What is an electrical diagram? Reading electrical drawings _ Reading electrical installation drawings _ Principles of drawing electrical diagrams

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This post was last edited by jennifer12580 on 2016-8-1 09:27. An electrical diagram is a schematic used to explain the working principles of electrical systems, describe the structure and functions of electrical products, and provide instructions for their installation and use. It uses graphic symbols, line drawings, or simplified representations to show the connections between the various components within an electrical device or system. An electrical schematic diagram is a representation used to illustrate the electrical working principle of a device, as well as the function of various electrical components and their interrelationships. Using the methods and techniques of electrical schematics is highly beneficial for analyzing electrical circuits and troubleshooting faults in machine tool circuits. An electrical schematic diagram generally consists of several parts, such as the main circuit, control circuit, protection circuits, and power distribution circuits. Commonly used electrical diagrams include: electrical schematic diagrams, layout diagrams of electrical components, and electrical installation wiring diagrams. The standard paper sizes for various drawings are generally 297×210, 297×420, 297×630, and 297×840 mm; for special requirements, other sizes can be selected in accordance with the GB126—74 \"Mechanical Drawing\" standard. An electrical schematic diagram is a diagram that uses graphic symbols, textual symbols, and item codes to represent the relationships between various electrical components in a circuit as well as their operating principles. Electrical schematic diagrams have a simple structure and clear hierarchy; they are suitable for studying and analyzing the working principles of circuits, and can help in identifying faults. They also serve as a basis for creating electrical installation wiring diagrams, which is why they are widely used in design departments and on production sites. An electrical schematic diagram shows the various components of an electrical element separately, while in some cases a circuit diagram is created by grouping all the components of the same device together and depicting them according to their actual positions. The method of drawing schematic diagrams makes it easier to identify electrical appliances, facilitating installation and maintenance. However, when the wiring is complex and there are many electrical appliances used, it becomes difficult to see the wiring clearly. Because although the various components of the same electrical appliance are mechanically connected, they are not necessarily interconnected electrically. Principles for drawing electrical schematic diagrams: 1. The electrical components in the schematic diagram are shown in their state when they are not powered and under no external forces. At different operating stages, various electrical devices operate in different ways, with their contacts opening and closing as appropriate. However, an electrical schematic can only represent one situation. Therefore, it is stipulated that the contacts of all electrical appliances shall be shown in their original positions, i.e., the positions when there is no electrical current or no mechanical movement. For a contactor, it is the position when the coil is not powered and the contacts are not in operation ； For a button, it is the position of the contacts when the finger is not pressing the button ； For thermal relays, it refers to the position of the normally closed contacts when no overload has occurred, and so on. 2. The drawing position of the contacts. The direction of the external force that acts on the contacts must be from left to right when the diagram is placed vertically; in other words, the contact on the left side of the vertical line is a normally open contact, while the contact on the right side is a normally closed contact ； When the diagram is placed horizontally, it reads from bottom to top; that is, the contacts below the horizontal line are normally open contacts, while the contacts above the horizontal line are normally closed contacts. 3. The main circuit, control circuit, and auxiliary circuit should be drawn separately. The main circuit is the drive circuit of the device, and it is the path through which large currents flow from the power source to the motor ； The control circuit is a logical circuit composed of contactor and relay coils, as well as the contacts of various electrical devices, which carries out the desired control functions ； Auxiliary circuits include signal, lighting, and protection circuits. 4. The power supply circuit of the power circuit is drawn as a horizontal line, while the powered equipment (motor) and its protective electrical components should be connected perpendicularly to the power supply circuit. 5. The main circuit is drawn with vertical lines on the left side of the diagram, while the control circuit is drawn with vertical lines on the right side. The energy-consuming components in the control circuit are placed at the bottom of the circuit. 6. The order of operations in the diagram is indicated from left to right or from top to bottom, with an effort to minimize lines and avoid line intersections. 7. The connection points of cross-wires that are in direct electrical contact (i.e., where the wires intersect) in the diagram should be indicated with black dots. For cross wires without direct electrical connection, no black dots should be drawn at the intersection. 8. Above the schematic diagram, divide it into several sections, and indicate the purpose and function of the circuit in each section ； Below the relay and contactor coils, a contact table is provided to illustrate the hierarchical relationship between the coils and contacts. The electrical component layout diagram shows the actual locations of all electrical components in an electrical device, providing necessary information for its installation and maintenance. The layout diagram of electrical components can be drawn collectively or separately, depending on the complexity of the electrical equipment. Dimensions do not need to be indicated in the diagram, but the codes for each electrical component must match those of all components listed in the relevant drawings and component lists. The diagram should usually include more than 10% spare area as well as space for wire conduits (trays) to allow for design improvements. Principles for drawing electrical layout diagrams: 1. When drawing the layout diagram of electrical components, the outline of the machine tool is represented by thin solid lines or dotted lines, while the simple outer shapes of the electrical components are drawn using thick solid lines. 2. When drawing the layout diagram of electrical components, the motor should be drawn together with the mechanical device it drives ； The travel switch should be drawn at the location where the information is obtained ； The control handle should be placed in a location that is easy to operate. 3. When drawing the layout diagram of electrical components, a certain distance should be maintained between them in all directions – above, below, to the left, and to the right. Additionally, factors related to heat generation and dissipation by these components must be taken into account, to ensure that wiring, connection, and maintenance are facilitated. The electrical wiring installation diagram is primarily used for the installation and wiring of electrical equipment, as well as for checking circuits, performing repairs on them, and dealing with faults. The diagram should show the actual wiring connections between various electrical devices and components, and indicate the data required for external wiring. In the electrical installation wiring diagram, the textual symbols of various electrical components, the order in which they are connected, and the numbering of the circuits must all be consistent with those in the electrical schematic diagram. Principles for drawing electrical installation diagrams: 1. When drawing electrical installation wiring diagrams, each electrical component is represented according to its actual position on the installation base plate. The area occupied by each component on the drawing is scaled according to its actual size using a uniform ratio. 