Soft starter or VFD: which one do you need?

What each one does

• Soft starter: ramps the motor voltage up and down for a smooth start and stop, then usually bypasses itself at full speed. Speed stays fixed at the supply frequency.
• VFD: changes the output frequency and voltage, so the motor speed can be set, ramped and controlled during operation.

Choose a soft starter when

• The load runs at full speed most of the time.
• You want to limit inrush current and reduce stress on belts, couplings or pipes.
• Budget and panel space matter and speed control is not needed.

Choose a VFD when

• The process needs different speeds, or precise torque control.
• The load is a centrifugal pump or fan, where reducing speed gives large energy savings.
• You need controlled braking, positioning or integration with a PLC over a fieldbus.

Practical checks

Whichever you choose, size it from the motor's rated current, check the duty cycle and starts per hour, and confirm the protection and short-circuit ratings. For a VFD, also plan an EMC filter, shielded motor cable and, for long cables, an output reactor.

Frequently asked questions