There are too many electrical components in the contactor control circuit of transmission system, which reduces the reliability of equipment and transportation, increases the workload of maintenance personnel and increases the maintenance cost. In this paper, PLC programmable controller is used to replace the contactor control system of transmission system, and the operation and protection requirements of central air conditioning system are expounded. Key words: PLC, ladder diagram, central air conditioning. Introduction to the application of PLC in central air conditioning control system With the continuous development of China's economy, the society is highly informatized, and high technology is constantly applied in all aspects, it has become an inevitable trend to develop intelligence. Since the development of the automation system, our company has installed four single-screw water chillers in 1996, with R22 as the refrigerant and water cooling as the cooling mode. 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 too many components are used in the equipment, such as contactors, intermediate relays, time relays, etc. The control circuit is complex, and the system often fails to operate normally after five or six years of use, and the maintenance process is complicated. Maintenance costs are soaring. In order to make the equipment run normally, reduce the workload of maintenance personnel, reduce maintenance costs and improve the working environment, it is decided to reform its control system. Because the PLC control tool has the advantages of high reliability, strong anti-interference ability, strong versatility, easy charging and expansion, no need to change the circuit, compact structure, small volume, light weight, low power consumption, less maintenance workload and convenient field connection, the company decided to change the original control circuit into a PLC control system. 1. Analysis of protection and requirements for air conditioning operation First of all, the electric heater of the air conditioning unit must be preheated before the single-screw water chiller is started, so that the oil temperature of the cold refrigeration oil is above 40℃ and the viscosity of the cold refrigeration oil is reduced. At the same time, the cold refrigeration oil and refrigerant are separated (about 24 hours). Because of the low working rate of the heater, the prepared metal shell absorbs heat and dissipates heat, so the heater can be heated for a long time without worrying about the high oil temperature. When the motor is running, the electric heater is cut off, and the cold refrigeration oil absorbs the heat generated by the motor. The power supply must have phase sequence protection, the power supply phase sequence must be correct, the compressor motor must not be reversed, and the compressor motor must find Y-? When starting, the thermal relay should be activated when the motor is overloaded, and the motor temperature should not be higher than 90℃. When the motor is overheated, the thermal protection in the motor should be activated by high and low pressure protection. The suction pressure and exhaust pressure are between 0.45MPa and 1.5MPa. When air conditioning, there must be cold water flow and cold chilled water flow. The pressure is about 0.3MPa to 0.4MPa, and the pressure difference between inlet and outlet water is about 0. 1MPa. Freezing point protection must be provided. After standby operation, when the temperature of cold water and chilled water is 2℃ lower than the set water temperature, the equipment should stop running. If the system fails and the water temperature drops to 4℃, the equipment must be stopped to prevent ice blocking. After standby operation, when the temperature of cold and chilled water reaches the set temperature of +2℃, the equipment can be started and stopped automatically, and the unit can't deliver it twice within 10 minutes after each shutdown. When the start button is pressed, the solenoid valves MV 4 and MV 1 will be opened. After a delay of 240 seconds, the motor Y will start for 5 seconds. For type B transportation, after 6 seconds of transportation, the energy solenoid valve MV 1 should be closed and the energy solenoid valve MV 2 should be opened at the same time. After 5 minutes of transportation, when the electromagnetic valve MV 3 is opened and closed at the same time, the electromagnetic valve MV 2 will run at 70% motor load, and after 10 minutes, the electromagnetic valve MV 3 will be closed at 100% motor load. When the temperature of cold and chilled water is 2℃ higher than the set water temperature, it