Application of the hottest solid state relay in th

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Application of solid state relay in fresh air unit

Abstract: This paper introduces the method of controlling the supply air temperature of fresh air air conditioning unit by using solid state relay and programmable logic controller. The air conditioner control system designed with this method can effectively control the air temperature of the unit and greatly save the production cost. 1 reasons for controlling the air supply temperature of air conditioning units

in the traditional air conditioning system using unit air conditioners, the air system is usually a circulating system. After being treated by the air conditioning unit, the air is sent to the controlled environment by the air supply pipe, mixed with the air in the controlled environment, and then returned to the air conditioning unit by the return air pipe. If there are certain requirements for the air quality of door 214, the quantitative fresh air can be introduced into the air inlet of the air conditioning unit, and then the quantitative air can be discharged through the exhaust pipe at the air outlet of the controlled environment. In this case, the temperature control of the controlled environment is usually to control the return air temperature of the air conditioner, and its control method has been very mature

however, in some specific use environments, such as automobile engine laboratories, animal experiment cabins, etc., because the air will become toxic or have peculiar smell after flowing through the controlled environment, it cannot be recycled and must be completely discharged. At this time, there is no return air in the air system, which is called the fresh air system. In the fresh air system, if the controlled environment is very small and the air volume is very large, that is, there are many ventilation times, in this case, the temperature control of it can only be to control the air supply temperature of the air conditioning unit

2 difficulties in controlling the air supply temperature of the air conditioning unit

the air conditioning unit using the compressor refrigeration should control its air supply temperature, which mainly has the following difficulties:

first, the experimental environment has high requirements for the control accuracy of the air conditioning unit, and generally requires that the temperature accuracy fluctuation range is 1 ℃ or even 0.5 ℃. Even in the air-conditioning system with full return air, it takes a lot of effort to achieve this requirement

secondly, the compressor of the unit air conditioner is a typical switching component, and for the consideration of cost and manufacturing process, the unit air conditioner is usually equipped with only one or two compressors, which makes the difference between the air supply temperature when the compressor is running and when it is stopped up to more than 10 ℃. Moreover, due to the need of pressure balance and oil return of the refrigeration system, there are strict time limits for the operation and shutdown of the compressor. Therefore, it is almost impossible for the unit air conditioning unit to control the supply air temperature simply by cooling the compressor. This is also the main reason why the former air conditioning system rarely used unit type cabinet machines when it needed to control the air supply temperature, and used more water chillers with continuously adjustable refrigerating capacity and terminals

to solve the above difficulties, the key is to try to keep the compressor of the unit air conditioning unit in a stable state for a long time, and at the same time, use other continuously adjustable control methods to fine tune the refrigerating capacity (or heating capacity), so as to effectively control the air outlet temperature of the air conditioning unit

3 air supply temperature control realized by solid-state relay and PLC

3.1 working principle of control system

for unit air conditioning units, due to the defects in compressor control mentioned above, it is not realistic to fine tune the refrigerating capacity of the compressor. Therefore, we can only start from the heater, another component of the unit that can adjust the temperature. If the heat compensation can be carried out by the heater to offset the excess cooling capacity of the compressor, the purpose of fine-tuning the cooling capacity can be achieved. Based on this theory, people began to use SCR power regulator to control the input power of electric heater through the transformation of conduction angle and enter the regulation of heating amount. However, the SCR power regulator itself has a large heating capacity, which requires large heat sinks and special cooling fans, so it is large in size and usually expensive, up to about 500 yuan/kW

through the in-depth study of the heater, we found that the resistance heater is usually used in the unit air conditioning unit. The relationship between the heating power and input power of the resistance heater is

q=kp formula (1)

k in the above formula is a constant value, indicating the efficiency of the heater

and the relationship between the input power and input voltage of the electric heater

formula (2)

the R value in the above formula is the resistance value of the heater, which is a constant

combining the above two formulas, we can get the following formula

q (U) formula (3)

, that is, the heating power of the resistance heater is a function of the voltage U loaded at both ends of the resistance

because the AC signal is equivalent to the DC signal whose voltage value is its effective value when loaded on the resistive load. When the current signal is switched off at a fixed frequency, the effective value of the output voltage is in direct proportion to its duty cycle (that is, the ratio of on time to wavelength time). Therefore, when the compressor of the unit air conditioning unit is in a stable operation state for a long time (i.e. running or stopping for a long time), as long as we try to change the duty cycle of the voltage signal loaded on both ends of the electric heater, we can fine tune the heating (or cooling) capacity of the air conditioning unit

3.2 working principle of solid state relay

AC solid state relays SSR (solid state relays) is a contactless on-off electronic switch, which is a four terminal active device. Two terminals are input control terminals, and the other two ends are output controlled terminals. Photoelectric isolation is adopted in the middle as the electrical isolation (floating) between input and output. By adding a DC or pulse signal to the input end, the output end can change from off state to on state (blocking state when there is no signal), so as to control a large load. The whole device has no movable parts and contacts, and can achieve the same function as the commonly used mechanical electromagnetic relay. Because solid state relay is a contactless switching element composed of solid components, it has a series of advantages compared with electromagnetic relay, such as reliable operation, not limited by switching life, small interference to the outside world, compatibility with logic circuit, anti-interference energy, strong zero, fast switching speed and convenient use. In addition, the cost of solid-state relay is low, and the cost of solid-state relay for 15kw heater is about 1000 yuan. Only one tenth of the cost of SCR power regulator

3.3 hardware composition and working process of the control system

the main controller of the control system adopts S7 224 PLC of German Siemens company. Because it needs to match with the input signal of solid-state relay, whether the PLC is damaged or not, the digital output point must be DC output type instead of relay output type. S7 224 is a small PLC with 14 digital input points and 10 digital output points. The first and second digital output points can be used as pulse width modulation (PWM) output. In practical application, we use the second output point as the control of electric heater. The man-machine interface adopts TD200 of Siemens, and the compaction density is 2 (2) 5 g/cm3 text display. This type of display can display 2x20 characters on the same screen, with background light and support Chinese display

two current type transmitters with strong anti-interference ability are selected for the temperature transmitter, which are respectively placed at the air supply outlet and air return outlet of the air conditioning unit. The output signal of the temperature transmitter is converted into digital temperature data after a/D conversion by the analog module and read into PLC. The on-off control of the main circuit of electric heater uses solid-state relay to replace the usual AC contactor

the main controller controls the startup and shutdown of the compressor according to the deviation between the fresh air inlet temperature and the set air supply temperature, as a rough adjustment of the air supply temperature. The purpose is to make the temperature of the air before reaching the heater slightly lower than the set air supply temperature. In order to prevent the compressor from frequently starting and stopping, the temperature difference between the compressor start-up temperature and shutdown temperature is set to a large value. The function of the heater is to fine tune the outlet temperature. Set the air supply temperature and measured air supply temperature, or on the basis of hydraulic loading mode, take two temperature values of electro-hydraulic servo valve or proportional valve as the input of a PID loop operation, and the result of PID operation is used for PWM output control of the heater output point, so as to control the solid-state relay, which has been highly praised by users. On off action. See Figure 1 for the flow chart of the control software

4 conclusion

this system adopts programmable logic controller and solid-state relay to design the control system. The hardware structure is simple, the response speed is fast, and the degree of automation is high. With a low investment cost, the purpose of controlling the air outlet temperature of the air conditioning unit is achieved. A 5HP unit air conditioner has been redesigned with this scheme. After nearly two years of on-site use in an engine laboratory of an automobile factory in Chongqing, the accuracy of the air supply temperature can be controlled within 0.5cc. And the operation is stable and convenient

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