Abstract: There are too many electrical components in the contact control circuit of the traditional system, which makes the reliability of the equipment and transportation decrease, the workload of maintenance personnel increase, and the maintenance cost increases. In this paper, PLC programmable controller is used to control the contact instead of the traditional system, and the operation and protection requirements of the central air conditioning system are expounded. Keywords: PLC, ladder diagram, central air conditioning. The preface of PLC's application in the central air-conditioning control system With the continuous development of China's economy, the society is highly informatized, and new high-tech technologies are constantly applied in all aspects, it has become an inevitable trend to develop intelligence. Since the establishment of the automatic system, our company installed four single-screw water cooling units in 1996, with R22 as the cooling agent and water cooling as the cooling method. Safety protection includes: phase sequence protection, steam generator water flow protection, condenser water flow protection, freezing point protection, overload protection, motor thermal protection, high and low voltage protection, etc. Because of too many components used in the equipment, such as contactors, intermediate relays, time relays, etc., the control circuit is complicated, and the system often fails to operate normally after five or six years of use, and the maintenance process is complicated. The maintenance cost is soaring. In order to make the equipment run normally, reduce the workload of maintenance personnel, reduce the maintenance cost and improve the working environment, it is decided to rebuild its control system. Because of the high reliability, strong anti-interference ability and strong universality of PLC control tools, The function of easy charging and expansion has the advantages of no need to change the circuit, compact structure, small volume, light weight, low power consumption, less maintenance workload, and convenient field connection, so the company decided to change the original control circuit into PLC control system. 1. Analysis of the protection and requirements of air conditioning operation First of all, the electric heater of the air conditioning unit must be pre-heated before the single screw rod cold water unit is started, so that the oil temperature of the cold frozen oil is above 4℃ and the viscosity of the cold frozen oil is reduced. At the same time, the cold frozen oil and the refrigerating agent are separated (about 24 hours). Because the work rate of the heater is small, the prepared metal outer shell absorbs heat and dissipates heat, so the heater can heat for a long time, and it is not necessary to worry that the oil temperature is too high. When the electric machine is running, the electric heater is cut off, and the cold frozen oil absorbs the heat generated by the electric machine. The power supply must have phase sequence protection, the phase sequence of the power supply must be correct, the compressor motor must not rotate reversely, and the compressor motor must seek Y-? When starting, the thermal relay should be activated when the motor is overloaded, and the temperature of the motor should not be higher than 9℃. When the motor is overheated, the thermal protection in the motor should be activated with high and low pressure protection, and the suction pressure and exhaust pressure should be between .45MPa and 1.5MPa. There must be cold water flow and cold frozen water flow when air conditioning is carried out, the pressure is .3MPa to .4MPa left and right, and the pressure difference between inlet and outlet water is .1MPa left and right, and there must be ice point protection. When the cold frozen water temperature is 2℃ lower than the set water temperature after the air conditioning is carried out, The equipment should be stopped. If the system fails to work properly and the water temperature drops to 4℃, it must be stopped to prevent ice blocking and freeze the equipment. When the cold frozen water temperature reaches the set constant temperature +2℃ after the equipment is transported, the equipment can start and stop automatically, and the unit cannot be transported twice within 1 minutes after each shutdown. After pressing the start-up button, the electromagnetic valves MV 4 and MV 1 will be opened. After a delay of 24 seconds, the motor Y will start for 5 seconds. After 6 seconds of transportation, the energy electro-magnetic valve MV 1 should be closed and the energy electro-magnetic valve MV 2 should be opened at the same time. The compression machine is loaded at 4%, and after 5 minutes of transportation, The electro-magnetic valve MV 3 is opened and closed at the same time. The electro-magnetic valve MV 2 is operated with 7% motor load, and after another 1 minutes, the electro-magnetic valve MV 3 is closed with 1% motor load. When the temperature of cold frozen water is 2℃ higher than the set value of water temperature, it is equipped with load shedding, and the motor of compressor 1 is operated at 7% negative load. When the temperature of cold frozen water reaches the set water temperature, it is equipped with load shedding again, and the motor of compressor is operated at 4% negative load. When the temperature of cold and frozen water is less than the set point of 2℃, the machine shall be automatically stopped. When the temperature of cold and frozen water rises to more than the set point of 2℃ and the machine is stopped for 1 minutes, the machine shall be automatically started. When there is any situation in the system that may damage the equipment, the equipment is required to stop immediately and report to the police. When there is an abnormal phenomenon, you can press the emergency stop button SEM to prepare for a forced shutdown. 