This causes the forward start relay (MF) in the first rung to be de-energized, which in turn de-energizes the forward motor starter (F) in the third rung. At the same time, the reverse motor starter (R) is energized in the fourth rung to cause the motor to rotate in the reverse direction. Pressing the stop switch (SP) causes the motor to stop in either direction.
2.7 Analyzing Electrical Control Circuits
2.7.1Circuit Analysis
Often times you need to analyze an existing electrical circuit that may not be familiar to you. You may not have the necessary documents that show how the circuit operates or perhaps even the circuit drawings with which to work. The other scenario is that you need to verify the electrical circuit you just designed. It is always necessary to analyze and prove the circuit before it is hard-wired and implemented. The circuit analysis presented in this section involves sequentially listing the status of all devices in each operating step. The status of devices in one operation step determines the operating conditions of the next step. You can fully understand the operation of the circuit and the system it controls after all operating steps have been completed. Circuit analysis is contrary to circuit design. In circuit analysis, you already have an existing circuit and try to find out how it performs. In circuit design, you identify a complete list of tasks to occur and their sequence, then design the logic to implement them. The documents of circuit analysis are a useful reference for trouble-shooting, maintenance, and system operations.
2.7.2Circuit Analysis Procedure
An eight-step procedure can be followed to analyze an existing control circuit. The tool for circuit analysis is a state chart (or table) that lists all control components in the circuit, operation steps, and state of control components in each operation step. Figure 2.31 shows a blank state chart. The relay circuit shown in Figure 2.32 is used to illustrate this eight-step procedure.
Figure 2.31: Blank state chart
Figure 2.32: A control circuit
The procedure for creating a state chart consists of the following eight steps:
a.Identify all devices used in the circuit and place each device in one of the following four categories:
| Input devices: | Devices provide information to the control system. |
| Logic devices: | Devices perform logic decision making. |
| Output devices: | Devices execute the control action. |
| Actuation devices: | Devices perform actual work in the physical system. |
In this example circuit, there are four input switches, three output devices, three logic elements, and one actuator. They are:
| Input: | push buttons, PS and PT |
| limit switches, LS1 and LS2 | |
| Output: | solenoid S, relay R and relay S |
| Logic: | contacts R-1, R-2, and S-1 |
| Actuator: | cylinder |
Figure 2.33: Reference Legend
b.Assign a legend list for reference.
The four-column table shown in Figure 2.33 can be used to create a legend list. It consists of four items: legend, device type, normal state, and function description. The legend is the text symbol of the device. It can be taken directly from the electrical circuit diagram, if available. If not available, give a proper legend to each device. The device type indicates what kind of device it is, such as limit switch, solenoid, etc. The normal state is used to indicate the non-actuated state of the input devices. It can be either a NO (normally open) or a NC (normally closed) state. The function description describes the intended function to be performed by this device.
c.Construct an analysis form.
Use the blank state chart as the analysis form. Fill in the symbols of these devices in the top horizontal row of the table. Follow the order of input devices, logic devices, output devices, and actuation devices. The first column from the left is the operation step column. All other columns display the status (or state) of the devices in the specified operation steps. Figure 2.34 shows a circuit analysis form with twelve control elements in this sample circuit.
Figure 2.34: Circuit analysis form
Table 2.2: Initial state of elements
| Device | State of Device |
|---|---|
| Solenoids, relay coils, timer coils, and other output devices | Energized (EN)De-energized (DE) |
| Contacts and switches | OpenClosed |
| Motors | ForwardStopReverse |
| Cylinder | ExtendRetractStop |
d.Determine the initial state of each device when the system is at rest.
The system at rest means that the system is in its non-operative condition. In other words, the system is not yet started. In most cases, the initial state of devices is the normal state at their non-actuated condition. Write the initial state of each device in the first row. Table 2.2 summarizes the legends used to describe the state of a device:
At rest, the state of each element in the sample circuit can be readily read from the circuit drawings. They are listed below:
| PS: | open |
| PT: | closed |
| LS1: | held to closed |
| LS2: | closed |
| R: | de-energized |
| R-1: | open |
| R-2: | open |
| S: | de-energized |
| S-1: | open |
| S-2: | open |
| Sol: | de-energized |
| Cyl: | fully retracted |
Fill in these states to the circuit analysis form to complete the step at rest, as shown in Figure 2.35.
e.Determine which input device starts the operation of the system.
Most likely every electrical control system has a start switch to start the system