2. When drawing electrical installation wiring diagrams, all components of a single element are drawn together and enclosed within a dashed line frame; sometimes multiple electrical components are also enclosed in dashed line frames to indicate that they are installed on the same mounting base. 3. When drawing the electrical installation wiring diagram, the connections between the electrical components inside and outside the mounting base are made through terminal blocks. Since there are several leads on the mounting base that lead to the external circuit, the terminal block should show the connection points for those wires. 4. When drawing electrical installation wiring diagrams, adjacent wires with the same path can be drawn as a single wire. Electrical secondary circuit diagram (1) DC circuit from positive to negative: including control circuits, signal circuits, etc. Starting from the positive DC terminal of a circuit, follow the direction of current flow until the negative terminal is reached. (2) AC circuits from the live wire to the neutral wire: examples include current and voltage circuits, as well as the air-cooling circuits of transformers. Start from the live wires of a circuit (phases A, B, C), and follow the direction of current flow until reaching the neutral wire (N). (3) When seeing a contact, look for the coil; when seeing a coil, look for the contact: Once a contact is seen, one must locate the coil of the relay or contactor that controls that contact. The circuit in which the coil is located is the contact control circuit, used to analyze the conditions for the contact to operate. Find all the contacts of the coil in order to identify all the contacts (targets) controlled by that relay. (4) Use Ohm’s law to analyze the relay in order to determine whether it operates: the criterion for judgment is that a sufficiently large voltage is applied across the terminals of the voltage-type coil, and a sufficiently large current flows through the terminals of the current-type coil. For the coil circuit of a voltage-type relay, when the two ends of the coil are connected to the positive and negative poles of the power supply via the contacts of several relays or current coils, it is considered that the relay (contactor) is activated (excited). On the other hand, if there are open contacts in the circuit, or if a relatively large resistance is connected in series in the coil circuit, or if the coils are short-circuited by connected contacts, then it is considered that the relay (contactor) is not activated (not excited). For example, in the switch opening circuit, when the switch is in the closed position, a closed-position relay (with high resistance) is connected in series with the positive terminal of the opening coil; in such a case, it is considered that the circuit does not function. When the protection trip contact closes and the coil is connected directly to the positive terminal of the power supply, it is considered that the opening coil is activated. For current-type relays (such as anti-jump relays in trip circuits), when the ends of the coil are connected to the positive and negative poles of the power supply via the contacts of several relays or through a coil with low resistance, it is considered that the relay (contactor) has activated (is energized). When there is a short-circuited contact in the circuit, or a relatively large resistance is connected in series with the coil circuit, or the contacts to which the coils are connected are short-circuited, it is considered that the relay (contactor) does not operate (is not energized). (5) Check all branches: When examining a circuit from the positive pole to the negative pole, if there are multiple branches leading to the negative pole along the way, each of those branches must be checked. Otherwise, the analysis circuit will miss some important aspects. (6) Use the relative numbering method and circuit labeling to clarify the correspondence between the devices in the wiring schematic diagrams of the installation diagram and the expanded diagram: The main purpose of verifying this correspondence is, first, to check whether the installation diagram corresponds to the expanded diagram. Second, determine the locations of each device in the layout on site. To determine the location of a terminal on a specific terminal block in the expanded diagram, based on the installation diagrams (such as the wiring diagram for the protection panel terminals), one first identifies the circuit label associated with that terminal, and then checks the circuit labels in the expanded diagram. Terminal blocks with the same circuit label belong to the same circuit; thus, it is possible to quickly locate that circuit in the expanded diagram and understand its role within the entire circuit. If only the installation diagram is available, or if it turns out that the principles shown in the installation diagram do not correspond to those in the wiring diagram, then using the numbers marked on each device terminal in the installation diagram, and by means of the relative numbering system, one can identify the terminal of the other device to which that terminal is connected. From there, it is possible to determine the other device connected to that terminal, until the positive and negative poles of the DC power supply, or the live and neutral wires in the AC circuit, are identified. Finally, identify the entire relevant circuit, draw it in a diagram, and then analyze whether the connections conform to the operating principle. When trying to determine the location of devices on the layout diagram, one approach is to use the locations provided in the device list on that diagram, and then check them against the corresponding installation diagram. Secondly, identify the terminal symbols in the layout diagram to determine which ones belong to the terminal strips of the control panels and cabinets, and which ones belong to the terminals of the protection or automatic devices; then search directly in the relevant control panels and terminal boxes. (7) Solutions to special problems in reading drawings. A. How to describe the operating conditions of a circuit or relay using the actual status of the equipment (the status of the equipment as it can be observed on-site): First, describe the conditions of the circuit based on the open and closed states of its contacts; then, using the relationship between these contact states and the status of the equipment, replace those descriptions (for example, use the fact that the \"remote/local control switch\" in the switchgear is in the \"remote\" position to represent the contact state of that switch in the remote control circuit). This capability must be developed gradually; otherwise, reading the diagrams will remain at a basic level – allowing only observation of whether contacts are open or closed and whether relays are energized – without the possibility of integrating this with the monitoring and operation of equipment in operation. B. How to determine the connection between the devices represented by boxes in the expansion diagram and the other external components? First, determine the terminal numbers of the device used for drawing boxes. Then, use the device’s instruction manual or the manufacturer’s diagrams, which show the internal wiring diagram of the device, to find the terminal numbers that lead to external connections. Connect these to the internal circuits, and then use those external connection terminals to link up with the external circuits.