How to read electrical control circuit diagrams? How to interpret electrical control circuit diagrams? For electrical engineering beginners, here are some methods for understanding circuit diagrams.
The general method for reading electrical control circuit diagrams is to first look at the main circuit, then the auxiliary circuit, and use the loops of the auxiliary circuit to study the control program of the main circuit.
Steps to examine the main circuit
Step 1
Identify the electrical equipment in the main circuit. Electrical equipment refers to appliances or devices that consume electrical energy. When reading a diagram, first identify how many appliances are used, their types, uses, wiring methods, and any different requirements.
step2
It's important to determine which electrical components control the equipment. There are many ways to control electrical equipment; some are controlled directly by switches, some by various starters, and some by contactors.
Step 3
Understand the control and protection devices used in the main circuit. The former refers to control components other than conventional contactors, such as power switches (changeover switches and air circuit breakers) and universal changeover switches. The latter refers to short-circuit protection devices and overload protection devices, such as the specifications of electromagnetic trip units and thermal overload trip units in air circuit breakers, the uses and specifications of fuses, thermal relays, and overcurrent relays. Generally, after analyzing the main circuit as described above, the auxiliary circuit can be analyzed.
Step 4
Check the power supply. You need to know the power supply voltage level, whether it is 380V or 220V, and whether it is supplied from the busbar, the distribution panel, or the generator set.
Steps to examine the auxiliary circuit
The auxiliary circuit includes control circuits, signal circuits, and lighting circuits.
Analyze the control circuit. Based on the control requirements of each motor and actuator in the main circuit, identify the other control links in the control circuit one by one. Divide the control circuit into several local control circuits according to their different functions for analysis. If the control circuit is complex, you can first exclude circuits such as lighting and display that are not closely related to control, so as to concentrate on the analysis.
Step 1
Check the power supply. First, identify whether the power supply is AC or DC. Second, determine where the auxiliary circuit's power supply comes from and its voltage level. The power supply is generally connected to the two phase lines of the main circuit, with a voltage of 380V. Alternatively, it can be connected to one phase line and the neutral line of the main circuit, with a single-phase voltage of 220V. Furthermore, it can be connected to a dedicated isolation power transformer, with voltages of 140V, 127V, 36V, 6.3V, etc.
When the auxiliary circuit is DC, the DC power supply can be drawn from a rectifier, generator set, or amplifier, and its voltage is typically 24, 12, 6, 4.5, or 3V. The rated voltage of the coils of all electrical components in the auxiliary circuit must match the auxiliary circuit power supply voltage. Otherwise, the circuit components will not operate when the voltage is too low; and the coils of the electrical components will burn out when the voltage is too high.
step2
To understand the purpose of the various relays and contactors used in the control circuit, and if some relays with special structures are used, you should also understand their operating principles.
Step 3
The operation of the main circuit is studied based on the auxiliary circuit.
By analyzing the above content and combining it with the requirements of the main circuit, we can analyze the operation process of the auxiliary circuit.
The operation process of the auxiliary circuit
Control circuits are always drawn in sequence between two horizontal or two vertical power lines. Therefore, they can be analyzed from left to right or top to bottom. For complex auxiliary circuits, the entire auxiliary circuit forms a large loop, which is further divided into several independent smaller loops. Each smaller loop controls an appliance or an action. When a smaller loop forms a closed loop and current flows through it, the electrical components (contactors or relays) in the loop activate, connecting or disconnecting the power to the appliance. In auxiliary circuits, buttons or selector switches are generally used to connect the circuit. The analysis of control circuits must be combined with the operational requirements of the main circuit. Only by fully understanding the requirements of the main circuit for the control circuit can one truly grasp the operating principle of the control circuit. The operating principles of each part should not be viewed in isolation; attention should be paid to whether there are inter-restraint relationships between the various actions, such as interlocking between the forward and reverse rotation of the motor.
Step 4
The study of the interrelationships between electrical components. All electrical components in a circuit are not isolated but interconnected and mutually restrictive. This mutual control relationship sometimes manifests in a single circuit and sometimes in several circuits.
Step 5
Study other electrical equipment and components, such as rectifiers and lighting fixtures.
Key points of line-checking and chart-reading method
In summary, the key points of the wiring and diagram interpretation method for electrical control circuit diagrams are:
Analyze the main circuit: Starting with the main circuit, analyze the control content of each motor and actuator according to the control requirements of each motor and actuator, such as basic control links such as motor starting, steering control, and braking.
Analyze the auxiliary circuit: Examine the power supply of the auxiliary circuit, and clarify the function of each electrical component in the auxiliary circuit and their mutual constraints.
Analysis of interlocking and protection links: Production machinery has high requirements for safety and reliability. In order to achieve these requirements, in addition to the reasonable selection of drive and control schemes, a series of electrical protections and necessary electrical interlocks are also set up in the control circuit.
Analyzing Special Control Components: Some control circuits also include relatively independent special components that are not closely related to the main circuit or control circuit. Examples include product counting devices, automatic detection systems, thyristor triggering circuits, and automatic temperature control devices. These components often form a small system on their own. The method for reading and analyzing these components can refer to the analysis process described above, and you can flexibly apply your knowledge of electronics, AC technology, automatic control systems, detection, and conversion to analyze them one by one.
Overall Inspection: After breaking down the circuit into smaller parts and analyzing the working principle of each section and the control relationships between them, it is necessary to use the method of "integrating the parts" to check the entire control circuit for any omissions. Finally, it is also necessary to further check and understand the connections between each control link from a holistic perspective, so as to clearly understand the function, working process, and main parameters of each electrical component in the circuit diagram.
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