A high-pressure control is wired in series with the compressor common terminal, and it is also connected to the discharge line. Its main function is to shut down the unit when the pressure in the discharge line rises to a dangerous level. The cut-in and cut-out settings on the control are manually adjustable. When the pressure goes higher than a preset cut-out point, a switch within the control snaps and shuts off the power to the compressor.
If the cut-in and cut-out settings are adjusted too close, the unit will short-cycle. If the unit is overcharged or if a restriction occurs in the sealed system, the discharge line pressure will rapidly rise higher than normal, causing the high-pressure control to react and shut down the system.
During the off cycle, the pressures in the sealed system tend to equalize (the head pressure decreases and the low-side pressure increases). As soon as the discharge line pressure drops to a cut-in point, the switch within the pressure control snaps closed and restores power to the compressor, causing the unit to resume operation. As long as this happens at short intervals, the short-cycling continues. (See the pages concerning pressure controls for more details.)
In a particularly restricted system, the unit will short-cycle as a result of the reaction of the low-pressure control to the lower-than-normal pressure in the suction line. The compressor keeps drawing from the suction line. It creates partial vacuum in the system from the restriction and keeps pumping into the discharge line against the restriction creating above normal pressure in the discharge line and the condenser.
Another reason for short-cycling in a system equipped with a low-pressure control is the occurrence of general low pressures in the suction line as a result of an undercharged system (or a leak in the system causing a loss of
refrigerant). In this case, as soon as power is restored to the compressor, the lower-than-normal pressure in the suction line drops to the cut-out point causing the low-pressure control to react and shut down the system. Thus, short-cycling continues as long as the unit operates under this condition.
A low-pressure control is connected to the suction line to shut off power to the compressor as soon as the pressure in the suction line drops below its predetermined setting. As soon as the compressor is de-energized, the evaporator warms and pressure in the suction line increases, causing the bellows within the control to expand and restore power to the compressor. Sometimes the cut-in and the cut-out settings on the control are adjusted too close, In which case, as soon as the compressor is activated, the pressure in the system drops to the cut-out point and the low-pressure control shuts the unit off in short intervals. In this case, the settings must be recalibrated. (See fig. 93).