is equipped with load shedding, and the motor of compressor 1 runs at 70% negative load. When the temperature of cold and chilled water reaches the set water temperature, the load is reduced again, and the compressor motor runs at 40% negative load. When the temperature of cold water and chilled water is lower than 2℃ of the set value, the machine should stop automatically. When the temperature of cold water and chilled water rises to 2℃ of the set value, the machine will stop for 10 minute, and the machine will start automatically. When there is any situation in the system that may damage the equipment, the equipment is required to stop immediately and give an alarm. In case of abnormal phenomena, you can press the emergency stop button SEM to prepare for forced shutdown. 2. The diagram is designed as 1, the main circuit diagram as shown in figure 12, the ladder diagram as shown in figure 23, the main components as shown in table 14, the input and output relays and controlled objects of PLC as shown in table 25, and the action and energy comparison table of solenoid valve with energy as shown in table 36. See table ④ 3 for the program list. Transportation of air conditioning equipment 3. 1. The standby operation is 1. The power supply of the air conditioner is sent to the electric heater one day before the air conditioner is started, so that the electric heater can heat the frozen oil to 40℃ (about 24 hours). When the oil temperature reaches 40℃, the normally open contact of the oil temperature protector is closed, which opens the PLC input relay X502. 2. The phase of the host power supply is correct, and the phase relay operates to disconnect the PLC input relay X50 1. 3. Open all water flow valves. 4. Adjust the air duct valve to make the blower of the gas collecting box circulate air (less air can be used to supply new air). 5. Start the gas collecting box fan, cooling tower fan, cold chilled water pump and 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 X503 and X504 are turned off. 6. The water temperature is always on and off. When the water temperature is lower than the set temperature, it will always open and close. The overheat protection and thermal relay of the motor are always on. The setting of high-voltage protection is 1.6MPa, the setting of high-voltage protection is greater than 1.6MPa, the setting of low-voltage protection is 0.35MPa, and the setting of low-voltage protection is lower than 0.35 MPa. 7. Set the on-off setting of cold and chilled water temperature to the required value (temporarily set the on-off setting of cold and chilled water temperature to 7℃, and set the other two on-off settings, and set the upper and lower limits of water temperature to 9 respectively. 3.2 When the air conditioner is ready to start, the PLC auxiliary relay M 10 1 loses power and the alarm light goes out. At this time, the equipment can be started. Because the relay T455 was energized as early as the preparation for heating and started after 10 minutes, the normally open contact of the relay T455 has been closed and started. Press the start button SB 1, the normally open contact of the input relay X400 of PLC is closed, so that the auxiliary relay is powered on from M 100, and the normally open contact of M 100 is closed and locked. At the same time, the output relay Y530 is electrically self-locked. Y530 is normally open and normally closed, so that the output relays Y430 and Y43 1 are electrically locked. Y530 is normally open and normally closed, so that the AC contact KM 3 is electrified and the star contact is closed. Y43 1 is normally turned on and off to turn on the power of time relay T450. When T450 is started, the power supplies of relays Y430 and Y43 1 are output, and the power supplies of solenoid valves MV 4 and MV 1 are turned on. When the time relay T450 is delayed for 240 seconds, the normally open contact of T450 is closed, so that the output relay Y432 is electrically self-locked. When Y432 is energized, the AC contact KM 1 is activated, and the compressor motor is started. Y432 2 The normally open contact is closed and the start indicator light H 2 is turned on, which causes the time relay T455 to lose power and reset, and the normally closed contact of a group of controlled heaters connected with contactor KM 1 is disconnected, which stops the electric heater. The cold refrigeration oil absorbs the heat generated by the motor, and the power-on time relay T45 1 of Y432. Five seconds after the compressor motor starts, the time relay T45 1 turns on and the output relay Y530 loses power. The AC contactor KM 3 is powered off and