GB1402905A - Motor control system for governing the operation of the compressor motor of a refrigeration system - Google Patents

Abstract

1402905 Control of A.C. motors; protective arrangements CARRIER CORP 25 Sept 1972 [12 Oct 1971 (2)] 44232/72 Headings H2J and H2K A control system for a motor includes two tripping circuits for operating a relay controlling the supply of power to the motor each of which is responsive to signals exceeding a predetermined magnitude and a reset time delay operable to prevent re-energization of the relay for a period after the operation of a trip circuit. In the circuit shown the motor is used to drive a refrigerator compressor and has a first trip circuit which is responsive to a motor overload and a second trip circuit which trips the motor supply if there is a rise in refrigerant temperature. The arrangement is designed to prevent relay contact damage when chatter occurs caused by thermostat contact bounce. Relay coil 23 is connected in series with a triac 56 and thermostat contacts 22 and to a supply through terminals 90, 91 and thus the motor operates whenever contacts 22 are closed and the triac 56 is conducting. A triggering voltage for triac 56 is provided by the normally conducting darlington pair 57 and this pair can be switched off by making the darlington pair 71 conductive to form the first trip circuit or by making either of transistors 83, 88 conductive as the second trip circuit. Motor overload current tripping.-Terminals 60, 61 are in series with the common lead of the motor windings and a D.C. voltage is produced by rectifier bridge 63 from current transformer 62. The voltage is limited by diode 68 and causes capacitor to rapidly discharge through resistor 65 and thus makes pair 71 conductive to operate the tripping circuit. When triac 56 switches off the voltage across resistor 67 and across resistors 73, 74 will change to increase the current through pair 71 and reduce the reset level of pair 71. Capacitor 66 will slowly charge through resistor 76 until the reset voltage for pair 71 is reached when pair 71 is made non- conducting and pair 57 will conduct causing triac 56 to conduct if thermostat contacts 22 are closed. Refrigerant temperature tripping.-A change in the resistance of thermistor 80 causes tranistor 83 to conduct and switch off the pair 57 as above. The resetting time in this case will be reduced since capacitor 66 has not been discharged. Thermostat contact operation.-Opening of contacts 22 causes relay 23 to drop and also, because of the voltage across resistor 92, causes transistor 88 to conduct and the pair 57 are switched off as above. Integrator circuit 93, 97 ensures that a time delay is imposed to prevent contact chatter causing relay operation when the contacts 22 close.