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The electrical control system is composed of an electric motor and various control devices such as contactors, relays, resistors, switches, and more. A diagram that illustrates the interconnections between these components is known as an electrical control system diagram. These diagrams typically include electrical schematics, layout diagrams, and installation diagrams. This section focuses specifically on electrical schematics.
An electrical schematic is a diagram that represents the main and auxiliary circuits using standardized graphic and textual symbols, following the operational sequence of each component. To ensure clarity for design, analysis, installation, and maintenance, it is essential to use the official symbols defined by national standards. Graphic symbols represent physical components like motors and switches, while textual symbols indicate their names, functions, and characteristics.
To make the control circuit easier to read and analyze, the electrical schematic is usually drawn in an expanded form, showing all conductive parts and connection points of each component. The following description will focus on the electrical schematic of the CW6132 horizontal lathe, as shown in Figure 1.
Figure 1: Electrical schematic of the CW6132 lathe
When drawing an electrical schematic, several principles should be followed:
1. The main circuit labels consist of text symbols and numbers. The text symbol identifies the main function of the component or circuit segment, while the numerical suffix differentiates between separate segments, such as UVW, U1V1W1, and U2V2W2 in the figure.
2. Control circuits are labeled with Arabic numerals, usually containing three or fewer digits. In vertically arranged circuits, labels are numbered from top to bottom and left to right. Any segment separated by coils, contacts, resistors, or capacitors should have its own unique label.
3. The main circuit, which carries high current, is drawn with thick lines, while the auxiliary circuits (including control, lighting, and signal circuits) are drawn with thin lines due to lower current levels.
4. All components are arranged in order—top to bottom and left to right—based on their function, whether they are placed vertically or horizontally.
5. The voltage, polarity, frequency, and number of phases for each power supply circuit must be clearly marked.
6. Relay and contactor contacts are drawn in their de-energized state, while pushbuttons and limit switches are shown in their normal, non-actuated position.
7. Conductive parts of the same device (such as coils and contacts of a contactor) are drawn separately but share the same text symbol. If multiple similar devices are used, numbers are added to the lower right corner of the text symbol to distinguish them.
8. Power circuits are drawn as horizontal lines, with motors and protective devices represented as vertical lines. Auxiliary circuits are placed between horizontal power lines, with coils, lamps, and indicators connected to the grounded line, and control contacts connected to another power line.
9. Where wires cross and have direct electrical contact, a black dot is used to indicate the connection; otherwise, no dot is needed.
Dividing the schematic into sections helps identify the purpose of each part. The numbers at the bottom of the diagram correspond to the area numbers, making it easier to trace wiring and understand the circuit.
A contact table is often placed below the coil of a relay or contactor, indicating the area numbers of its corresponding contacts. For example, the first column under the contactor KM coil shows the area number of the main normally open contact, the second column shows the auxiliary normally open contact, and the third column shows the auxiliary normally closed contact.
For intermediate and current relays, the first column indicates the normally open contact, and the second column shows the normally closed contact. Time relays have four columns, representing time-delay normally open, normally closed, momentary normally open, and momentary normally closed contacts, respectively.
Each contact’s corresponding coil area number is also marked beneath the contact.
Finally, technical data such as the rated current of fuses, the operating range and setting values of thermal relays, and the cross-sectional area of conductors should be included in the electrical schematic for reference and proper implementation.