the star contact is disconnected. At the same time, the relay T45 1 is always on and off, so that the output relay Y433 is powered on, and the power supply of Y433 is self-locked, so that the relay T452 is powered on, and the time is counted when T452 is turned on. Y433 energizes the AC contactor KM 2, and the compressor motor starts to operate normally. Y433 activates and turns off the start indicator H 2, and at the same time turns on the run indicator H 3. Intermittent relay T452 is energized. After the compressor motor runs for 6 seconds, the intermittent relay T452 is activated, and the contact of T452 is normally closed, so that the output relay Y43 1 loses power. Y43 1 loses power to close the solenoid valve MV 1, and T452 normally open contact closes to energize the output relay Y434 and energize the solenoid valve MV 2, and the compressor motor load is 40%. Another group of normally open contacts of time relay T452 is closed, and time relay T453 is energized. After the compressor motor runs for another 300 seconds, the time interval relay T453 runs to turn on the power of the output relay Y435. The enable solenoid valve MV 3 is powered on, and the operation of T453 turns off the output relay Y434, and the enable solenoid valve MV 2 is powered off and closed. The load of the compressor motor is 70%, and other normally open contacts of T453 are closed. After the motor of the time relay T454 is operated for another 600 seconds, the time relay T454 is activated, and the time relay T454 is turned off and output to the relay Y435. Y435 power-off enabling solenoid valve MV 3 is turned off, and the compressor motor is loaded with 100%. After running for a period of time, when the chilled water temperature drops to 9℃, the water temperature of 9℃ is turned on and off, which makes the input relay X402 run. The normally closed contact of X402 is opened, the time relays T45 1, T452, T453 and T454 are disconnected and reset, the normally open and normally closed input relay X402 is opened, and the energized solenoid valve MV 3 of the output relay Y435 is opened. When the temperature of cold water and chilled water drops to 7℃, the temperature of 7℃ is controlled to open and close, the input relay X505 is activated, the contact of X505 is normally closed, and the output relay Y435 is disconnected. Y435 turns off the metering valve MV 3, X505 normally open contacts turn on the output relay Y434, and Y434 turns off the metering valve MV 2. At the same time, the motor load of the compressor is reduced to 40%. When the temperature of cold water and chilled water drops to 5℃, the 5℃ temperature is controlled to open and close, so that the input relay X403 is energized, the contact of X403 is normally closed and opened, and the output relay Y434 is powered off, so that the energy metering valves MV 4 and MV 2 are powered off and closed. X403 normally open contact is closed, so that the output relay Y43 1 is energized, Y43 1 is energized and self-locked, and the metering valve MV 1 is energized. When the normally open contact of X403 is closed, the output relay Y437 is energized. The normally closed contact of Y437 is disconnected, which makes the output relays Y432 and Y433 lose power, stops the motors of AC contactors KM 1 and KM 2, and the loss of Y432 also makes Y434, Y435 and Y437 lose power. Output relays Y432 and Y433 lose power, and turn off the operation indicator lamp H 3. When KM 1 is cut off, the normally closed contact is closed to make the electric heater work (for example, when the temperature of chilled water drops to 2℃ lower than the set value, the water-limiting thermostat will not move. The temperature of cold water and chilled water will continue to drop. When the temperature of cold water and chilled water drops to 4℃, the freezing point protection will always be on and off, and the input relay X406 will always be on and off. The auxiliary relay M 10 1 can be operated to disconnect the normally closed contacts of the relay, so that the auxiliary relay M 100 and the output relay Y530 lose power. M 100 loses power, which makes the output relays Y430, Y43 1, Y432, Y433, Y434 and Y435 lose power, which makes the AC contactor KM 1 2 lose power, and the compressor motor stops forcibly, which can close all solenoid valves. At the same time, the normally open contact of M 10 1 is closed when the KM voltage is connected in parallel, so that the alarm indicator light H 4 of the output relay Y534 is lit. At this time, there is no way to start the operation again, only waiting for the troubleshooting. Auxiliary relay M 10 1 loses power and resets, and alarm