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. Talk to Our Expert Team 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. Comparison of a soft starter, which only ramps voltage during start and stop, against a VFD, which continuously controls both voltage and frequency Soft starter: thyristors ramp voltage up at start, then the motor runs directly from fixed-frequency mains. VFD: rectifies AC to DC and inverts it to a variable-frequency output, controlling speed continuously while running. Soft Starter Mains Thyristors Motor Ramps voltage during start/stop only Fixed frequency while running No speed control once started VFD Mains AC→DC DC→AC Motor Rectifies then re-synthesises supply Variable frequency continuously while running Full speed control, not just start Side-by-Side Comparison Feature Soft Starter VFD Controls running speedNo — motor runs at line frequency once startedYes — continuously variable Reduces starting current/torqueYesYes, and throughout the run Typical cost vs VFDSignificantly lowerHigher, especially at larger kW Energy saving on part-load/variable-demand dutyMinimal — no running speed controlSignificant, especially on fans/pumps Harmonics/EMI generatedLowerHigher — often needs filtering Typical applicationPumps, fans and conveyors needing gentle start/stop onlyProcess needing variable flow, speed or torque control Three Questions That Settle Which One You Need 1 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. 2 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. 3 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 …