Soft Starter vs VFD vs Across-the-Line: Which Starter?
Every motor needs something to start it. The question is whether the application needs speed control, inrush control, or just a contactor and overload. The wrong answer either wastes budget on a drive the process never modulates - or hammers your gearboxes and demand meter with 600% inrush six times an hour.
The Three Options
| Across-the-Line | Soft Starter | VFD | |
|---|---|---|---|
| Inrush current | ~600% FLA | Ramped, typ. 250–400% | ~100–150% |
| Mechanical shock | Full torque snap | Controlled ramp | Fully controlled |
| Speed control | None | None (start/stop only) | Full range |
| Relative cost | $ | $$ | $$$ |
| Fits when… | Small motors, stiff supply, infrequent starts | Big inertia, weak supply, frequent starts, no speed need | Process needs variable flow/speed |
When a Soft Starter Is the Right Call
High-inertia loads - crushers, large fans, centrifuges - and generator- or rural-fed sites where across-the-line inrush sags the bus. A soft starter ramps voltage so torque builds smoothly, then closes an internal bypass and runs the motor at full speed with negligible loss. If the process runs at one speed, a soft starter delivers the mechanical benefits of a drive at a fraction of the cost and panel space. We stock chassis soft starters (Siemens 3RW40, 3RW50, and 3RW52-based WSSD lines) plus enclosed packages wired with disconnects.
When to Spend on the VFD
Any load whose demand varies - pump flow, fan CFM, conveyor rate. Speed reduction on centrifugal loads cuts power by the cube of speed, which no starter can do. See the VFD pairing guide for sizing. For plain small-motor duty, WWSXL general purpose starters and Siemens IEC contactors with thermal overloads remain the economical, bulletproof answer.
NEMA Starter Sizes at a Glance
| NEMA Size | Max HP @ 230V 3φ | Max HP @ 460V 3φ |
|---|---|---|
| 00 | 1.5 | 2 |
| 0 | 3 | 5 |
| 1 | 7.5 | 10 |
| 2 | 15 | 25 |
| 3 | 30 | 50 |
| 4 | 50 | 100 |
| 5 | 100 | 200 |
Frequently Asked Questions
Does a soft starter save energy like a VFD?
No. Once up to speed, a bypassed soft starter passes utility power straight through - it saves demand charges and mechanical wear at start, not running energy. Only speed reduction on a variable load saves running energy, and that requires a VFD.
Can a soft starter reduce my demand charges?
Often, yes. Utilities bill peak demand, and 600% across-the-line inrush on a large motor can set that peak. Ramping the start to 300% or less flattens the spike - one of the most common paybacks on 50+ HP motors started multiple times per day.
How do I pick between NEMA and IEC starters?
NEMA starters are conservatively sized by standard sizes (00-5) and tolerate application abuse; IEC contactors are sized tighter to the specific load and cost less per amp. Both are stocked - NEMA-style WWSXL for general purpose, Siemens IEC lines where panel space and price rule.
What protection does a starter actually provide?
The contactor switches the motor; the overload relay protects it. Thermal or electronic overloads track motor heating from current draw and trip before the windings cook - set them to nameplate FLA. Short-circuit protection comes separately from fuses or the breaker.
Can I retrofit a soft starter into an existing starter panel?
Usually. Chassis soft starters in the 3RW40/3RW50 families mount in place of (or beside) the contactor and reuse existing overload and control wiring; verify enclosure heat rise and keep the bypass contactor if the panel already has one.
Related Resources
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