Apparatus for controlling condenser pressure in a refrigeration system

Claim 1

A control system for modulating the speed of a plurality of parallel-connected, variable speed fan motors, of an air-cooled condenser coil in a refrigeration system, in response to the temperature of the refrigerant in the condenser coil in order to maintain a substantially constant condenser pressure despite wide variations in condenser cooling air temperature, comprising: an AC power supply for providing an alternating line voltage of relatively high magnitude; a triac for coupling said AC power supply to the condenser fan motors to effect simultaneous operation thereof in response to gate current supplied to said triac; a variable resistance network including a full wave bridge rectifier, having a pair of AC terminals and a pair of DC terminals, and a photosensitive transistor having its collector and emitter connected respectively to the two DC terminals of said bridge rectifier, the resistance between the AC terminals of said network being determined by and being inversely proportional to the amount of light received by said transistor; a timing capacitor coupled in series with the AC terminals of said bridge rectifier; a frequency compensated capacitive voltage divider coupled to said AC power supply, via a series-connected resistor, for providing a small replica of the line voltage from said AC power supply, said capacitive voltage divider in conjunction with said resistor constituting a dv/dt suppression circuit across said triac; means for coupling said variable resistance network and said timing capacitor to said capacitive voltage divider to derive therefrom the small replica of the AC line voltage, said capacitor charging during each half cycle of the alternating voltage in response to charging current flowing through said network and of an amplitude determined by the resistance of said network; a DC power supply, isolated from said AC power supply, for providing a direct voltage of relatively low magnitude; a temperature sensing thermistor for sensing the refrigerant temperature in the condenser coil; means, including a differential amplifier, coupled to said DC power supply and to said temperature sensing thermistor for developing a control signal which varies as a function of the difference between a desired set point temperature and the actual sensed temperature of the refrigerant in the condenser coil; a light emitting diode optically coupled to said photosensitive transistor; means responsive to said control signal for translating current through said light emitting diode to effect illumination thereof in an amount directly proportional to the sensed temperature of the refrigerant, said timing capacitor thereby charging at a rate directly proportional to the sensed temperature; and means coupled to said timing capacitor for supplying gate current to said triac during each half cycle at a phase angle inversely proportional to the amplitude of the charging current, thereby simultaneously driving each of the condenser fan motors at a speed directly proportional to the temperature of the refrigerant in the condenser coil.