indicator light H 4 goes out, so it can only be started when it is ready. ) When the temperature of chilled water rises to 5℃, the temperature control at 5℃ is turned on and off, and the normally open contact of the input relay X403 is turned off. The output relay Y437 is powered off, and the contact of Y437 is reset. When the water temperature rises to 7℃, it will reset at 7℃, reset at 9℃ when the water temperature rises to 9℃, and reset the incoming relay X402 when it loses power. If the machine has stopped for 10 minutes and relay T455 has been activated, the output relay Y530 will be electrically activated, so that the AC contactor KM 3 will be electrically closed and ready for the next round of operation. Please note that during the operation of the equipment, if any protector acts or the operating conditions of the equipment change beyond the allowable value, the auxiliary relay M 10 1 will be activated to stop the equipment and give an alarm, and the alarm indicator H4 will be on. Only after the fault is eliminated can the equipment run. 3.3 Stop the air conditioning equipment. After the air conditioner compressor motor automatically stops, it is best to press the stop button SB 2. This can reduce the damage to equipment and electrical components caused by the large current generated when the compressor motor stops suddenly. Press the stop button SB 2 to turn on the input relay X40 1, turn on the output relay Y436, and turn off the auxiliary relay M 100. When the contact of M 100 is normally open, when the compressor stops, the electromagnetic valves of relays Y430, Y43 1, Y432, Y433, Y434 and Y435 that output all energy are closed, and the operation indicator light goes out. When the compressor stops, the header fan, cooling tower, cooling water pump and chilled water pump should continue to run without special circumstances. 4. Conclusion After the transformation, the equipment has been put into operation for more than one year without any fault, and the operation is stable, achieving the expected effect, reducing the workload of maintenance personnel, saving a lot of maintenance expenses for the company and improving the working environment of the company. 5. List of main components: List of main components ① Component symbol PLC MV1-MV 4 SEM FR1R/R CP Component name Sanling F 1-40MR energy solenoid valve emergency stop button thermal relay electric heater phase sequence protector component symbol Fu1-FU5. KM1—KM3H1—H4TQ1Component Name Fuse AC Contactor Compressor Motor Indicator Lamp Transformer Air Switch PLC Input and Output Relay and Control Object Table ② Input Relay Symbol X400 X40 1 Control Object Start Button SB 1 Stop button SB2 Input relay symbol Y430 Y43 1 Controlled object energy solenoid valve MV4 energy solenoid valve MV14x07x500x501x502x503x504x505 Start temperature control shutdown, temperature control motor internal insulation relay freezing point protection high voltage protection low voltage protection phase sequence protection oil temperature switch evaporation water flow switch condensate water flow switch temperature setting switch Y432Y434Y4305. KM 1 AC contactor KM2 energy solenoid valve MV2 energy solenoid valve MV3 AC contactor KM3 start indicator H2 alarm indicator H4 energy solenoid valve action and energy comparison table ③ energy solenoid valve action and energy comparison table ③ open or close MV 1 000 65438. +200mv20 1 0000mv3001000mv41000 energy start 40% 70% 100% operation stop Note:1,"65438" MV 1, MV2, MV3 and MV4 are energy solenoid valves. 3. Operation stop refers to the automatic stop of the equipment after the chilled water temperature reaches the set value. 5 6 7 Program List ④ Program List ④ Serial Number 0123456789101121314/kloc. 5438+0 22 23 24 25 26 27 28 29 303132 instruction LD or ANI ANI ANI OUT LD or ORI ANI OUT LD and OUT K LD or component number X400Y436m1kloc-0/m/ kloc-0/00 x 40 1y 436m 100t 455 x0y 430y 436 x 430m 100y 430y 530y 436 5438+0x 403t 452m 1 00Y436 5438+0Y436 5438+0Y530T450 240T450Y432 Serial number 33 34 35 37 38 39 40 465432. 