At a Glance. The Siemens SINAMICS G120C and the SIRIUS 3RW55 both sit next to a motor, but they solve different problems. A VFD (variable frequency drive) controls the motor continuously — speed, torque, ramp — and is the only choice when the process needs variable flow, pressure or speed. A soft starter only shapes the start and stop: it eliminates inrush current and mechanical jerk, then bypasses itself and lets the motor run at full speed. The selection rule is simple: need to change speed during operation → G120C; constant-speed motor that just needs a gentle start → 3RW55, at roughly a third of the price.
1. Head-to-Head Specification Table
| Specification | SINAMICS G120C (6SL3210-5BE27-5UV0) | SIRIUS 3RW55 (3RW5513-1HA14) |
|---|---|---|
| Device type | Variable frequency drive (VFD) | Soft starter |
| Power rating | 7.5 kW | 15 kW @ 400 V |
| Input voltage | 380-480 V, 3-phase | 400 V class, 3-phase |
| Speed control | 0-400 Hz, V/f and vector control | None (full speed after start) |
| Integrated bypass | — | Yes (SCRs out of circuit after start) |
| Built-in EMC filter | Class A | — |
| Safety function | Safe Torque Off (STO), SIL 3 | — |
| Communication | PROFINET, PROFIBUS, EtherNet/IP | PROFINET |
| Protection class | IP20 | IP20 |
| Standard | — | IEC 60947-4-2 |
| Stock page | G120C | 3RW55 |
2. What the VFD Does That the Soft Starter Cannot
The G120C regulates motor speed continuously between 0 and 400 Hz with V/f and vector control. This unlocks four capabilities no soft starter offers:
- Process control: holding a constant pressure, flow or tension by modulating speed in closed loop;
- Energy savings on centrifugal loads: for fans and pumps, power scales with the cube of speed — running at 80% speed cuts consumed power by roughly half;
- Precise stopping: controlled deceleration, braking and positioning;
- Integrated safety: STO to SIL 3 removes the need for a separate contactor-based safety chain.
In a full SIMATIC stack, the G120C talks PROFINET directly to a CPU 1511-1 PN or CPU 1513-1 PN and is operated from a KTP700 panel — one engineering environment, no gateway.
3. What the Soft Starter Does Better
Most motors in a plant — conveyors at fixed duty, mixers, saws, workshop pumps — run at one speed all day. For them, the 3RW55 delivers everything needed:
- Starting duty solved: voltage ramp and current limit remove the 6-8× inrush of direct-on-line starting, protecting belts, couplings and the upstream network;
- Integrated bypass: after the ramp, the SCRs are bypassed, so there are no drive harmonics on the network and no cooling requirement at steady state;
- Lower cost and panel space: a soft starter for the same motor costs roughly one third of a VFD and mounts in a fraction of the panel volume;
- Pump and fan stopping modes: the 3RW55 also provides controlled deceleration for water hammer prevention — the second-most common reason it is chosen.
For direct-on-line circuits that need only basic switching and protection, the pairing of the SIRIUS 3RT2026 contactor and 3RV2011 motor protector remains the reference build.
4. Selection Rule by Application
| Application | Recommended device |
|---|---|
| Fan or pump with varying demand (HVAC, water, process) | G120C — cube-law energy savings pay back the price gap |
| Conveyor, mixer, compressor at constant speed | 3RW55 — soft start, bypass, minimal cost |
| High-inertia start, long ramps | G120C (vector control holds torque at low speed) |
| Water hammer / pipe stress on stop | 3RW55 soft stop mode |
| Safety-rated stopping required at drive level | G120C (STO SIL 3) |
| Budget DOL replacement with network protection | 3RV2011 + 3RT2026 |
5. Replacement and Migration Notes
Two recurring replacement requests are worth noting:
- 3RW30/3RW34 legacy soft starters: the 3RW55 is the direct modern successor — same SIRIUS mounting philosophy, added PROFINET and integrated bypass, so panel modification is minimal.
- G120 (PM240-2) compact alternative: where the G120C's fixed 7.5 kW frame does not match the motor, the G120 family offers modular power ratings; for standard 380-480 V fans and pumps up to 7.5 kW the G120C is usually the lowest total-cost choice because the filter, choke and communication are built in.
Send Glorient your motor rating (kW, voltage, duty) and panel layout, and we will quote the matching G120C order number or 3RW55 frame from stock.
6. FAQ
Can a soft starter save energy like a VFD?
No. Once bypassed, a soft starter has no influence on running speed or power. Only a VFD saves energy on centrifugal loads by slowing the motor. If the motor runs at constant speed, there is no saving to harvest — and the soft starter's bypass even eliminates drive losses.
Does the 3RW55 replace the motor protector?
No. The 3RW55 handles starting duty; overload and short-circuit protection is still provided by a motor starter protector such as the 3RV2011 upstream, per IEC 60947-4-2 coordination.
Which one for a 5.5 kW water pump with a pressure setpoint?
The G120C: a pressure setpoint is a process-control task requiring continuous speed modulation, and the energy saving at partial flow will usually recover the price difference within one to three years.
Need a quote? Browse in-stock Siemens drives, soft starters and control gear — SINAMICS, SIRIUS, SIMATIC and SCALANCE — or send your motor list for a same-week quotation.