SIRIUS 3RW40 soft starter

Both are two-phase and have an integral bypass. The 3RW40 adds soft stop (up to 20 s), current limiting by potentiometer, ramp-up detection, electronic overload protection with trip classes 10, 15 and 20, intrinsic thyristor protection and, on the 24 V version, a thermistor input. The reference conditions for normal starting also differ: 10 s and 5 starts per hour for the 3RW40 against 3 s and 20 starts per hour for the 3RW30; the Siemens catalogue presents the 3RW30 for easy starting conditions and high switching frequencies.

For overload protection, yes. The motor current is measured by integral current transformers and the device trips on a thermal model and the selected class; after a trip a recovery time of 5 minutes is needed. Siemens recommends combining the thermal model with the motor thermistor (type A PTC or Thermoclick, on the 24 V AC/DC version) for full protection. Short-circuit protection is still external: fuses or a motor starter protector, and SITOR semiconductor fuses for type of coordination 2.

The 3RW40 now stops at 55 kW. Sizes S6 and S12 (3RW405, 3RW406 and 3RW407) are listed as phased out in Industry Mall and Siemens names the SIRIUS 3RW5 as their successor: 3RW4055-6BB44 leads to 3RW5055-6AB14 and 3RW4075-6BB34 to 3RW5075-6AB14, for example. The successor depends on the code suffix (3RW4055-6BB45 leads to 3RW5055-6AB15) and current and control supply are not exactly equal, so check the data sheet before swapping. The 3RW50 goes up to 315 kW; for more power or heavier duty the 3RW55 is the better fit.

According to the Siemens manual the device must be at least one power class larger than the motor (reference conditions: at most 20 s, 300 % current limiting, 5 starts per hour, 30 % ON time). At CLASS 20 the settable motor current is also lower; on the 3RW4047 with 106 A it is at most 77 A. For motors with a high starting current (about 8 times rated current or more) Siemens suggests product version E07 or later in sizes S0, S2 and S3 and asks for a support request above that. Beyond CLASS 20 (CLASS 30) only the 3RW55 is recommended.

Sizes S0 to S3 are approved for starting explosion-proof motors with the increased-safety type of protection Ex eb (BVS 05 ATEX F 002 X and IECEx BVS 24.0016 X), and its overload protection is used for that purpose. The device itself is not suitable for installation in a hazardous area and must sit in a cabinet of at least IP4x outside it; the Auto RESET setting is not permitted for motors in hazardous areas, and approved devices may be repaired only at the manufacturer.

With the ramp-down potentiometer (0 to 20 s) the motor torque falls gradually and water hammer is reduced compared with direct or star-delta stopping. In the Siemens selection table, however, a pump with a special stop against water hammer is recommended only for the 3RW55 with its pump stop function. During soft stop the current exceeds the rated motor current, so the starter, cables and feeder protection may need to be sized higher.

When the motor has to run at different speeds in operation, or the process needs speed and torque accuracy. A soft starter only softens starting and stopping. When variable speed is not needed Siemens regards the soft starter as the first choice: less capital, space and maintenance, and simpler installation.