A workshop's air demand changes all the time: an impact wrench, a break, three blow guns at once, then nothing. A fixed speed compressor answers these changes in an all-or-nothing way. A variable speed compressor constantly adjusts what it produces. Here is what that changes, and when it is worth it.
The fixed speed compressor: all or nothing
Its motor always runs at the same speed. To follow demand, it switches between three states:
- Loaded: it produces air at full flow up to the cut-out pressure.
- Unloaded: the pressure is reached, the screw keeps turning but no longer draws in air. The motor still uses a large share of its power, around a quarter, while producing nothing.
- Stopped: if it stays unloaded long enough, it stops, then restarts when the pressure falls again.
When demand is irregular, the machine spends a lot of time unloaded. That is electricity paid for to produce nothing. The pressure swings between the cut-out and cut-in pressures.
The variable speed compressor: it follows demand
A frequency inverter controls the motor speed. The controller measures the pressure continuously: if demand falls, the motor slows down; if it rises, the motor speeds up. The compressor produces roughly the air the workshop uses, at the moment it uses it.
What you gain:
- Less unloaded running: when the workshop uses little air, the machine runs slowly instead of switching between loaded and unloaded.
- More stable pressure: the pressure stays close to the set point instead of swinging. You can often set a lower pressure, and every bar less reduces consumption.
- Soft starting: the inverter brings the motor up to speed gradually, with no surge on the electrical installation.
When variable speed pays off
| Your situation | The right choice |
|---|---|
| Demand varies a lot during the day: workstations stopping and restarting, changing shifts | Variable speed |
| Compressor often unloaded when you look at the controller | Variable speed |
| Demand will grow over time: the machine will be a little large at first | Variable speed |
| Stable, high demand, compressor loaded nearly all the time | Fixed speed works, provided it is correctly sized |
| A few minutes of use per hour | A piston compressor is often enough: see piston or screw |
The more hours the compressor runs each year, the more the difference in consumption matters. Over a machine's life, electricity weighs far more than the purchase price: our guide on what a compressor really costs explains why, and the electricity cost calculator gives an estimate for your case.
What about the permanent magnet motor?
On most of our screw compressors, variable speed comes with a direct drive permanent magnet motor: the motor drives the screw without a belt, and it keeps good efficiency even when running slowly. That is what lets the machine slow down without losing efficiency.
AIRSTAHL variable speed compressors
- S series, standalone screw compressors: S 55, S 75, S 110, S 150 and S 220 PM, with permanent magnet motor and frequency inverter.
- K series, 4-in-1: K 75, K 110 and K 150, with air receiver, refrigerated dryer and filtration on the same frame.
- L series, 16 bar all-in-one units for laser cutting.
Correct sizing is still essential
Variable speed does not fix everything. A compressor that is far too big still spends time at its minimum speed or stopped, and one that is too small runs flat out without keeping up. You also need a suitable air receiver to absorb peaks, and a leak-free network: a leak makes any compressor run for nothing.
The configurator calculates your demand from your tools and suggests the right model. You can also describe your workshop in a quote request: we check the sizing with you before you order.
For your trade: Compressor for industry and production, Compressor for plastics processing, Compressor for joinery workshops.