2. Diagram design 1. See Figure 12 for the main circuit diagram, Figure 23 for the ladder diagram, Table 14 for the main components, Table 25 for the input and output relays and control objects of PLC, and Table 36 for the actuation and energy comparison of the electromagnetic valve with energy. See Table ④ 3, Operation of Air Conditioning Equipment 3.1 Pre-standby Operation 1. Send the air conditioning power supply to the electric heater one day before the air conditioning is started, so that the electric heater can heat the frozen oil to 4℃ (about 24 hours). When the oil temperature reaches 4℃, the contact point of the oil temperature protector is normally open and closed, and the PLC input relay X52 is disconnected. 2. The phase of the main machine power supply is correct, and the phase relay acts to disconnect the PLC input relay X51. 3. Open all the water flow valves. 4. Adjust the valve of the air duct to make the air blower of the gas collecting box circulate air (new air can be supplied with less air). 5. Start the air blower of the gas collecting box, the air blower of the cooling tower, the cold frozen water pump and the cold chilled water pump. When there is water flow between the steam generator and the cold condenser and the pressure difference of the water flow reaches the set allowable value, the water flow of the steam generator is turned on and off and the water flow of the cold condenser is turned on and off, so that the PLC input relays X53 and X54 are turned off. 6. The water temperature is always on and off, and when the water temperature is lower than the set temperature, it will be on and off. The electric machine overheating protection and thermal relay are always on. The high-pressure protection is set to 1.6MPa, the high-pressure protection is set to 1.6MPa, the low-pressure protection is set to .35MPa, and the low-pressure protection is set to .35MPa. 7. Set the on-off setting of cold frozen water temperature to the required value (for the time being, set the on-off setting of cold frozen water temperature to 7℃, and set the remaining two temperatures on-off, and set the upper limit value and lower limit value of water temperature to 9℃ and 5℃ respectively). 3.2 When the air conditioning is started, the equipment can be started at this time when the PLC auxiliary relay M11 loses power and the alarm lamp goes out. Because the time relay T455 was powered on as early as when the equipment was heated and started after 1 minutes, the normally open contact point of the time relay T455 had already been closed and started. Press the start button SB 1, and the normally open contact of the input relay X4 of PLC will be closed to make the auxiliary relay get power from M1, and the normally open contact of M1 will be closed and self-locked, at the same time, the output relay Y53 will be self-locked. Y53 normally opens and closes to connect the output relays T45 and Y431 with electricity, Y53 normally opens and closes the contact point to connect the alternating current contactor KM 3 with electricity, and Y431 normally opens and closes to connect the star-shaped contact point with electricity. When T45 is started, the power output from relay Y43 and Y431 enables the electricity of electromagnetic valves MV 4 and MV 1 to be opened. When the relay T45 is delayed for 24 seconds, T45 normally opens the contact point and closes it. The output relay Y432 is electrified and self-locked, the electrified Y432 activates the alternating current contactor KM 1, the compressor motor starts, the normally open contact point Y432 2 closes, and the starting indicator light H 2 is turned on. When the relay T455 loses power and resets, the normally closed contact point of a group of controlled heaters connected with the contactor KM 1 is broken, so that the electric heaters stop working, and the cold frozen oil absorbs the heat generated by the electric machine. When Y432 is powered on, the time relay T451 is powered on. After the compressor motor starts for 5 seconds, the time relay T451 starts to operate, and the output relay Y53 loses power, which makes the alternating current contactor KM 3 lose power and open the star-shaped junction. At the same time, the relay T451 is always on and off, which makes the output relay Y433 power-on, and the power of Y433 is self-locked, which makes the relay T452 power-on. When T452 is started, the power of Y433 makes the AC contactor KM 2 power-on. When the compressor motor starts to run normally, Y433 turns on and turns off the starting indicator lamp H 2, and at the same time, the traveling indicator lamp H 3 turns on, and the intermittent relay T452 is powered on. After the compressor motor runs for 6 seconds, the intermittent relay T452 turns on. When T452 normally closed contact is opened, the output relay Y431 loses power; when Y431 loses power, the electro-magnetic valve MV 1 loses power and closes; when T452 normally opened contact is closed, the output relay Y434 gets power. When the electromagnetic valve MV 2 is energized and opened, the compressor motor is operated at 4% load, another group of normally open contact points of the intermittent relay T452 are closed, the intermittent relay T453 is energized, and the compressor motor is operated for 3 seconds. The time relay T453 makes the output relay Y435 powered on, and the enabling electromagnetic valve MV 3 is powered on, and the output relay Y434 is powered off and the enabling electromagnetic valve MV 2 is powered off. When the load of the compressor motor is 7%, the other group of normally open contact points of T453 are closed. After the compressor motor is powered by the intermittent relay T454 for another 6 seconds, the intermittent relay T454 is activated, and T454 turns off the output relay Y435, which loses power and enables the electromagnetic valve M.