Operating Principle - Siemens SIRIUS 3RW55 Equipment Manual

3RW55 Failsafe soft starter
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3.3

Operating principle

Soft starters are used to start three-phase induction motors with reduced torque and reduced
starting current.
The 3RW55 and 3RW55 Failsafe soft starters start the motor as soon as the switch-on command
is issued (t
semiconductors (switching elements), which start the motor smoothly.
The 3RW55 and 3RW55 Failsafe soft starters feature internal start-up recognition. If the 3RW55
or 3RW55 Failsafe soft starter detects a motor start-up before the ramp-up time elapses, the
motor voltage is immediately increased to 100% of the line voltage (t
contacts close and the power semiconductors are bypassed. The 3RW55 or 3RW55 Failsafe soft
starter is in bypass mode.
SIRIUS 3RW55 and 3RW55 Failsafe Soft Starters
Equipment Manual, 02/2022, A5E35630887002A/RS-AF/006
Main circuit connection (line power supply)
3RW5 HMI High Feature
Diagnostics LEDs
Eye for lead seal
Slot for 3RW5 communication module (accessory)
Connectable conductor cross sections
You will also find the conductor cross-sections that can be connected in the Technical specifica‐
tions (Page 285).
Main circuit connection (motor)
Control terminals (inputs/outputs)
Control cable duct with cover
). During the ramp-up time (t
1
to t ), the current is conducted via power
1 3
Description
3.3 Operating principle
). The internal bypass
2
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