In this guide
Both devices stop a motor from slamming onto the supply when it starts. They solve different problems, and choosing the wrong one means paying for features you will not use or missing one you need.
What each one does
A motor started direct on line typically draws about six to eight times its full-load current for a moment. A soft starter limits that by raising the voltage to the motor gradually, then bypasses itself once the motor is up to speed. The motor still runs at the speed set by the supply frequency.
A variable frequency drive (VFD) changes both the frequency and the voltage it supplies, so it controls speed all the time the motor runs, as well as starting and stopping it smoothly.
Side by side
| Feature | Soft starter | VFD |
|---|---|---|
| Controls speed while running | No | Yes |
| Limits starting current | Yes, to a set level | Yes, usually to about rated current or less |
| Smooth stopping | Yes, with a stop ramp | Yes |
| Energy saving on fans and pumps | No | Yes, when the load does not need full speed |
| Typical cost | Lower | Higher |
| Harmonics and cable considerations | Few once bypassed | More: filters, shielded motor cable and cable length limits may apply |
When a soft starter is enough
- The motor runs at full speed whenever it runs
- The problem is inrush current, belt or gearbox shock, or water hammer when a pump stops
- Budget and simplicity matter, and the supply cannot cope with direct on line starting
When a VFD is the better choice
- You need to vary speed to suit the process, for example flow, pressure or line speed
- The load is a fan or centrifugal pump. Power falls steeply as speed falls, so running slower saves energy
- The load is hard to start and needs high torque at low current
- You want the motor protected and monitored by the drive
Check before you order
- Motor nameplate: power, voltage, full-load current and speed
- Starting duty: how often it starts, how heavy the load is, how long the ramp must be
- Cooling and ventilation of the panel, because both devices produce heat
- The cable run between the drive and the motor, if a drive is chosen
Send us the motor nameplate and a line about the application. We will check availability of a suitable soft starter or drive and tell you what to confirm before you buy.