With the gradual improvement of people’s awareness of power supply safety, dual power supply circuit must be used in many occasions to ensure safe and reliable power supply. In this paper, the double power automatic switching system based on PLC uses undervoltage relay to detect, which ensures the safety and reliability of the circuit and avoids many unnecessary losses.
In the design of dual power switch, power A is connected to the load to detect the phase gap. If there is a fault, it is automatically switched to power B. If there is no fault, it is also necessary to detect the overvoltage and undervoltage. After 24-hour test, AB of power supply automatically switches, and then automatically determines whether B power supply is out of phase.
Then, overvoltage and undervoltage detection are carried out.
If there is no problem, switch detection is continued, and then repeated detection is carried out. The flow chart is shown in Figure 1. Power AB connects to PLC.
First, power A carries out phase-absence detection. If there is no phase-absence in the detection, the logic switch of PLC is closed, and the PLC display of FX3U-32MR has no phase-absence.
Subsequently, the overvoltage and undervoltage are detected. If the indicator lamp is in good condition, the power supply A is connected to the load without any problem. If the indicator lights alarm, then immediately use power B, B power supply also needs to carry out phase-gap detection, and then overvoltage and undervoltage detection, if connected with the load, no problem. When power A is connected to the load for 24 hours, even if power A is normal, it should be switched to power B. After 24 hours, it will continue to switch repeatedly.
I/0 address, as shown in Table 1. Detection of kv1, kv2, Kv3 coil undervoltage, as shown in Figure 2, if there is no undervoltage, kv1, kv2, Kv3 switch closed, KA1 coil power, KM1 coil power, KM1 switch closed load power. If one of the switches of kv1, Kv2 and Kv3 is not closed, thermostatic element then KA1 can’t get electricity, KM1 coil switch can’t get electricity and load can’t get electricity. The first case: first enter the initialization state of PLC, and then determine whether power A is under-voltage, if there is under-voltage, it proves that power A can not be used.
Then determine whether the power supply B is under-voltage or not, and return to the initialization state of PLC if under-voltage also occurs. The second situation: after the initialization of PLC, determine whether power A is under-voltage, if no under-voltage is found, then use power A to work for 24 hours, then judge whether power B is under-voltage, and if no under-voltage occurs, power B will continue to judge whether power A is under-voltage after 24 hours, so as to cycle. The third case: after the initialization of PLC, determine whether the power A is under-voltage. After discovering that the power A is not under-voltage, the power A works 24 hours, and then judge whether the power B is under-voltage.
If there is under-voltage, continue to judge whether the power A is under-voltage, so as to cycle. The dual-power automatic switching system designed in this paper realizes the power supply A and B voltage suddenly stop or under-voltage, ensures the safe power supply of the power supply, improves the power supply efficiency and realizes the power supply 24-hour automatic switching function.