A direct-on-line start applies full mains voltage to the motor the moment the contactor pulls in. That is simple and inexpensive, but it also delivers six to eight times the rated current instantly, produces a hard torque jolt through the drivetrain, and stresses the supply. On larger pumps, fans, and compressors those stresses add up to belt wear, pipe hammering, torque-tube fatigue, and dips in the mains. A soft starter solves this by ramping the voltage up and down in a controlled way. The Siemens SIRIUS 3RW4047-2TB04 is a size S3 soft starter rated for 106 A and 55 kW at 400 V. This guide covers its verified ratings, how it starts and stops a motor, wiring, and where it fits against a direct-on-line starter.
What Is the 3RW4047-2TB04?
The 3RW4047-2TB04 belongs to the SIRIUS 3RW40 range of basic-performance soft starters, built for standard starting and stopping of three-phase asynchronous motors. Instead of switching the line directly, it uses back-to-back thyristors to control the voltage applied to the motor during start and ramp-down, then carries the running current through an integrated bypass contactor. This means the power semiconductors only work during the short start and stop phases, which keeps heat loss low in continuous operation. The "-2TB04" variant uses spring-type terminals and a 24 V AC/DC control supply, and includes thermistor motor protection.
Verified Specifications
| Parameter | Specification |
|---|---|
| Order number | 3RW4047-2TB04 |
| Product family | SIRIUS 3RW40 soft starters |
| Frame size | S3 |
| Rated operational current Ie (40 °C) | 106 A |
| Rated operational current (50 °C / 60 °C) | 98 A / 90 A (derated) |
| Rated operational voltage Ue | 200 ... 480 V AC |
| Rated motor power, inline (400 V) | 55 kW |
| Rated motor power, inline (230 V) | 30 kW |
| Control supply voltage | 24 V AC/DC |
| Power circuit terminals | Spring-type terminals |
| Controlled phases | Two-phase controlled |
| Starting current limit | Approx. 3 ... 5 x Ie (adjustable) |
| Integrated bypass contactor | Yes |
| Thermistor motor protection | Integrated |
| Degree of protection | IP00 (open device, panel-mounted) |
| Network type | Three-phase (3 AC) |
How Soft Start and Soft Stop Work
On a start command, the 3RW40 ramps the motor voltage up from a set starting level over an adjustable start time rather than applying full voltage at once. Because torque scales with the square of the voltage, the motor accelerates gently and the inrush current is limited to roughly three to five times rated current instead of six to eight. Once the ramp is complete, the internal bypass contactor closes and carries the running current, taking the semiconductors out of circuit. On stop, a soft-stop ramp brings the voltage down gradually, which is especially useful on pumps where it reduces water hammer and on conveyor belts where it avoids a sudden halt. The adjustable ramp times and starting voltage are set with the onboard dials, and the built-in electronic overload protection plus thermistor input monitor the motor during running.
Wiring and Control
The power circuit is connected in series with the motor on a three-phase supply within 200 to 480 V. The control circuit runs from a 24 V AC/DC supply, so the start/stop commands and status contacts sit on a safe extra-low control level rather than the main voltage. Spring-type terminals speed up, vibration-resistant wiring without screwdriver tightening. For correct protection, coordinate the upstream short-circuit protection with the soft starter's rated short-time withstand, and connect the motor's PTC thermistor (where fitted) to the dedicated input so over-temperature switches the motor off. As with any power-electronic device, observe the minimum and maximum motor current allowed and the clearance between the starter and adjacent heat-sensitive components.
Soft Starter vs Direct-on-Line
| Factor | Direct-on-line (DOL) | SIRIUS 3RW40 soft starter |
|---|---|---|
| Starting current | 6 ... 8 x Ie | 3 ... 5 x Ie, limited |
| Starting torque | Full torque immediately | Ramped up gradually |
| Mechanical stress | High torque jolts | Low, smooth acceleration |
| Supply demand | Large inrush / voltage dip | Lower peak demand |
| Pump stop | Abrupt (risk of water hammer) | Adjustable soft stop |
| Running losses | Contactor only | Bypass carries running current |
| Cost / complexity | Lower, simpler | Higher, more functions |
A soft starter is worthwhile wherever the inrush, torque shock, or abrupt stop causes real problems; for small, light loads a simple contactor remains the economical choice.
Typical Applications
- Centrifugal and positive-displacement pumps where water hammer must be avoided
- Fans and blowers with long acceleration ramps and belt drives
- Compressors, conveyors, and agitators that must start smoothly
- Applications on weak supplies where inrush dips are a concern
- Retrofits where a DOL start is causing mechanical or supply issues
You can compare related motor-control hardware in our Siemens range and the wider drives and soft-starter collection.
Lifecycle and Spare Parts
The 3RW40 is an established SIRIUS range used on a great many installed machines. Soft starters on critical pumps and fans are worth holding as spares, because a failed unit can strand a high-value load. When a specific 3RW4047 variant is no longer readily available, our Discontinued Products range and sourcing team can help locate replacements; send us the part number and we will confirm what we can supply and any compatible alternatives.
Frequently Asked Questions
How big a motor does the 3RW4047-2TB04 control?
It carries 106 A at 40 °C, corresponding to a 55 kW three-phase motor at 400 V (about 30 kW at 230 V). At higher ambient temperatures the current is derated to 98 A at 50 °C and 90 A at 60 °C.
Does it need a separate bypass contactor?
No. The 3RW4047-2TB04 has an integrated bypass contactor that carries the running current after starting, so no external bypass contactor has to be wired in.
What control voltage does it use?
This variant is controlled from a 24 V AC/DC supply, which keeps the start/stop and status circuits on a safe control level separate from the 200 to 480 V main circuit.
How is it different from a variable-frequency drive?
A soft starter only controls voltage during start and stop, then runs the motor at full line speed. A VFD varies both voltage and frequency to control speed the whole time. For starting and stopping without speed control, the soft starter is the simpler and lower-cost device.
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