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Siemens Soft Starters

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Siemens SIRIUS soft starter: 3RW30, 3RW40 or 3RW44 for your motor

Customers buy Siemens from us in two camps. First: plants that already run other Siemens automation (S7-1200/1500, ET 200, Sinamics drives) and need to keep one ecosystem through TIA Portal. Second: those who buy SIRIUS specifically because the cabinet already has a Siemens 3RT contactor and the wiring logic must stay. We stock three series: 3RW30 (up to 55 kW), 3RW40 (up to 250 kW), and 3RW44 (top, up to 710 kW). This year's typical customer: a plant modernization engineer whose old magnetic contactor keeps burning out from heavy mixer starts.

How to choose between the series. We ship 3RW30 when you need a cheap reliable compact for a fan; a low-kW pump; a belt conveyor without frequent starts. No fieldbus, just a relay output; setup via DIP switches in 5 minutes. We ship 3RW40 when you need serious motor protection (overload, imbalance, rotor lock) plus adjustable ramp-up/stop times; it has RS-485 for basic SCADA. We ship 3RW44 when the load is heavy (crusher; ball mill; piston compressor) or when you need Profibus DP / Profinet for full TIA Portal integration. 3RW44 has built-in bypass, so the panel runs cooler plus takes less space.

What we ask before the order

First: motor FLA from the nameplate plus power. Not the spec sheet, the actual nameplate, because real values drift by 5-15%. Second: what automation you already run. If it is TIA Portal with ET 200, we go straight to 3RW44 with Profinet, because reworking other series later costs more. Third: starts per hour (3RW30 likes rare; 3RW44 handles up to 6 per hour at S4 50%). Fourth: panel temperature, because above 40°C you need to derate.

This year's best-sellers from our warehouse for typical jobs: 3RW3016-1BB14 (9 A, 4 kW, domestic plus small pumps); the 3RW30 series for boiler rooms plus ventilation units; 3RW40 at 75-110 kW for compressors plus conveyors. Siemens warranty 24 months. 3RW30/3RW40 ship from our Kyiv warehouse in 1-3 days, some 3RW44 frame sizes ship to order (5-15 days). Datasheet available before payment, plus we walk you through parameter setup via the A button.

Frequently Asked Questions

What is the difference between 3RW30, 3RW40 and 3RW44?

Current rating, plus what each one can actually do. Our 3RW30 range covers 6 to 106 A. DIP-switch setup, a relay output, and that is it. The 3RW40 runs 12 to 432 A and adds overload protection, phase-imbalance protection, locked-rotor detection, plus RS-485. The 3RW44 starts at 551 A and holds up to 1214 A, with a built-in bypass (cooler cabinet) and Profibus DP / Profinet. If the cabinet already runs TIA Portal with ET 200, go straight to 3RW44. Retrofitting later costs more.

How do I size a Siemens soft starter by motor current?

Read the motor nameplate, not the machine datasheet. Real figures drift by 5-15%, and we see it almost weekly. The device rated current must be at least the motor current. For heavy starts (crusher, ball mill, piston compressor) go one frame size up. In our catalogue the 3RW30 covers 6-106 A, the 3RW40 gives 12-432 A, and the 3RW44 works in the 551-1214 A band.

How do I set up a Siemens SIRIUS soft starter?

On the 3RW30 it is two DIP switches: ramp time and initial voltage. About five minutes of work. On the 3RW40 you turn potentiometers and set the protection parameters. The 3RW44 is configured from its display panel or straight out of TIA Portal. A sensible starting point for a pump: 10-15 s ramp, 30-40% initial voltage. For a loaded conveyor raise the initial voltage, otherwise the motor simply will not break away. We send the datasheet before payment.

What is the starting current with a soft starter?

A direct-on-line start draws 6-8 times rated current. A soft starter brings that down to 3-4 times. On light loads such as a fan or centrifugal pump you get 2.5-3. It will never reach unity. The motor still needs torque to break the machine away. If you need to hold current at rated level, that is a job for a VFD.

Does SIRIUS replace a variable frequency drive?

No. A soft starter removes the inrush, gives a soft stop, and there its job ends. Once up to speed the motor runs straight off the mains at 50 Hz. It cannot vary speed, hold pressure with a PID loop or save energy at partial load, by design. If you need speed control, take a Sinamics drive or any VFD from our catalogue.

How is a Siemens soft starter wired and is a contactor needed?

The layout is standard. Breaker or fuses on the line side, then the soft starter, then the motor. A line contactor gives galvanic isolation and emergency disconnect: it is external on the 3RW30 and 3RW40, while the 3RW44 has a built-in bypass. Control voltage is 24 V or 230 V depending on the version. We supply the wiring diagram and the series manual with the invoice.

How many starts per hour does SIRIUS handle?

It depends on the series and cabinet temperature. The 3RW30 is built for infrequent starts, that is its ceiling. The 3RW40 handles more thanks to better heat dissipation. The 3RW44 with its bypass manages up to 6 starts per hour at S4 50%. One more catch. Above 40 °C inside the enclosure you need derating, so we ask about temperature before quoting.

What is in stock and what is the lead time?

Most 3RW30 and 3RW40 frame sizes ship from stock in 1-3 days. Some 3RW44 positions are brought in to order, that is 5-15 days. Catalogue prices run from UAH 7,039 for the 3RW3013-1BB14 at 6 A up to UAH 134,784 for the 3RW4076-6BB44 at 432 A. Siemens warranty is 24 months.