Description
1.3 Operating principle
SIRIUS 3RW50 soft starter
Manual, 09/2019, A5E35628455002A/RS-AA/001
11
1.3
Operating principle
Soft starters are used to start three-phase induction motors with reduced torque and reduced
starting current.
The 3RW50 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
(switching elements) which start the motor up smoothly.
The 3RW50 soft starter features internal run-up recognition. If a successful motor run-up is
detected before the ramp up time elapses, 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 3RW50 soft starter is 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. As long as the stopping time is active (t
4
to t
5
), energy is still supplied to
the motor. It may take longer for the motor to actually coast down to a standstill (t
6
).
①
Speed
t
5
End of the set stopping 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 operating speed n
e
reached
U
e
Rated operational voltage
t
3
End of the set ramp up time
(t
3
-t
1
)
n
e
Rated operating speed of the motor
t
4
Switch-on command is cancelled, motor is
switched off