42 43 44 45 46 47 48 49 50 5 1 52 53 54 55 56 57 58 59 60 6 1 62 63 64 65 instruction and ANIANI OUT LDANI OUT KLDOR and ANIANI OUT LDANI O. Ut k ld or Aniani OUT LD KLD or ani ani ani and component number m 1 00y 436y 437y 432 x402 t 4565438+33y 433 x 402t 452 6t 452 x 505 453 x 403y 43 1 Y 432 Y 434 T 452 T 453 300 T 453 X 402 T 454 X 505 Serial number 66 67 68 69 70 7 1 72 73 74 75 76 77 78 79 80 86 5438+0 82 83 84 85 86 87 88 90 96548406x407x500x501x502x503x504m1kloc-0/8 Abstract With the continuous development of China's economy, the society is highly developed. Intelligentization also began with the establishment of automation system. Based on our graduation project "Intelligent Small Central Air Conditioning", this paper expounds the relationship between the design of PLC control system and intelligent central air conditioning (cold storage station) system. Now briefly introduce the realization of system transformation: the system debugging in this paper is divided into two steps, namely, the preparation of electrical control system and central network system. Let's explain the design, test and use of the complete electrical control system of the equipment: requirements for the laboratory: needle adjustment should ensure the stability of the electrical and pneumatic systems, high temperature sensitivity, long service life of maintenance personnel and online management. In addition, the design of the fault alarm part of the system is not perfect in practical use, and many functions have not been developed. Based on the investigation of equipment and students' knowledge of central air conditioning, this paper thinks that the A series PLC of Sanling Company can be used as the core of equipment control system. It not only has all the advantages of ordinary PLC programmable controller, but also can use Ethernet network module (B2/B5) to build MELSECNET network, and finally achieve a distributed control system, which not only realizes networking among various devices, but also realizes remote control and management. In this design, we have two sets of central air conditioning systems, which are composed of three cooling water pumps, three cold chilled water pumps, one cooling tower fan and two water chillers, and assembled into two sets of cooling systems (due to the small system and high power consumption rate of cooling towers, we need to ask the laboratory, etc.). ). This system is the same. The difference between the two refrigeration systems is that only one cooling tower is selected for the two chillers. After calculation and verification, this does not affect its effect. ) The cold water unit is supplied by a set of production equipment. According to the requirements of the actual laboratory in this design, we chose 2*5 totally enclosed compressor chillers. Generally, it is automatically controlled by microprocessor according to the principle and law of air conditioning. The cold water unit consists of compressor, condenser and evaporator. The compressor compresses the refrigerant and the compressed refrigerant enters the condenser. After compression, it becomes liquid after being cooled by cold water, and the extracted heat is taken away by cold water and discharged into the cooling tower. The liquid refrigerant enters the evaporator from the condenser to evaporate and absorb heat, so that the temperature of cold chilled water is reduced, and then the chilled water enters the cold fan coil to absorb the heat in the air. This cycle will bring out the indoor heat and achieve the purpose of reducing the ambient temperature. When the foundation of natural system meets the design requirements, it not only has the function of backup logic control, but also has the function of networking communication and management. Compared with the old control system, the external control system has the advantages of stable operation, low failure rate, automatic alarm, simple operation and maintenance, and greatly reduced maintenance cost (standby time, etc.). In the intelligent central air conditioning refrigeration system, it is feasible to adopt PLC control system, which can effectively ensure its stable and reliable work, convenient maintenance and high performance, energy and price ratio. At the same time, the highly reliable monitoring system with PLC as the core embodies the control system of the air-conditioning host and the coordinated control system between the two hosts, which has the remarkable characteristics of advanced, reliable, economical and flexible. 9 references 1. Design and construction of central air conditioning engineering, edited by Wu and Li Zuozhou, Higher Education Press 2. Automatic control of refrigeration and air conditioning, edited by Zhang Zihui, Science Press 1 3. Mitsubishi Corporation, Mitsubishi Microprogrammable Controller Programming Manual, April 2000. Principle and application of programmable controller, Gu Zhansong, Zhang Zihui, etc. 1996 5. Xiao et al. , realizing the communication between microcomputer and PLC in Ethernet, electrical automation, 200 1.5 6. Song Bosheng, editor-in-chief, Configuration, Programming and Networking of Programmable Controller, China Meteorological Press,