Compressor control, start/stop, load/unload or variable speed

A compressor rarely produces exactly the air the workshop uses. Its control system bridges the gap. There are three main families: start/stop, where the motor stops once pressure is reached; load/unload, where the motor keeps running without producing air; and variable speed, where the motor slows down to follow demand. All three rely on the same thing: a pressure band and a reserve of air in the receiver.

This guide describes how each mode works. To find out whether variable speed pays off in your case, read variable speed vs fixed speed compressor.

The starting point: two pressure thresholds

Every controlled compressor works between two pressures:

• the cut-out pressure (or unload pressure): when it is reached, the compressor stops producing;
• the cut-in pressure (or load pressure): when demand has brought the pressure down to it, the compressor starts again.

The gap between them is called the differential. It is measured by a pressure switch on piston compressors, and by a pressure sensor linked to the controller on screw compressors. The lower pressure must stay above what the tools need, network losses included.

Mode 1: start/stop

The motor starts at the lower pressure, runs at full speed up to the upper pressure, then stops completely. This is how receiver-mounted piston compressors work.

• Advantage: no energy used while stopped. Simple and robust.
• Limit: every start puts stress on the motor and the electrical supply. The number of starts allowed per hour is limited, see the motor manual. With strongly varying demand, the receiver must be generous to space out the cycles.

This mode suits intermittent use, a few minutes per hour: it is the logic of our E series, fitted with a pressure switch and a safety valve on the receiver.

Mode 2: load/unload

Used on fixed-speed screw compressors. At the upper pressure, a valve closes the intake: the screw keeps turning but no longer draws in air. The compressor runs unloaded. At the lower pressure, the valve reopens and it goes back on load.

• Advantage: the motor does not stop at every cycle, which protects it and allows closely spaced cycles.
• Limit: unloaded, the motor still draws a significant share of its power, typically around a quarter, without producing any air.

Most of these machines combine both modes: after a set unloaded time, the controller stops the motor, then restarts it when pressure drops. This is sometimes called dual or timed control.

Mode 3: variable speed

A frequency inverter controls motor speed according to measured pressure. If demand drops, the motor slows down; if it rises, the motor speeds up. Pressure stays close to a set point instead of swinging within a differential.

Variable speed has a range: below the motor's minimum speed, the machine cannot slow down any further. When the workshop uses less than that minimum, it rises to the set pressure, stops, then restarts when pressure drops. This is what our S series screw compressors do, with their permanent magnet motor and frequency inverter: you set the working pressure, the compressor rises to it, stops by itself and restarts automatically below the set threshold.

The receiver sets the cycle rate

In start/stop as in load/unload, the receiver determines the time between two cycles. The air it releases between the two thresholds is, in litres of free air:

usable reserve ≈ receiver volume (L) × differential (bar)

An example, with values to adapt:

1. Fixed-speed compressor delivering 1,100 l/min, 500 L receiver, band from 7 to 8 bar. Usable reserve: 500 × 1 = 500 litres.
2. The workshop uses 600 l/min. Pressure falls from 8 to 7 bar in 500 ÷ 600 ≈ 50 seconds.
3. The compressor refills with a surplus of 1,100 − 600 = 500 l/min: it needs 500 ÷ 500 = 1 minute.
4. A full cycle lasts about 1.8 minutes, or 33 cycles per hour.

The worst case is when the workshop uses roughly half the compressor's flow: that is when cycles are closest together. Two ways to space them out:

• A larger receiver: with 1,000 L, you drop to about 16 cycles per hour.
• A wider differential: it also spaces out cycles, but means producing at a higher pressure. Each extra bar costs roughly 7% more energy, as a common order of magnitude.

The air receiver size calculator does this calculation for your installation.

What about several compressors?

When two machines feed the same network, they are set up in cascade: a base-load machine runs continuously, and the second only starts for peaks. Their thresholds are offset so that they do not keep taking over from each other. A variable speed machine used as the trim unit, alongside a base-load machine, follows variations without piling up unloaded running.

The right mode for the job

To put a figure on unloaded running, use the compressor electricity cost calculator. The configurator starts from your tools to suggest a compressor and a receiver, and our technicians check the settings with you in a quote request.