SB821 ABB

It can be known from the working principle of PLC that when the input terminal is connected to the moving contact,when the PLC is working,if the moving contact of the input is closed,the input relay coil corresponding to the input terminal is energized,and its moving contact is closed.,The moving break contact is disconnected;when the input end is connected to the moving break contact and the PLC is working,if the moving break contact of the input end does not act,the input relay coil corresponding to the input end is energized,and its moving closing contact Closing and breaking contacts are disconnected.If the break contact is connected in series with the output relay coil,the output relay coil cannot be energized.Therefore,PLC is used to control the start and stop of the motor.If the stop button uses a moving break contact,the PLC output relay coil connected to the contactor that controls the motor should be connected in series with the moving contact corresponding to the input terminal connected to the stop button..In the relay contact control,both the stop button and the thermal relay use the moving break contact.In order to be consistent with the control circuit of the relay contact control,the moving break contact is also used in the PLC ladder diagram.The stop button and the thermal relay contact connected to the input end must use the moving contact.This must be taken into account when reading programs.

由PLC的工作原理可知，当输入端接动合触点，在PLC工作时，若输入端的动合触点闭合，则对应于该输入端子的输入继电器线圈得电，它的动合触点闭合、动断触点断开；当输入端接动断触点且在PLC工作时，若输入端的动断触点未动作，则对应于该输入端的输入继电器线圈得电，它的动合触点闭合、动断触点断开。如果该动断触点与输出继电器线圈串联，则输出继电器线圈不能得电。因而，用PLC控制电动机的启停，如果停止按钮用动断触点，则与控制电动机的接触器相接的PLC输出继电器线圈应与停止按钮相接的输入端子相对应的动合触点串联。在继电接触控制中，停止按钮和热继电器均用动断触点，为了与继电接触控制的控制电路相一致，在PLC梯形图中，同样也用动断触点，这样一来，与输入端相接的停止按钮和热继电器触点就必须用动合触点。在识读程序时必须注意这一点。