Steps to examine the main circuit
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.
2. It is important 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.
3. Understand the control and protection electrical appliances used in the main circuit. The former refers to control components other than conventional contactors, such as power switches (changeover switches and air/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, as well as the uses and specifications of components like fuses, thermal relays, and overcurrent relays. Generally, after analyzing the main circuit as described above, the auxiliary circuit can be analyzed.
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.
II. Steps for examining 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 eliminate circuits that are not closely related to control, such as lighting and display circuits, in order to concentrate on the analysis.
1. Check the power supply. First, identify the type of power supply: AC or DC.
Secondly, it's important to determine the source and voltage level of the auxiliary circuit's power supply. The power supply is typically 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 a neutral line of the main circuit, resulting in a single-phase 220V voltage. 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.
2. Understand the purpose of the various relays and contactors used in the control circuit. If some relays with special structures are used, you should also understand their operating principles.
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.
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 from 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 controlling an appliance or an action.
When a small loop forms a closed circuit and current flows through it, the electrical components (contaminants or relays) in the loop will activate, connecting or disconnecting the power supply to the electrical equipment. In auxiliary circuits, push-buttons or selector switches are typically used to connect the circuit.
The analysis of the control circuit must be carried out in conjunction with the operating requirements of the main circuit. Only by fully understanding the requirements of the main circuit on the control circuit can the operating principle of the control circuit be truly grasped. The operating principles of each part should not be viewed in isolation, but attention should be paid to whether there are mutual constraints between each action, such as the interlock between the forward and reverse rotation of the motor.
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.
5. Study other electrical equipment and components, such as rectifiers and lighting fixtures.
An experienced electrician shared his steps for checking wiring and interpreting diagrams in electrical control circuits:
① First, you need to know how this equipment is operated, used, and works;
② Look at the circuit block diagram of this device, understand the connections, relationships, and interactions between the blocks, and use your knowledge of the device's operation, use, and function to understand and analyze the block diagram;
③ Step into the specific circuit of each block diagram. Locate the power supply terminal, signal input and output terminals, etc.
④ Open the actual circuit of the device, first find the corresponding part of the circuit according to the block diagram, and divide the actual circuit into several parts according to the block diagram;
⑤ Go into each section of the circuit, refer to the circuit diagram, find the location of the corresponding components, and repeatedly understand and memorize the function, operation, and possible fault manifestations of each major component in the actual circuit;
⑥ Ultimately, this will enable you to understand circuit diagrams and actual circuits, and troubleshoot problems!
To help everyone fully understand this article and for your convenience in saving it, we're reposting the content from Series 1 from a few days ago. You can also take a look back.
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1. 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 situation of the drawing and the key points it is trying to express.
2. Look at the schematic diagram and block diagram. Since the schematic diagram and block diagram only roughly represent the basic components, interrelationships and main characteristics of the system or subsystem, you need to look at the circuit diagram in detail to understand their working principle.
Schematic diagrams and block diagrams mostly use single-line diagrams, with only some schematic diagrams of 380/220V low-voltage power distribution systems using multi-line diagrams.
3. Reading circuit diagrams is both the key and the difficult part of reading drawings. Circuit diagrams are the core of electrical drawings and are also the most comprehensive and the most difficult to understand electrical drawings.
(1) You need to see what graphic symbols and text symbols are used, understand the function of each component of the circuit diagram, and distinguish between the main circuit and auxiliary circuit, AC circuit and DC circuit.
(2) Look at the diagram in the order of first looking at the main circuit and then the auxiliary circuit. The main circuit is the part of the electrical control circuit through which a large current flows, including the electrical components connected from the power source to the motor. When looking at the main circuit, you should usually look from bottom to top, that is, start from the electrical equipment, then look through the control electrical components, and look towards the power source in sequence.
By examining the main circuit, you need to understand how the load obtains power, which electrical components the power lines pass through to reach the load, and why they pass through these electrical components.
The auxiliary circuit is the circuit in the control circuit other than the main circuit, and the current flowing through it is relatively small. When examining the auxiliary circuit, look at it from top to bottom and from left to right, that is, first look at the main power supply, then look at each branch in sequence, analyze the operation of the electrical components in each branch and their control relationship with the main circuit, and pay attention to the connection relationship between electrical and mechanical mechanisms.
By examining the auxiliary circuit, one should understand its composition, the interrelationships and control relationships between the various electrical components, and their operational status.
At the same time, it is also necessary to understand the relationship between the auxiliary circuit and the main circuit, and to use the loop of the auxiliary circuit to study the control program of the main circuit, so as to understand the working principle and ins and outs of the entire circuit.
4. Comparing the wiring diagram and circuit diagram together can help clarify the wiring diagram. When reading the wiring diagram, follow the terminal markings and circuit numbers sequentially from the power source to understand the wiring route and circuit connection method, and how each branch circuit forms a closed loop through various electrical components. Internal and external circuits within the distribution panel (panel) must be interconnected through terminal blocks.
Generally, the terminal block has the same number of wires as the distribution panel, and the wires of external circuits simply need to be connected to the same numbered terminals on the terminal block. Therefore, when reading wiring diagrams, to understand the circuit layout inside and outside the distribution panel (panel), it is essential to carefully understand the wiring of the terminal block.
Starting control of three-phase wound-rotor asynchronous motor
Next, let's talk about some factors to consider when analyzing each route:
Analyzing the main circuit: Whether designing or analyzing a circuit, the first step is always to examine the main circuit. The main circuit's function is to ensure the machine tool's drive requirements are met. From the main circuit's structure, we can analyze the type and operating mode of the motor or actuator, as well as the control and protection requirements for starting, steering, speed regulation, braking, etc.
Analysis of the control circuit: The control requirements of the main circuit are realized by the control circuit. Using the principles of "breaking down the whole into parts" and "following the clues", the control circuit is divided into several local control circuits according to their functions. Starting from the power supply and the master signal, the control flow is written out through logical judgment, so as to express the automatic working process of the circuit in a simple and clear way.
Analysis of auxiliary circuits: Auxiliary circuits include displays of the operating status of the actuators, power supply displays, parameter measurements, lighting, and fault alarms. This part of the circuit is relatively independent, playing an auxiliary role without affecting the main functions. Many parts of the auxiliary circuits are controlled by components in the control circuit.
Analysis of Interlocking and Protection: Production machinery has high requirements for safety and reliability. To achieve these requirements, in addition to the rational selection of drive and control schemes, a series of electrical protections and necessary electrical interlocks are also set up in the control circuit. Electrical interlocking and electrical protection are important aspects that cannot be overlooked in the analysis of electrical control schematic diagrams.
Overall Inspection: After breaking down the entire circuit into its components and analyzing the working principle of each part and the control relationships between them, it is necessary to use the method of "integrating the parts" to check the entire control circuit to see if there are any omissions. In particular, it is necessary to further check and understand the connections between each control link from a holistic perspective in order to correctly understand the function of each electrical component in the schematic diagram.
