In the ladder diagram, two start and stop circuits are used to respectively control the forward and reverse rotation of the motor. Press and hold the forward rotation start button SB2, X0 becomes ON, its normally open contact is connected, and the coil of Y0 is "powered on" and self holding, so that the coil of KM1 is powered on, and the motor starts to move forward. Press the stop button SB1, X2 becomes ON, and its normally closed contact is broken, so that Y0 coil "trips" and the motor stops.

In the ladder diagram, the normally closed contacts of Y0 and Y1 are connected in series with each other's coils to ensure that they will not be ON at the same time, so the coils of KM1 and KM2 will not be energized at the same time. Such safety measures are called "interlocking" in relay circuits. In addition, in order to facilitate operation and ensure that Y0 and Y1 will not be ON at the same time, "key interlock" is also set in the ladder diagram. The normally closed contact of the startup key X1 to be turned over is connected in series with the coil of Y0 to control the forward rotation, and the normally closed contact of the forward rotation startup key X0 is connected in series with the coil of Y1 to control the reverse rotation. If Y0 is set to ON, the motor will rotate in the forward direction. At this time, if you want to change to the overturning operation, you can directly press the overturning start button SB3 instead of pressing the stop button SB1, and X1 will become ON. Its normally closed contact will be broken, making the coil Y0 "trip". At the same time, the normally open contact of X1 will be connected, making the coil Y1 "energized", and the motor will turn from the forward direction to the overturning direction.

The interlock and key interlock circuits in the ladder diagram can only ensure that the normally open contact points of the hardware relays corresponding to Y0 and Y1 in the output module are not connected at the same time. Because of the time-delay effect of the inductance in the conversion link, it is possible that one contactor has not broken the arc, but the other has been covered, thus causing a transient short circuit fault. This problem can be solved by using the time delay after the forward and reverse conversion, but this scheme will increase the workload of programming, and can not deal with the power short circuit accident caused by the contactor contact failure that is not described. If the main contact of a contactor is bonded by arc fusion welding when the power is cut off due to excessive current of the main circuit or poor quality of the contactor, the main contact is still connected after the coil is cut off. At this time, if the wiring diagram of another contactor is energized, it will still lead to a three-phase power supply short circuit accident. In order to avoid this situation, it is necessary to set a hardware interlock circuit (see Figure 2) composed of KM1 and KM2 auxiliary normally closed contacts outside the PLC. Assuming that the main contact of KM1 is arc welded, at this time, the auxiliary normally closed contact connected in series with KM2 coil is disconnected, so the coil of KM2 cannot be powered on.

FR in Figure 1 is a thermal relay used for overvoltage protection. When the asynchronous motor is seriously overloaded for a long time, after a certain delay, the normally closed contact of the thermal relay is broken and the normally open contact is closed. The normally closed contact is connected with the coil of the contactor in series. When the load is on, the coil of the contactor is powered off, and the motor stops running to protect it.

Some thermal relays must be reset manually, that is, after the thermal relay acts, press the reset switch it has, and its contacts can return to normal, that is, the common open contacts are broken, and the normally closed contacts are closed. The normally closed contact of this type of thermal relay can be connected to the output circuit of PLC as shown in Figure 2, and still connected to the coil of the contactor in series. This scheme can save one input point of PCL.

Some thermal relays have the function of automatic reset, that is, the motor stops after the thermal relay acts, the thermal elements of the thermal relay connected in series in the main circuit are cooled, and the contacts of the thermal relay automatically return to normal. If the normally closed contact of this type of thermal breaker is still connected to the output circuit of the PLC, the motor will automatically run again after a period of time after turning and stopping because the contact of the thermal relay returns to normal, which may lead to equipment and safety accidents. Therefore, the normally closed contact of the thermal relay with the function of automatic reset cannot be connected to the output circuit of the PLC. It must be connected to the input terminal of the PLC (normally open contact or normally closed contact can be connected). The ladder diagram is used to achieve over-voltage protection of the motor. If the electronic motor overload protector is used to replace the thermal relay, pay attention to its reset method.