Fan speed control
Also known as: condensing pressure control, head pressure control
Short definitionAn arrangement that changes fan speed according to pressure or temperature. On a condenser it stops condensing pressure from falling too far in winter; the fans use less electricity and make less noise.
A condenser is sized for the hottest day of the year, so in winter that surface is too much and condensing pressure falls. A small fall suits the compressor. If it falls too far, the expansion valve cannot find enough pressure difference and too little refrigerant reaches the evaporator. The control throttles the air and holds the pressure above a lower limit. The signal comes from a pressure sensor on the discharge pipe or a temperature sensor on the liquid pipe.
A quote can go four ways: step control that switches fans on and off in turn, a cheap phase-cut controller suited to single-phase fans, a frequency inverter that drives three-phase fans together, and the EC fan, the most efficient. If the set point follows the outdoor air from 8–12 K above, this is called floating condensing and compressor consumption drops. Throttling speed on an evaporator reduces moisture loss from the product but also lowers capacity, and frosting may increase.