Many compressors operate intermittently. Some pump refrigerant in refrigerators, others use compressed air to drive various pneumatic mechanisms. But why is it necessary to periodically turn off the compressor motor?
First, you need to familiarize yourself with the principle of the so-called pulse width modulation. To do this, imagine a children's electric car. In most of these cars, it is impossible to adjust the speed smoothly, because a conventional switch is hidden under the gas pedal. You can only either turn on the engine at full power, or turn it off. Now imagine that a small driver decides to go at a slower speed than his electric car is capable of. He will soon realize that the engine can be periodically turned on and off, changing the ratio between the duration of the on and off states. This ratio is called the duty cycle. If the duty cycle is multiplied by the nominal value of the parameter, you get its average value.
Feedback is used to keep this or that parameter constant using pulse width modulation. So, in an ordinary iron, the feedback sensor and at the same time the switch is a bimetallic regulator. When the temperature is higher than the first preset value, it turns off the heater, when it is below the second, it turns it on again. The sole of the iron has thermal inertia, so there are no sudden changes in temperature. And the difference between the first and second temperature values is called hysteresis. Almost all automatic regulators have this property. It is necessary so that switching does not occur too often. Using the knob of the regulator, you can simultaneously change both temperature values (at which the heater turns on and at which it turns off), and hence the duty cycle, and, ultimately, the average temperature.
The refrigerator also has a thermostat, it only measures the temperature in the refrigerating chamber, and turns the compressor on and off. It also has hysteresis and uses air and food stored in the refrigerator to provide thermal inertia.
Compressors used to supply compressed air to various pneumatic mechanisms are equipped with so-called receivers - large and durable metal tanks. It is they who provide inertia, but in this case, not in temperature, but in pressure. When it exceeds the first limit, a sensor is triggered and the compressor is turned off. If air is consumed for the operation of the pneumatic mechanism, the pressure gradually drops. As soon as it falls below the second limit, the sensor will again force the compressor to turn on.
