Dc Braking; Dc Braking With External Braking Contactors - Siemens SIRIUS 3RW55 Equipment Manual

Functions
8.2 Run-down
8.2.6

DC braking

8.2.6.1

DC braking with external braking contactors

Operating principle
In the case of DC braking, coasting down or the natural stopping of the load is shortened by
electrical braking using a brake contactor. The 3RW55 soft starter impresses a (pulsating) direct
current in phases L1 and L2 on the motor stator. This current generates a permanent magnetic
field in the stator. Since the rotor is still rotating due to its mass inertia, currents are induced in
the short-circuited rotor winding that generate a DC braking torque. If the braking process is not
completed on expiry of the stopping time, the motor coasts down.
To guarantee a premature cancellation of the braking process, the "External motor standstill
detection (Page 162)" or "Sensorless motor standstill detection (Page 162)" function can be used
to detect motor standstill before the stopping time has elapsed. As motor protection, the use of
the thermistor motor protection with temperature sensor (Page 169) is recommended.
If you use a 3RW55 soft starter with article number "3RW551.‑.....", "3RW5521‑....." or
"3RW5524‑....." with a rated operational voltage U
stopping mode "DC braking" with an external braking contactor. Use 2 external braking
contactors for 3RW55 soft starters from article number "3RW5525‑....." or for a rated operational
voltage U
Note the technical data of the braking contactors. You will find further information on braking
contactors in Catalog IC 10 (https://support.industry.siemens.com/cs/ww/en/view/109747945).
Size
Size 1
Size 2
Size 3
Size 4
Size 5
Application
• Stopping in applications with large mass inertia (centrifugal masses): J
• Lathes (e.g. on a tool change)
• Circular saws
• Punches
• Conveyor belts
154
larger than 400 V.
e
Article number of the
3RW55 soft starter
3RW551.-.....
3RW5521-.....
3RW5524-.....
3RW5525-.....
3RW5526-.....
3RW5527-.....
3RW553.-.....
3RW554.-.....
3RW555.-.....
up to 400 V, you can only implement the
e
Number of required external
braking contactors up to 400 V
1 or 2
2
SIRIUS 3RW55 and 3RW55 Failsafe Soft Starters
Equipment Manual, 02/2022, A5E35630887002A/RS-AF/006
Number of required exter‐
nal braking contactors
higher than 400 V
2
≤ 5 x J
load motor