Analysis of Low-temperature Out-of-control Faults in Alternating-cycle High-low Temperature Test Chamber
After long-term use of the high-low temperature alternating cycle test chamber, the temperature of the low temperature test rises during a test and fails to meet the low temperature test requirement. The general reason is analyzed based on the working principle and fault phenomenon of the test box, and the analysis conclusion is based on the test. The box was inspected and it was found that the failure of the output terminal of the solid state relay resulted in the failure. The faulty component was replaced. The function of the equipment was restored to normal and the temperature test was satisfied.
The high and low temperature alternating cycle test chamber has accurate temperature control and reliable operation during long-term use, and has played an important role in engine temperature testing for many years. However, at a low temperature of -40°C in July 2010, the temperature did not continue to drop at -30°C and there was a slight recovery. The room leader organized a temperature test technician to troubleshoot the equipment. Saved maintenance funds and completed engine test tasks on time.
2. Fault phenomenon
The high and low temperature alternating cycle test chamber carries out the low temperature test on the engine. The temperature measured on the computer is no longer about -30°C, and the temperature of the test chamber rises slowly with time. Observe that both compressor groups are in working condition, and the exhaust pressure and suction pressure of the high-temperature and low-temperature stages are all within the normal range. The frosting condition of the compressor head is normal. Turn on the lighting and observe through the glass window of the chamber door. There was no frost in the test chamber, and the entrance and exit of the air duct did not receive too much frost.
3 high and low temperature alternating test chamber composition and temperature control principle
3.1 Test Chamber Composition
The test chamber consists of a heating system, a refrigeration system, a circulating air system, a control system, a protection system, and a box body.
Heating system consists of electric heaters and other components;
The refrigeration system consists of a high-temperature compressor refrigeration unit, a low-temperature compressor refrigeration unit, a condenser, an evaporator refrigeration piping system, and a cooling capacity control solenoid valve;
The circulating air system consists of fans, air ducts, etc.
The control system consists of detection sensors, PLCs, touch screens, and relays, contactors, etc. that are output.
The protection system consists of fault detection circuits, modules, and PLCs.
The box consists of steel plate, heat insulation material, door, observation window and so on.
3.2 Control principle of high and low temperature alternating cycle test chamber
Set the target temperature value through the touch screen. After the device is running, the temperature detected by the sensor installed in the air duct is detected. The PLC calculates the deviation between the two. The PID algorithm calculates the output according to the size, direction, and rate of the deviation. The range is -100% to +100%. When the output is less than 0, the refrigeration unit is working, and the cold capacity control valve adjusts the amount of cooling delivered to the evaporator according to the output, and the temperature in the test box is controlled at the set temperature under the effect of circulating wind; when the output is When it is greater than 0, the electric heater works, and the solid state relay adjusts the amount of heat generated by the electric heater according to the output, and controls the temperature in the test box at the set temperature under the action of the circulating wind. The temperature control principle of the test chamber is shown in Figure 1.
1: Circulation wind 2: Cooling output 3: Heat output 4: Test chamber
4. Causes of out-of-control at low temperature
The control principle of the high-low temperature alternating cycle test box shows that because the product is subjected to a low temperature test, in order to obtain a low temperature, the temperature deviation is a positive deviation, and the PLC should output an output value less than 0, ie, the refrigeration unit can work to obtain the test box. Low temperature, and because the test chamber controls the heating system and the refrigeration system are independent of each other, the electric heater should be stopped. So the loss of control at low temperatures should cause problems for the refrigeration system. However, it can be seen from the phenomenon of failure, the refrigeration unit compressor head has a small amount of frost, the compressor suction and discharge pressure is normal, indicating that the refrigeration system is working properly, and through the door and window observation in the test chamber and the air inlet and outlet does not endure too much frost According to experience, it can be judged that the evaporator should not have too much frost, and the temperature rise in the test box is not due to poor evaporator heat transfer caused by too thick evaporator frost, and the blocking of the air duct affects the circulation of the wind. The frosting conditions in the test chamber and at the inlet and outlet of the air duct also indicate that the test chamber is in good sealing condition.
From the above analysis of the refrigeration system there should be no problem, only the idea to the electric heating part. According to the control principle of the high-low temperature alternating test box, if the equipment is under normal operating conditions, the heating system is in a stopped state, but if some components of the equipment have a failure, such as contactor contact adhesion, solid state relay output Breakdown short circuit causes the heating system to be in an error state. At this time, the heat generated by the electric heater cancels out a part of the cooling capacity, and thus the low-temperature out-of-control failure phenomenon occurs.
5. Fault check
View the schematic diagram of the high and low temperature alternating cycle test chamber. The heating system and the circulating fan of the test chamber are controlled by a contactor. The control of the electric heater is controlled by a circuit breaker and solid state relay in addition to the contactor control. It is normally closed. When the test box is running, the fan works, and the power supply terminal of the solid state relay output part is powered. It can be known from the analysis of the causes of the low temperature out of control that when the equipment is running, the power supply end of the output part of the solid state relay is energized, and the breakdown of the solid state relay is damaged. Even if the control terminal of the solid state relay does not receive a control signal, electric energy can be added to the electric heater. So you can check whether the solid state relay breakdown, with a clamp meter current, there is a group of heaters have a current, the device power off measurement corresponding to the solid state relay output terminal has broken down. The solid state relay was replaced. The test run of the test chamber, low temperature control returned to normal.
6. Conclusions and recommendations
The low-temperature out-of-control of the high-low temperature alternating cycle test chamber is caused by the erroneous operation of the electric heater due to breakdown breakdown at the output terminal of the solid state relay.
Suggestion: The temperature test involves the knowledge of electrical control theory, compressor mechanics, refrigerants, and circulating wind fluid mechanics. Only the usual targeted learning, combined with the usual maintenance experience, can solve the problems that occur during the subsequent temperature test. Various problems.
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