In the broad discipline of electrical engineering, many types of electrical circuits are involved, such as, for example, the electrical circuits of basic electrical components (R, L, and C), electronic circuits, power circuits, telephone circuits, and integrated circuits, and many more. However, the electrical circuits being utilized for the study and implementation of an industrial manufacturing plant can be divided into the following categories:
- Power circuit
- Control circuit
The power circuit (also called “main circuit”) indicates the type of power supply for the utilized motors and all other related power devices.
As an example, Figure 1 depicts:
(a) the single-line, three-phase circuit of a motor with two directions of rotation;
(b) the complete multi-line circuit of the same motor; and
(c) the power circuit of a direct starting motor.
The control circuit (which can also be referred to as an automation circuit, auxiliary circuit, secondary circuit, or schematic circuit) represents the operational logic and control of the power devices.
It is indicated in Figure 2, for a start/stop operation of the previously depicted motor in Figure 1c.
Wiring diagrams are circuits representing both the power circuit and the control circuit while, at the same time, representing the actual positioning of all the devices and components in the industrial installation, which is ideal information for the technician executing the wiring and overall installation. In Figure 3, the wiring diagram produced from the synthesis of the power circuit in Figure 1c, and the control circuit shown in Figure 2, is displayed.
It should be noted that in the wiring diagram of Figure 3, the wires can intersect each other, and thus it is very difficult to follow the route of each wire and the overall functionality of the design, even in this very simple case, where we have only four devices (one relay, two buttons, and one overload protection). Having this in mind, it can be easily generalized how this complexity will grow in cases with more intersections, e.g., in wirings with 50 devices. In contrast to this, in the control circuit of Figure 2, there are no wiring intersections and thus it is very easy to follow and understand the overall operation logic.
For this reason, control circuits provide an overview of the automation functionalities that are most commonly utilized during the development, installation, and operation of an automation system. Control circuits are being developed in branches (sectors), which are presented in Figure 4.
Each branch denotes the operational function of a corresponding relay, solenoid, or actuator, while the whole control circuit denotes the operational logic of the overall industrial process automation. Each branch in the control circuit can have multiple parallel sub-branches, depending on the complexity of the implemented logical function. In Figure 4, the indicating branches have the simplest form. Each one implements the logic “If the rotary switch RSi is closed, then the corresponding “i” motor will be in operation”. An industrial system with a control circuit of the form presented in Figure 4 has a manual operation. In reality, this is not common, since the start and stop operations of an industrial process with similar machines are executed in an automated manner, based on the sequence of the sensing signals and the corresponding status of the machines. In these cases, which are dominant in industrial automation, control circuits are becoming more complex and thus a proper methodology for designing such automation circuits is needed.