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1.3
Operating principle
Soft starters are used to start three-phase induction motors with reduced torque and reduced
starting current.
The 3RW51 soft starter starts the motor as soon as the switch-on command is issued (t
During the ramp-up time (t
starts up the motor smoothly.
The 3RW51 soft starter features internal run-up recognition. The following operation occurs
when the motor reaches its rated operating speed before the ramp-up time expires:
● The motor voltage is immediately increased to 100 % of the line voltage (t
● The external bypass contactor close
● The power semiconductors are bypassed and the soft starter is 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
3RW51 Soft Starter
Manual, 06/2019, A5E37108631002A/RS-AB/002
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
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
-t
)
n
3
1
e
1.3 Operating principle
) until the end of the
5
End of the set ramp-down time (t
Motor at standstill
Set starting voltage
Rated operational voltage
Rated motor speed
Description
).
1
)
2
) and
4
-t
)
5
4
9
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