp-down time is reached (t
4
to t
5
). It may take longer for the motor to actually coast down
to a standstill (t
6
).
①
Speed t
5
End of the set ramp-down time
(t
5
-t
4
)
②
Voltage
t
6
Motor at standstill
t
1
Motor start-up with set starting voltage U
S
U
S
Set starting voltage
t
2
Rated speed n
e
achieved
U
e
Rated operational voltage
t
3
End of the set ramp-up time
(t
3
-t
1
)
n
e
Rated motor speed
t
4
Switch-on command is cancelled, motor is
switched off