Soft Starter vs VFD: Which One Does Your Motor Actually Need?
The short version: if you only need to control how a motor starts and stops, a soft starter does that job for a fraction of the cost and complexity of a variable frequency drive. If you need to control speed while the motor is running — not just during start-up — you need a VFD, full stop. Buying a VFD purely for a soft-start function is a common and expensive over-specification; buying a soft starter when the process actually needs variable speed control simply won't work.
This guide compares what each device actually does, where the cost and complexity trade-offs genuinely matter, and how to decide which one fits your application before you spec either one.
What Each One Actually Does to the Motor's Supply
A soft starter works by controlling the voltage applied to the motor during start and stop, using thyristors to ramp voltage up gradually instead of slamming full voltage on at once. The motor still runs directly from the fixed-frequency mains supply once started — the soft starter's job is finished at that point, and speed is whatever the motor's natural running speed is at that frequency. A VFD works completely differently: it converts the incoming AC supply to DC and then synthesises a new AC output at a frequency and voltage it controls, which is what lets it vary motor speed continuously while running, not just during start-up.
Side-by-Side Comparison
| Feature | Soft Starter | VFD |
|---|---|---|
| Controls running speed | No — motor runs at line frequency once started | Yes — continuously variable |
| Reduces starting current/torque | Yes | Yes, and throughout the run |
| Typical cost vs VFD | Significantly lower | Higher, especially at larger kW |
| Energy saving on part-load/variable-demand duty | Minimal — no running speed control | Significant, especially on fans/pumps |
| Harmonics/EMI generated | Lower | Higher — often needs filtering |
| Typical application | Pumps, fans and conveyors needing gentle start/stop only | Process needing variable flow, speed or torque control |
Three Questions That Settle Which One You Need
Does the Process Need Variable Speed, or Just a Gentle Start?
If the equipment only ever needs to run at one fixed speed once it's up and running — many conveyors, some fixed-duty pumps — a soft starter delivers the mechanical stress reduction and reduced inrush current at a fraction of the cost. If the process genuinely needs to vary flow, speed, or torque during operation, only a VFD can do that.
Is Energy Saving on Part-Load Operation a Real Driver?
Centrifugal fans and pumps follow the affinity laws, where power consumption drops roughly with the cube of speed reduction — a VFD running such a load at 80% speed can cut energy use dramatically compared with a fixed-speed motor throttled by a damper or valve instead. If the load spends significant time at less than full demand, a VFD often pays for the cost difference over a soft starter through energy savings alone.
Can the Installation Tolerate the Extra Harmonics and EMI a VFD Introduces?
A VFD's switching output introduces harmonic distortion and electromagnetic interference that a soft starter largely avoids, since a soft starter doesn't synthesise a new waveform. Sensitive nearby equipment, long cable runs to the motor, or a supply already close to harmonic distortion limits can all push the balance back toward a soft starter, or require additional filtering if a VFD is genuinely needed.
Frequently Asked Questions
Can a VFD do everything a soft starter does, just at higher cost?
Functionally, yes — a VFD can ramp a motor up gently just like a soft starter, plus control speed while running. If cost, panel space, and minimising harmonics aren't concerns, a VFD is never the wrong technical choice — it's a question of whether that extra capability and expense is actually needed.
Does a soft starter save any energy at all?
Some soft starters include an energy-saving mode that reduces voltage to a lightly loaded motor, but the saving is modest compared with what a VFD achieves by actually reducing speed on a variable-demand load. Don't specify a soft starter expecting VFD-level energy savings.
Do I need extra filtering with a soft starter like I might with a VFD?
Generally no — a soft starter doesn't synthesise a switched waveform the way a VFD does, so harmonic distortion and EMI are much lower and additional filtering is rarely needed. This is one of the genuine simplicity advantages of a soft starter where variable speed isn't required.
Can I retrofit a VFD onto a motor currently running direct-on-line or via a soft starter?
In most cases yes, provided the motor is inverter-duty rated or the installation includes appropriate cabling and, where cable runs are long, output filtering to protect the motor windings from voltage reflections. Older motors not rated for inverter duty may need insulation checks before being run from a VFD.
Which one reduces mechanical stress on the driven equipment better?
Both reduce start-up mechanical shock significantly compared with direct-on-line starting. A VFD has a slight edge because it can also control acceleration and deceleration ramps with more precision, and can hold a load at reduced speed rather than only ramping through to full speed.
Related Reading
Motor starting method affects how the rest of the motor circuit is protected — these guides cover the components either option is typically paired with:
Shop the Parts
| Product | Use | Link |
|---|---|---|
| ABB PSR Soft Starter, 5.5kW, 3-Phase | Gentle start/stop on smaller fixed-speed motors | View product → |
| ABB PSTX Soft Starter, 90kW, 3-Phase | Larger motors needing controlled start/stop only | View product → |
| ABB ACS480 Inverter Drive, 11kW, 3-Phase | Full variable speed control for pumps, fans and process loads | View product → |
| Full soft starter range | Browse by motor power, voltage and enclosure rating | Browse soft starters → |
| Full inverter/VFD range | Browse by power rating, phase and control features | Browse inverters → |
| All genuine ABB products | Soft starters, drives and motor control components from the same manufacturer | View all ABB products → |
Disclaimer: ABB, and any other manufacturer or brand names referenced in this article, are used solely for identification and compatibility purposes. Z&F Corporation is not affiliated with, authorised by, or an official representative of these manufacturers; all trademarks are the property of their respective owners.
