Articles / compressors

VSD retrofit: run the numbers on your plant before the vendor does

2026-07-04 · by Cas, air-compressor-guide.com · every number below is a real simulation output

The variable-speed pitch is everywhere, and it's not wrong — it's just not universally right. A VSD saves money in exact proportion to how badly your fixed-speed machine is matched to your demand. On our simulated half-load plant: −€6,487 a year, −21%. On a plant running near capacity: close to nothing, for the same capex.

Where the 21% comes from

A fixed-speed screw at partial load lives in load/unload cycling. When loaded, it's efficient. When unloaded, it still burns a substantial idle power producing zero air — and at 50% duty, that's half its life. Our demo plant's machine cycles between 6.0 and 7.0 bar sixty times an hour doing exactly this.

A VSD instead slows the machine to match demand, holding a steady 6.5 bar. Almost no unloaded running, almost no start events, and — a bonus that's often missed — a lower average pressure than the top of a fixed-speed band, which itself saves energy.

Assumptions: €0.25/kWh · 6,000 h/yr · ~50% average load. Retrofit capex is site-specific; at typical costs this profile pays back in 2–3 years (illustrative).

When the answer is "don't"

Run the same swap on a plant averaging 90% load and the picture collapses: the fixed-speed machine barely unloads, so there's barely any idle waste for the VSD to eliminate — while the drive itself adds a couple of percent of conversion losses. The load profile decides everything, and the load profile is exactly what a vendor's one-week logging exercise is designed to measure for the purpose of quoting their machine.

Simulating first flips the sequence: you learn whether your duty is in the VSD sweet spot — and what the saving is worth — before anyone's sales engineer is in the building. If the answer is yes, you also now have the evidence to negotiate with, and a neutral number to put next to whatever the quote claims.

The starts number matters too

60 starts/hour versus ~1 isn't just an energy story: it's contactor wear, winding stress, and maintenance interval math. If your plant can't justify a VSD on kWh alone, the honest alternative is often cheaper: more storage plus an auto-stop timer — the poor man's VSD — which kills the same idle waste for a fraction of the capex.