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Both soften the start, but in different ways. A soft starter uses thyristors to ramp the voltage applied to the motor from a low value up to supply voltage; frequency does not change. Once the start is complete, the motor runs directly from the supply at fixed speed. An inverter (frequency converter) rectifies the supply and regenerates it at the desired frequency and voltage, so motor speed can be adjusted during operation.
| Feature | Soft starter | Inverter |
|---|---|---|
| What it changes | Voltage at starting and stopping | Frequency and voltage together |
| Running speed | Fixed, at supply frequency | Adjustable |
| Starting torque | Falls with voltage | Can deliver high torque at low speed |
| After starting | Usually taken out of circuit by a bypass | Always in circuit; cable, filter and cooling must be considered |
Selection criterion: if the machine runs at fixed speed and only starting current, water hammer or belt shock needs reducing, a soft starter is enough; it is common on pumps, fans and conveyors. If flow, pressure or line speed must be adjusted during the process, an inverter is needed. With a heavy starting load, a soft starter may struggle because it also reduces torque.
Our three-phase ranges are suitable for speed control with a frequency inverter; for long periods at low speed and full torque a separate fan may be needed, and with long cables a dv/dt or sine filter is considered. Details are in the motor with inverter guide and on the inverter-compatible electric motors page.