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The electrical control system is composed of an electric motor and various control devices, such as contactors, relays, resistors, and switches. A diagram that illustrates the connection between these components is known as an electrical control system diagram. These diagrams include electrical schematics, layout diagrams, and installation drawings. This section focuses specifically on electrical schematics.
An electrical schematic is a diagram that shows the main and auxiliary circuits using standardized graphic and text symbols, arranged according to the operational sequence of each component. To ensure clarity for design, analysis, installation, and maintenance, it is essential to use the national-standard symbols when drawing these schematics. Graphic symbols represent the physical components like motors and relays, while text symbols identify their names, functions, and characteristics.
To make the control circuit easier to read and analyze, the schematic is typically drawn in an expanded form, showing all conductive parts and wiring terminals. In this section, we will explain the drawing principles and precautions using the electrical schematic of the CW6132 lathe shown in Figure 1.
Figure 1: Electrical schematic of the CW6132 lathe
1. Principles for Drawing Electrical Schematics
(1) The main circuit is labeled with a combination of text symbols and numbers. The text symbol indicates the primary function of the component or segment, while the numerical part distinguishes different segments, such as UVW, U1V1W1, and U2V2W2 in the figure.
(2) Control circuits are numbered with Arabic numerals, usually consisting of three or fewer digits. In vertically oriented circuits, labels are numbered from top to bottom and left to right. Each segment separated by coils, contacts, resistors, or capacitors should have a unique number.
(3) The main circuit, which carries high current, is drawn with thick lines, while the auxiliary circuits (including control, lighting, and signal circuits), which carry low current, are drawn with thin lines.
(4) All components are arranged in order from top to bottom and left to right based on their motion sequence, whether vertically or horizontally placed.
(5) The voltage, polarity, frequency, and phase count of each power supply circuit must be clearly marked.
(6) Relay and contactor contacts are shown in their de-energized state, while pushbuttons and limit switches are drawn in their normal, non-actuated position.
(7) Conductive parts of the same device (like a contactor’s coil and contacts) are drawn separately but share the same text symbol. For similar devices, a number is added to the lower right of the text symbol to differentiate them.
(8) The power circuit is drawn as horizontal lines, with motors and protective devices shown as vertical lines. Auxiliary circuits are drawn vertically between power lines, with coils, lamps, and signals connected to the grounded line, while control contacts connect 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.
2. Division of the Diagram Area
The schematic is divided into sections, with each area labeled to indicate its function. The numbers below the schematic (e.g., 1, 2, 3...) help in identifying and retrieving specific wiring connections easily.
3. Index of Coil and Contact Positions
A contact table is provided below each relay and contactor coil, indicating the area number where each contact is located. For example, under the KM contactor coil, the first column shows the area number of the main normally open contact, the second column the auxiliary normally open contact, and the third column the normally closed auxiliary contact.
For intermediate and current relays (KA), the first column shows the normally open contact, and the second column the normally closed contact. For time relays (KT), the first column indicates the delayed normally open contact, the second the normally closed contact, the third the momentary normally open contact, and the fourth the momentary normally closed contact.
Each contact’s corresponding coil area is also marked beneath the contact itself.
4. Annotation of Technical Data
Technical specifications of each electrical component should also be included in the schematic. This includes the rated current of fuses, the operating range and setting value of thermal relays, and the cross-sectional area of the conductors.