A soft starter and a frequency converter both connect to the motor, but they do different jobs. Knowing the difference prevents an expensive mismatch in an air compressor system.
What a Soft Starter Does
A soft starter limits inrush current during start-up by ramping voltage, reducing mechanical and electrical stress. It does not change running speed.
What a Frequency Converter (VFD) Does
A frequency converter varies motor speed continuously by changing output frequency, so the compressor can match air delivery to demand and save energy at partial load.
When to Use Each
- Soft starter: fixed-speed compressors that only need smooth starting
- VFD: variable load profiles where energy savings justify the higher cost
Summary
Use a soft starter for gentle starts on a fixed-speed unit; use a VFD when load varies and you want speed control and part-load energy savings.