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VFD vs soft starter: which motor starter to spec
Both soften the start. Only the VFD controls the run and saves energy at part load.
Short answer
Pick a soft starter if the load runs at one fixed speed and just needs a gentle start or a soft stop. Pick a VFD if the load benefits from running slower part of the time. The single deciding factor is speed control: a soft starter only ramps voltage to start and stop, then the motor runs at full line speed, while a VFD varies frequency so it runs the motor at any speed and saves energy on variable pump and fan loads. Do not buy a VFD only to soften the start on a constant-speed load, and do not expect a soft starter to give variable speed, because it cannot.
VFD vs Soft starter: side by side
| Factor | VFD | Soft starter |
|---|---|---|
| Speed control | Full variable speed by changing frequency | None; starts and stops only, then runs at full line speed |
| Starting torque | Full torque at low speed, almost no inrush | Reduced, ramped; torque falls with voltage squared |
| Inrush at start | Almost none; little more than running current | Limited and adjustable, commonly set 150 to 350 percent FLA |
| Upfront cost | Highest of all starting methods | Cheaper, smaller, simpler than a VFD |
| Install complexity | Reflected-wave, cable length, bearing currents, shaft grounding, inverter-duty motor, cooling | Nothing on the output; after bypass the motor runs on a clean line sine wave |
| Energy savings | Large on centrifugal loads; power drops roughly with cube of speed | None; motor runs at full speed once started |
| Soft stop | Yes, plus controlled deceleration | Yes; ramps voltage down to kill pump water hammer |
| Frequent starts | Nearly eliminates start heating; highest starts-per-hour headroom | Limits start current, so easier on the motor than DOL |
| Best use | Variable-speed pumps, fans, and processes that run at part load | Constant-speed loads needing a gentle start or soft stop |
Which should you pick?
Choose VFD when
- The load benefits from running slower part of the time, such as a pump, fan, or process that varies.
- Part-load energy savings will pay for the drive, since power on a centrifugal load drops roughly with the cube of speed.
- You need full breakaway torque at low current that no reduced-voltage method can deliver.
- You can accept the install work: cable-length limits, bearing currents and shaft grounding, an inverter-duty motor, and cooling.
Choose Soft starter when
- The load runs at one speed and only needs a gentle start.
- You want a soft stop to prevent water hammer when a pump check valve slams.
- You want a controlled, adjustable ramp in a small package for less money than a VFD.
- The load starts easily, like a centrifugal pump or fan that unloads at low speed.
Bottom line
It depends on whether the load ever needs to run at less than full speed. If it does, the VFD wins because the soft start comes free with the speed control and the energy savings usually cover the extra cost on a variable load. If the load runs at one fixed speed, the soft starter starts it for less money, less panel space, and nothing on the output to worry about. Both soften the start; only the VFD controls the run. Buying a VFD just to soften a constant-speed start is paying for capability you will never use, and asking a soft starter to run at variable speed is asking for something it physically cannot do. Note that neither device is the motor protection: the overload relay still has to be set to the nameplate and a separate short-circuit device sized for the available fault current, per NEC Article 430.
FAQ
What is the main difference between a soft starter and a VFD?
Speed control. A soft starter only ramps the voltage to start and stop the motor gently at the fixed line frequency, then the motor runs at full line speed. A VFD varies the frequency, so it starts with full torque and almost no inrush and then runs the motor at any speed you set, which saves energy on variable loads. Both soften the start; only the VFD controls the run.
When should I use a soft starter instead of a VFD?
Use a soft starter when the load runs at one speed and only needs a gentle start, or a soft stop to kill pump water hammer. It is cheaper, smaller, and simpler, with nothing on its output to worry about because after bypass the motor runs on a clean line sine wave. Use a VFD only when the load benefits from variable speed and the energy savings.
Can a soft starter save energy like a VFD?
No. A soft starter limits current during the start, then bypasses to full line power and the motor runs at full speed like any other. Energy savings come from running slower, and only the VFD does that. On a centrifugal pump or fan, slowing the motor cuts power roughly with the cube of speed, which is where the VFD earns its keep on a variable load.