Figure 1 shows the main circuit and relay control circuit diagram for the forward and reverse rotation control of a three-phase asynchronous motor. Figures 2 and 3 show the external wiring diagram and ladder diagram of a PLC control system with the same function. KM1 and KM2 are AC contactors that control forward and reverse rotation, respectively.
In the ladder diagram, two start-stop circuits are used to control the forward and reverse rotation of the motor, respectively. Pressing the forward start button SB2 turns X0 ON, its normally open contact closes, energizing the Y0 coil and holding it in place, thus energizing the KM1 coil, and the motor begins to run forward. Pressing the stop button SB1 turns X2 ON, its normally closed contact opens, de-energizing the Y0 coil, and the motor stops running.
In a ladder diagram, connecting the normally closed contacts of Y0 and Y1 in series with each other's coils ensures they will not be ON simultaneously. Therefore, the coils of KM1 and KM2 will not be energized at the same time. This safety measure is called "interlocking" in relay circuits. In addition, to facilitate operation and ensure that Y0 and Y1 will not be ON simultaneously, a "button interlock" is also included in the ladder diagram. This involves connecting the normally closed contact of the reverse start button X1 in series with the coil of Y0, which controls forward rotation, and connecting the normally closed contact of the forward start button X0 in series with the coil of Y1, which controls reverse rotation. If Y0 is ON and the motor is rotating forward, to change it to reverse operation, the reverse start button SB3 can be pressed directly without pressing the stop button SB1. X1 becomes ON, its normally closed contact opens, de-energizing the Y0 coil. Simultaneously, the normally open contact of X1 closes, energizing the Y1 coil, and the motor changes from forward to reverse.
The interlocking and push-button interlocking circuits in the ladder diagram can only ensure that the normally open contacts of the hardware relays corresponding to Y0 and Y1 in the output module will not be closed simultaneously. Due to the delay effect of the inductance during the switching process, it is possible that one contactor has not yet broken the arc while the other has already closed, resulting in a momentary short circuit fault. This problem can be solved by using the delay during forward and reverse switching, but this solution will increase the programming workload and cannot solve the power supply short circuit accident caused by the contactor contact fault mentioned above.
If, due to excessive main circuit current or poor contactor quality, the main contacts of one contactor are welded together by the arc generated when the power is cut off, the main contacts will remain closed after the coil is de-energized. In this case, if the coil of another contactor is energized, a three-phase power short circuit will still occur. To prevent this from happening, a hardware interlock circuit consisting of the auxiliary normally closed contacts of KM1 and KM2 should be installed outside the PLC (see Figure 2). Assuming that the main contacts of KM1 are welded together by the arc, the auxiliary normally closed contact connected in series with the coil of KM2 will be in the open state, so the coil of KM2 cannot be energized.
In Figure 1, FR is a thermal relay used for overload protection. When an asynchronous motor is severely overloaded for a long period of time, after a certain delay, the normally closed contact of the thermal relay opens and the normally open contact closes. Its normally closed contact is connected in series with the coil of the contactor. When overloaded, the contactor coil is de-energized, and the motor stops running, thus providing protection.
Some thermal relays require manual reset. This means that after the thermal relay trips, you must press its built-in reset button for the contacts to return to their original state: the normally open contact opens, and the normally closed contact closes. The normally closed contact of this type of thermal relay can be connected in series with the contactor coil in the PLC output circuit, as shown in Figure 2. This solution saves one input point for the PLC.
Some thermal relays have an automatic reset function. This means that after the thermal relay trips, the motor stops, the thermal element of the relay connected in series in the main circuit cools down, and the relay contacts automatically return to their original state. If the normally closed contact of this type of thermal relay is still connected to the PLC's output circuit, the motor will automatically restart after a period of time due to the thermal relay contacts returning to their original state, potentially causing equipment and personal injury accidents. Therefore, the normally closed contact of a thermal relay with an automatic reset function cannot be connected to the PLC's output circuit; it must be connected to the PLC's input terminal (either normally open or normally closed contacts can be connected), using a ladder diagram to implement motor overload protection. If an electronic motor overload protector is used instead of a thermal relay, its reset method should also be carefully considered.
