I. General steps for reading electrical diagrams
1. Please refer to the drawing instructions carefully.
After receiving the drawings, you should first carefully read the main title block and related instructions, such as the drawing index, technical specifications, electrical component list, and construction instructions. Combine this with your existing electrical knowledge to gain a clear understanding of the type, nature, and function of the electrical drawing, and to understand the overall overview and key points of the drawing.
2. Look at the outline and block diagram.
Since schematic diagrams and block diagrams only roughly represent the basic components, interrelationships, and main characteristics of a system or subsystem, a detailed examination of the circuit diagram is necessary to understand their working principles. Schematic diagrams and block diagrams mostly use single-line diagrams; only some schematic diagrams of 380/220V low-voltage power distribution systems partially use multi-line diagrams.
3. Reading circuit diagrams is both the key and the challenge in interpreting diagrams.
Circuit diagrams are the core of electrical diagrams, and also the most comprehensive and difficult to understand. To read a circuit diagram, first identify the graphic and textual symbols, understand the function of each component, and distinguish between main and auxiliary circuits, AC and DC circuits. Next, read the diagram in the order of main circuits first, then auxiliary circuits. When reading the main circuit, usually work from bottom to top, starting with the electrical equipment, passing through the control components, and sequentially looking towards the power source. When reading the auxiliary circuit, work from top to bottom and from left to right, starting with the main power source, then sequentially examining each branch, analyzing the operation of the electrical components in each branch and their control relationship to the main circuit, paying attention to the connections between electrical and mechanical mechanisms. By reading the main circuit, understand how the load obtains power, which electrical components the power lines pass through to reach the load, and why they pass through these components. By reading the auxiliary circuit, understand its structure, the interrelationships and control relationships between the electrical components, and their operation. At the same time, it is also necessary to understand the relationship between the auxiliary circuit and the main circuit, and then to understand the working principle and ins and outs of the entire circuit.
4. Compare the circuit diagram and wiring diagram.
Comparing wiring diagrams and circuit diagrams side-by-side helps in understanding the wiring diagram. When reading a wiring diagram, follow the terminal markings and circuit numbers sequentially from the power source to understand the wiring path and connection methods, clarifying how each branch circuit forms a closed loop through various electrical components. Interconnections between internal and external circuits in the distribution panel (panel) must be made through terminal blocks. Generally, the terminal block has the same number of wires as the internal wires in the distribution panel; external wires simply need to be connected to the corresponding terminals on the terminal block. Therefore, to understand the wiring path inside and outside the distribution panel (panel), it is essential to carefully examine the wiring on the terminal block.
II. Methods for reading electrical control 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.
1. Steps to examine the main circuit
Step 1: Identify the electrical equipment in the main circuit. Electrical equipment refers to electrical appliances or equipment that consume electrical energy. When reading a diagram, first identify how many electrical appliances are used, their categories, uses, wiring methods, and any different requirements.
The second step is to determine what 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.
2. Examine the steps of 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 the type of power supply: 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. It can also be connected to one phase line and a neutral line of the main circuit, with a single-phase voltage of 220V. Alternatively, it can be connected to a dedicated isolation transformer, with voltages of 140V, 127V, 36V, 6.3V, etc. When the auxiliary circuit is DC, the DC power supply can be connected to a rectifier, generator set, or amplifier, with voltages typically 24V, 12V, 6V, 4.5V, 3V, etc. The rated voltage of the coils of all electrical components in the auxiliary circuit must match the auxiliary circuit's power supply voltage. Otherwise, if the voltage is too low, the circuit components will not operate; if the voltage is too high, the coils of the electrical components will burn out.
The second step is to understand the purpose of the various relays and contactors used in the control circuit. If some relays with special structures are used, their operating principles should also be understood.
Step 3: Study the operation of the main circuit based on the auxiliary circuit. After analyzing the above content and combining it with the requirements of the main circuit, the operation process of the auxiliary circuit can be analyzed. The control circuit is always drawn between two horizontal or two vertical power lines according to the sequence of actions. Therefore, it can be analyzed from left to right or from top to bottom. For complex auxiliary circuits, the entire auxiliary circuit forms a large loop, which is further divided into several independent small loops. Each small loop controls an electrical appliance or an action. When a small loop forms a closed loop and current flows through it, the electrical components (contactors or relays) in the loop will actuate, connecting or disconnecting the power supply to the electrical equipment. In the auxiliary circuit, the circuit is generally connected by a button or changeover switch. The analysis of the control circuit must be combined with the operation requirements of the main circuit. Only after fully understanding the requirements of the main circuit for the control circuit can the operation principle of the control circuit be truly grasped. The operation principles of each part should not be viewed in isolation, but attention should be paid to whether there are mutual constraints between the various actions, such as interlocking between the forward and reverse rotation of the motor.
Step 4: Study 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: Investigate other electrical equipment and components, such as rectifiers and lighting fixtures.
III. Summary of key points for checking and interpreting electrical circuit diagrams
(1) 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 the basic control links of motor starting, steering control, braking, etc.
(2) Analyze the auxiliary circuit. Examine the power supply of the auxiliary circuit and understand the function of each electrical component in the auxiliary circuit and the mutual constraints between them.
(3) 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.
(4) Analyze special control components. Some control circuits also include certain special components that are not closely related to the main circuit or control circuit and are relatively independent. Examples include product counting devices, automatic detection systems, thyristor triggering circuits, and automatic temperature control devices. These parts often form a small system on their own. The method for reading and analyzing these components can refer to the above analysis process, and flexibly apply the knowledge of electronic technology, AC technology, automatic control systems, detection and conversion, etc., to analyze them one by one.
(5) Overall Inspection. After breaking down the whole into parts and gradually analyzing the working principle of each local circuit and the control relationship between each part, it is also necessary to use the method of "integrating the parts into a whole" to check the entire control circuit to see if there are any omissions. Finally, it is also necessary to further check and understand the connection between each control link from an overall perspective, so as to clearly understand the function, working process and main parameters of each electrical component in the circuit diagram.
