When a compressor system is designed, it is necessary that it can handle the peak demand. However, it is unlikely that you will experience this demand at all times. When demand is lower, compressor controls are utilized to coordinate a reduction in output that matches the lower need. This is essential to the efficiency of any compressor system. There are six primary types of compressor controls that I will cover here. Which one is right for you and your system could depend on a number of factors that I will elaborate on:

1. Start/Stop – This is the simplest and least expensive control. The start/stop function will turn off the motor in response to a pressure signal. This is a good control method for smaller, reciprocating compressors that aren’t adversely affected by cycling on and off.
2. Load/Unload – The motor will run continuously (unlike the start/stop), but the compressor unloads when a set pressure is reached. The compressor will then reload at a specified minimum pressure setting. These are used when cycling on and off can adversely affect the compressor, like with rotary screw models, where the finite start-up time can lead to header pressure dropping below usable levels. While unloaded, the motor uses about 1/3 the amount of energy it does when loaded.
3. Modulating – This is a variation of load/unload that restricts the air coming into the compressor to reduce compressor output to a specified minimum. This is also known as throttling or capacity control. This method is limited in range from 100% to 40% or rated capacity, so it can be somewhat inefficient.
4. Dual/Auto Dual – On small reciprocating compressors, this control allows the selection of either start/stop or load/unload. This is ideal when certain systems can benefit from both options, but at different times. This is often found in facilities where different shifts have noticeably different demand levels.
5. Variable Displacement – Gradually reduces the compressor displacement without reducing inlet pressure. These controls reposition the valve progressively, which reduces the working length of the rotors. This will lower the power draw considerably when at partial load.
6. Variable Speed – Controls the compressor capacity by adjusting the speed of the electric motor. This adjustment can be controlled to match air demand, greatly reducing energy costs.

In conjunction with the compressor controls are the starter motors. There are several types that are used with compressor systems:

1. Full Voltage Starters – The original and simplest starter. Full line voltage comes in, and amperage can peak at up to 8 times full load (normal operating) amperage during startup.  This can result in voltage dips in more than just the facility itself, but also in the surrounding neighborhood as well.  You may have seen a similar effect in movies when they throw the switch on an electric chair, and it causes the lights to dim.
2. Reduced Voltage Starters – As the name suggests, these electro-mechanical starters use reduced voltage. Commonly starting at 50% full voltage, and stepping up from there.  This prevents the current from drastically jumping during the startup, and reduces the stress on mechanical components like the motor shaft, bearings, and coupling to the compressor.
3. Solid State (or “Soft”) Starters – Like the reduced voltage types, these reduce the full line voltage coming in as well, but increase the power gradually, as opposed to stepping up incrementally.  As with the reduced voltage starters, these will provide less stress on mechanical components.

If you have questions about your compressed air system, or your compressed air products, give us a call.

Al Wooffitt
Application Engineer

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