Why the motor keeps running
Frequent stops and starts may be unsuitable for the drive and the required air availability. The controller may therefore unload first and stop only after a set time. The exact sequence, input power and blowdown depend on design. One universal unloaded-power fraction does not apply to every compressor.
How to identify costly waiting
Monitor loaded hours, total motor running time and actual input power in both states. A large difference between running and loaded hours warrants investigation, but is not automatic proof of a fault. Possible causes include oversizing, poor machine sequencing or unsuitable storage volume.
What to check in practice
- Record actual input power in each operating mode.
- Check unloading duration, pressure hysteresis and the number of starts.
- Address simultaneous operation of multiple machines before simply buying a variable-speed drive.
Example scenario
An example compressor draws 30 kW under load for 3,000 hours and 8 kW unloaded for 1,000 hours. It consumes 98,000 kWh in total, including 8,000 kWh unloaded. These figures are not a general ratio for other machines.
What to watch out for
- Assigning zero consumption to unloaded running.
- Reducing pressure hysteresis without checking cycle frequency.
- Confusing motor running time with actual air production time.
Will a larger air receiver solve unloaded running?
It may change cycle lengths and enable more suitable stopping, but it does not change production demand itself. The benefit depends on control, pressure band and operating profile. It cannot be promised from receiver volume alone.