PLC output contact failure and its elimination

PLC output contact failure and troubleshooting Programmable logic controller (PLC) is widely used in industrial control. As a switching device, its output contact is often used to connect or disconnect external relays , Contactor coil circuit. However, in practical applications, such failures often occur in PLC output contacts: PLC output contacts can connect external circuits under program control, but cannot disconnect external circuits under program control.
The cause of the failure needs to be analyzed from the output circuit of the PLC itself. The PLC output circuit includes a transistor output circuit, a bidirectional thyristor output circuit and a relay output circuit. Depending on the size of the load, select the PLC with the corresponding output circuit: when there is a load such as a contactor, a relay output circuit with a larger output power should be selected. The principle of the relay output circuit is shown in Figure 1.


Figure 1 Schematic diagram of output circuit and external circuit
KD is a small DC relay with a value of about 80VA and cannot carry a large load. In order to improve the load capacity of the contact, a series RC branch is often connected in parallel to eliminate the arc generated when the contact is disconnected, so as to improve the breaking capacity of the contact. K is an external contactor.
The basic working principle of PLC is that when the result of program logic operation makes the base of the transistor V in the PLC be at a high level, V is turned on, the KD coil is energized, its normally open contact is closed, and the external K of the PLC is pulled in. ; When the result of the logic operation of the program makes the base of V very low, V is cut off, KD is powered off, and the PLC external K trips. In this way, the PLC controls the on-off of the external K through a pre-programmed program.
From Figure 1, it can be seen that the reason why its output contact can be closed by an external K and cannot be opened is because the moving contact of KD is glued when it is broken. There are two reasons why the moving contact is glued: (1) The RC branch circuit component connected in parallel with it may be damaged, or the wire may be broken, and the arc extinguishing ability may be reduced, so that the KD moving contact may be glued when it is broken; ( 2) The external K capacity is too large. When the K capacity is large, the magnetic field energy stored in the coil will be large, and when the breaking capacity of the PLC output contact is exceeded, it will be broken http://www.diangon.com, and the moving contact of the PLC output relay may be glued. .
Regardless of the above reasons, the following methods can be adopted:


Figure 2 Capacitors connected in parallel with AC contactor coil
When the output terminal of PLC is connected with AC contactor, it can be connected between the contactor coil A capacitor (or a capacitor plus a resistor) is connected in parallel to the terminal, as shown in Figure 2. When the KD coil is powered off and the moving contact breaks, the K coil CI forms an oscillating loop, and the magnetic field energy stored in the coil can be released through the loop formed by RI and CI, which can weaken the arc intensity generated when the K coil loop is disconnected. It can prevent the moving contact of the relay from being stuck when breaking. For example, we use PLC to control the contactor CJ10-10 in the speed control equipment, the rated current is 10A, and the coil voltage is 220V. The load on the output relay of the PLC is too large. Therefore, the failure phenomenon that K cannot be opened often occurs. Later, we connected a capacitor of about 0.1uF in parallel at both ends of K, and the fault that could not be opened was eliminated immediately.

Figure 3DC relay coil anti-parallel diode
When the output terminal of the PLC is connected to the DC relay KD, the current flowing through the coil is DC. In order to eliminate the fault that cannot be opened, a diode can be connected in parallel at both ends of the coil, as shown in Figure 3 . When KD is de-energized, its moving contact breaks when the DC relay coil in Figure 3 is anti-parallel to the diode, the current in K continues to flow through the diode VD, which can reduce the high voltage generated when the K coil is de-energized, which is conducive to arc extinguishing and avoids The output contacts of the PLC are stuck.
In short, as long as the fault is analyzed correctly, the fault phenomenon will be eliminated quickly.

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