Explanation of the principle of the three-phase asynchronous motor forward and reverse circuit diagram

In the ladder diagram, two start-stop circuits are used to control the forward and reverse rotation of the motor respectively. When the forward start button SB2 is pressed, X0 becomes ON, its normally open contact closes, the coil of Y0 is energized and self-latches, energizing the coil of KM1, and the motor starts to rotate forward. When the stop button SB1 is pressed, X2 becomes ON, its normally closed contact opens, causing the Y0 coil to de-energize, and the motor stops running.

In the ladder diagram, the normally closed contacts of Y0 and Y1 are connected in series with each other's coils respectively, ensuring that they cannot be ON simultaneously, thus preventing the coils of KM1 and KM2 from being energized simultaneously. This safety measure is called "interlock" in relay circuits. In addition, for ease of operation and to ensure that Y0 and Y1 are not simultaneously ON, a "button interlock" is also set in the ladder diagram, which connects the normally closed contact of the reverse start button X1 in series with the coil of the forward control Y0, and the normally closed contact of the forward start button X0 in series with the coil of the reverse control Y1. If Y0 is ON and the motor is rotating forward, if you want to change to reverse rotation, you can directly press the reverse start button SB3 without pressing the stop button SB1. X1 becomes ON, its normally closed contact opens, causing the Y0 coil to de-energize, and at the same time, the normally open contact of X1 closes, energizing the Y1 coil, and the motor changes from forward to reverse rotation.

The interlock and button interlock 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 switching, there may be a phenomenon where one contactor has not yet broken the arc, but the other has already closed, resulting in a momentary short circuit fault. This problem can be solved by using a delay during forward and reverse switching, but this scheme will increase the programming workload and cannot solve the aforementioned contactor contact fault-induced power short circuit accident. If the main circuit current is too large or the contactor quality is poor, and the main contact of a contactor is welded due to the arc generated when the power is cut off, and the main contact is still closed after the coil is de-energized, if the coil of another contactor is energized at this time, a three-phase power short circuit accident will still occur. To prevent this situation, a hardware interlock circuit composed of auxiliary normally closed contacts of KM1 and KM2 should be set outside the PLC (see Figure 2). Assuming that the main contact of KM1 is welded by the arc, its auxiliary normally closed contact connected in series with the KM2 coil is in the open state, so the KM2 coil cannot be energized.

The FR in Figure 1 is a thermal relay used for overload protection. When an asynchronous motor is seriously overloaded for a long 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, playing a protective role.

Some thermal relays require manual reset, that is, after the thermal relay operates, press its built-in reset button, and its contacts will return to their original state, that is, the normally open contact opens, and the normally closed contact closes. The normally closed contact of this type of thermal relay can be connected to the PLC output circuit as shown in Figure 2, still connected in series with the contactor coil. This scheme can save one input point of the PLC.

Some thermal relays have an automatic reset function, that is, after the thermal relay operates and the motor stops, the thermal element in the main circuit connected in series is cooled, and the contacts of the thermal relay automatically return to their original state. If the normally closed contact of this type of thermal relay is still connected to the PLC output circuit, after the motor stops, it will automatically restart after a period of time due to the restoration of the thermal relay contacts, which may cause equipment and personal accidents. Therefore, the normally closed contact of a thermal relay with an automatic reset function cannot be connected to the PLC output circuit, and its contact must be connected to the PLC input end (a normally open or normally closed contact can be connected), and the ladder diagram is used to achieve motor overload protection. If an electronic motor overload protector is used to replace the thermal relay, its reset method should also be noted.