PLC three-phase induction motor fault alarm control circuit wiring diagram

1 Traditional electrical diagram

A conventional electrical diagram of a known three-phase induction motor fault alarm control circuit is shown in FIG1 :

Figure 1 Three-phase induction motor fault alarm control circuit diagram

2 Action description 1) When the power supply is normal, only the green light gl is on, and the motor does not move. 2) Press the start button pb1, the electromagnetic contactor mc is activated, the motor starts to run immediately, the indicator light rl is on, and the green light gl is off. 3) Press the stop button pb2, the electromagnetic contactor mc is powered off, the motor stops running, the indicator light rl is off, and the green light gl is on. 4) When the motor is running, due to overload or other faults, the thermal relay th-ry is activated, the motor stops running, the buzzer bz sounds an alarm, the indicator light rl is off, and the green light gl is on. 5) Press the button switch pb3, the buzzer bz stops the alarm, the white light wl is on, the green light gl is on, and the red light rl is off. 6) After the fault is eliminated, press the reset lever of the thermal relay th-ry, then the white light wl is off, the green light gl is on, and the red light rl is off, and the motor can be restarted.

3 I/O encoding

Using PLC means replacing hardware wiring with software programs. In traditional electrical diagrams, the main circuit cannot be replaced by PLC; the part that PLC can replace is the control circuit. The first step in converting traditional electrical diagrams to ladder diagrams is i/o coding, that is, determining the external input/output terminal number of the input/output components in the traditional electrical diagram and whether the external input component wiring method uses a/b contacts.

(a): External connection using a contact method (b): External connection using b contact method

4 PLC external wiring diagram

After the input/output components are coded by I/O and the external input components are determined to use A/B contact wiring, the PLC external wiring diagram is shown in Figure 2. The figure shows the Fengwei Vigor-VB series PLC model, which uses NPN wiring and can also be connected in parallel with the 24V terminal and S/S terminal.

Figure 2 PLC external wiring diagram

5 PLC Ladder Diagram The program design steps for converting the traditional electrical diagram to the ladder diagram are as follows: (1) Convert the control circuit in the electrical diagram directly into the corresponding ladder diagram. Because the PLC ladder diagram stipulates that the contacts are in the front and the output coil must be at the end of the loop. Therefore, the electrical diagram must be redrawn first, and the positions of the contacts and output coils in the diagram must be appropriately changed to meet the requirements of the PLC ladder diagram. The redrawn electrical diagram is shown in Figure 3.

Figure 3 Redrawn electrical diagram

(2) Replace the input/output components in the electrical diagram with the component numbers after the i/o coding. It should be noted here that the CA contact and CB contact of TH-RY should be separated and each become a control circuit, as shown in Figure 4.

Figure 4 Electrical diagram after i/o coding

(3) Rotate the electrical diagram shown in Figure 4 90° to the left and then flip it vertically (upsidedown) to become a PLC ladder diagram. However, since the y1 and x0 contacts and the output coils y1 and y2 do not conform to the general programming software format, they must be appropriately corrected, as shown on the right side of Figure 5.

Figure 5: The ladder diagram after appropriate flipping and correction

※If you use Visio to draw electrical diagrams, it will be easy to rotate 90° to the left and then flip it vertically.

(4) The ladder diagram drawn using the programming software, as shown in FIG6 , is exactly the same as the ladder diagram after being appropriately flipped and corrected.

Figure 6 Ladder diagram drawn by programming software

6 Instructions

Converting the ladder diagram into instructions is as follows:

7 PLC conversion wiring and ladder diagram

PLC external input/output wiring and ladder diagram after complete conversion of traditional electrical diagram. PLC external input/output wiring and ladder diagram after using software program to replace hardware wiring, as shown in Figure 7 below.

Figure 7 PLC after replacing hardware wiring with software program

8 Conclusion The purpose of PLC development is to replace sequential control based on relays, that is, to use software programs to replace hardware wiring. Therefore, the control sequence can be changed by simply changing the software program, and different control requirements can be easily achieved.

The general PLC is developed based on the traditional relay control circuit, and the contacts and coils of the relay are symbolized. When converted into a general ladder diagram or instructions, its control can be realized.