Introduction
PLC programming generally uses the intuitive method, relying on the circuit designer's experience. This approach is subjective and direct. It often requires a period of trial and error to debug the program before it meets the required functions and behaviors. As a result, programs vary by author and may be difficult for other users or maintenance personnel to understand, reducing program readability.
This article outlines a structured approach using the example of a three-phase induction motor fault alarm control circuit. It shows the conversion from a traditional electrical schematic to a ladder diagram, with the aim of making related circuit conversions and program design easier to follow.
01 Traditional electrical schematic
The known three-phase induction motor fault alarm control circuit, in its traditional electrical schematic, is shown below.
Figure 1. Three-phase induction motor fault alarm control schematic
02 Operation description
(1) When the power supply is normal, only the green indicator GL is on and the motor does not run.
(2) Press the start button PB1. The magnetic contactor MC operates, the motor immediately runs, the indicator RL is on, and the green indicator GL turns off.
(3) Press the stop button PB2. The magnetic contactor MC de-energizes, the motor stops, the indicator RL turns off, and the green indicator GL turns on.
(4) While the motor is running, an overload or other fault may cause the thermal relay TH-RY to operate. The motor stops, the buzzer BZ sounds an alarm, the indicator RL turns off, and the green indicator GL turns on.
(5) Press button PB3 to silence the buzzer BZ. The white indicator WL turns on, the green indicator GL remains on, and the red indicator RL stays off.
(6) After the fault is cleared, reset the thermal relay TH-RY using its reset lever. The white indicator WL turns off, the green indicator GL turns on, and the red indicator RL turns off. The motor can be restarted.
03 I/O assignment
Using a PLC replaces hardwired control with software. The main power circuit cannot be replaced by the PLC; the PLC can replace the control circuit. The first step when converting a traditional schematic to a ladder diagram is I/O assignment. Assign the external input and output terminal numbers in the PLC to the input/output components from the schematic, and determine whether external input components are wired as A or B contacts.
(a) External wiring uses A contact method.
(b) External wiring uses B contact method.
04 PLC external wiring diagram
After I/O assignment and determining the contact wiring method, the PLC external wiring diagram is shown below. The example uses the Vigor-VB series PLC with NPN wiring; it is also possible to parallel the 24V terminal and the S/S terminal.
Figure 2. PLC external wiring diagram
05 PLC ladder diagram
The steps to convert the traditional schematic into a ladder diagram are as follows:
(1) Directly convert the control circuit from the schematic into the corresponding ladder diagram. Because ladder diagrams require contacts to appear before the output coil at the end of the rung, redraw the schematic to reposition contacts and coils to meet ladder diagram requirements. The redrawn schematic is shown below.
Figure 3. Redrawn control schematic
(2) Replace the input/output components in the schematic with the I/O-assigned component numbers. Note that the TH-RY C-A and C-B contacts should be separated into independent control circuits, as shown below.
Figure 4. Control schematic with I/O assignment
(3) Rotate the schematic shown in Figure 4 90 degrees counterclockwise, then flip it vertically to form a PLC ladder diagram. Because some contact and coil placements such as Y1, X0 contacts and coils Y1 and Y2 may not match typical programming software formats, make appropriate adjustments as shown to the right in the figure below.
If using Visio to draw the schematic, rotate left 90 degrees then flip vertically to obtain the ladder layout.
(4) Draw the ladder diagram using programming software. The resulting ladder diagram matches the rotated and corrected diagram.
Figure 5. Ladder diagram created in programming software
06 PLC external wiring and ladder diagram
The complete PLC external I/O wiring and ladder diagram after conversion from the traditional schematic are shown below. The diagrams illustrate the external input/output wiring and ladder diagram when software replaces hardware wiring.
Figure 6. PLC external wiring and ladder diagram after replacing hardware wiring with software
07 Conclusion
PLCs were developed to replace relay-based sequential control, using software to replace hardwired connections. By changing the program, control sequences can be modified to meet different control requirements. PLCs evolved from traditional relay control circuits by symbolizing relay contacts and coils; once converted into ladder diagrams or instructions, the control can be implemented in software.
