# Soft starter (compressor motor)

A soft starter is, in WHO's technical specifications for oxygen generator plants (December 2024), a compressor feature to reduce load upon start-up, listed with variable speed drives among the energy-saving components of the plant's air feed.

WHO's table names the variable speed drive (VSD), or variable frequency drive (VFD), and the soft-starter feature, and notes that a soft-starter is also required if a downstream filling station compressor is in use. WHO's interim PSA specifications of 8 June 2020, which the 2024 document supersedes, listed a soft start or variable speed drive compressor as an option.

UNDP's REQ-OXY-001 (effective 1 January 2025) says the supplier must provide an electrical requirements chart detailing the starting current, minimum protection current and load curves, to determine the total power needs of the plant, and WHO's Foundations of medical oxygen systems (17 February 2023) says the supplier of the oxygen system should specify the total power needs, including the starting current, minimum protection current and load curve. See [VSD (variable speed drive)](https://library.ultracontrolo.com/glossary/vsd/) and [what power supply a PSA plant needs](https://library.ultracontrolo.com/answers/what-power-supply-does-a-psa-plant-need/).

## Sources

- [WHO, Technical specifications for health facility based medical oxygen systems (31 December 2024), oxygen generator plant table, line 18](https://iris.who.int/server/api/core/bitstreams/ff437601-add3-44c8-8149-b85f0cdb2b52/content)
- [WHO, Technical specifications for PSA oxygen plants, interim guidance (8 June 2020), line 1](https://iris.who.int/server/api/core/bitstreams/d53996d5-380c-44ae-9fe8-9e529d3270cd/content)
- [UNDP, REQ-OXY-001 v01 (effective 1 January 2025), section 6.4](https://healthimplementation.undp.org/wp-content/uploads/2025/01/REQ-OXY-001-v01-UNDP-TECHNICAL-REGULATORY-REQUIREMENTS-FOR-OXYGEN-PLANTS.pdf)
- [WHO, Foundations of medical oxygen systems (17 February 2023), section 3.5.3 (power supply)](https://iris.who.int/server/api/core/bitstreams/4ad6f5fd-089e-4142-8439-b64e98d816d8/content)

Reviewed: 2026-10-23. Ultra Controlo engineering. https://library.ultracontrolo.com/glossary/soft-starter/