PLC internal and external circuits

1 external circuit wiring
Figure 1 shows the electrical control circuit of the contactor for full voltage starting control of the motor. The control logic is realized by connecting the coil of AC contactor km, indicator lights HL1, HL2, normally closed contact fr of thermal relay, stop button SB2, start button SB1 and normally open auxiliary contact km of the contactor through wires.
After closing QS, press the start button SB1, then the coil km is energized and self-locking, and the auxiliary contact km of the branch where the indicator light HL1 is located and the main contact in the main circuit are connected, HL1 is on, and the motor M starts; Press the stop button SB2, then the coil km is powered off, the indicator light HL1 is off, and M stops rotating.


Figure 1 electric control circuit of motor full voltage starting
Figure 2 is the external wiring diagram of a S7 Series PLC of Siemens to realize the full voltage starting control of the motor. The main circuit remains unchanged. The normally closed contact fr of thermal relay, stop button SB2, start button SB1, etc. are connected to the input interface of PLC as the input equipment of PLC, while the coil of AC contactor km, indicator lights HL1, HL2, etc. are connected to the output interface of PLC as the output equipment of PLC. The control logic is realized by executing the user program written according to the requirements of motor full voltage control and stored in the program memory.


Fig. 2 PLC control wiring diagram of motor full voltage starting
a) Main circuit B) i/o actual wiring diagram