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The electrical control system, often referred to as the secondary control loop, plays a vital role in the operation of electrical equipment. Each type of equipment has its own unique control loop, and the methods used for controlling high-voltage and low-voltage devices differ significantly.
Key Functions
To ensure the safe and reliable operation of primary electrical equipment, various auxiliary devices are needed. These components work together to form a control loop or secondary circuit, which is responsible for specific functions. The main features of these systems include:
(1) Automatic Control. High-voltage and high-current switching devices are typically large in size, and manual operation is impractical. Instead, operating systems are used to control the opening and closing of switches. In case of a fault, automatic control devices must be in place to quickly isolate the circuit and protect the system.
(2) Protective Function. During operation, electrical equipment and lines may experience faults that cause current or voltage to exceed safe limits. To address this, protective devices such as fuses, thermal relays, and voltage loss coils detect these signals and automatically take action, like disconnecting the circuit or activating protection mechanisms.
(3) Monitoring Function. Since electricity is invisible, it's difficult to determine whether a device is energized just by looking at it. Therefore, visual and audio signals—like indicator lights and alarms—are used to monitor the status of the primary equipment in real time.
(4) Measurement Function. While indicators can show whether a device is on or off, they don’t provide detailed information. For precise monitoring, measuring instruments are used to track parameters like voltage, current, frequency, and power levels, offering quantitative data about the system’s performance.
Although modern systems have largely replaced traditional control components with computer-based and electronic solutions, some smaller devices and local control circuits still use conventional methods. This forms the foundation for microcomputer-based automation and control systems.
System Components
A typical control circuit consists of several key parts:
(1) Power Supply Circuit. This circuit usually operates on AC 380V or 220V, providing the necessary energy for the system to function.
(2) Protection Circuit. This part includes components like fuses, thermal relays, and voltage loss coils, designed to protect against short circuits, overloads, and voltage drops. It often uses single-phase 220V or 36V, or DC 220V or 24V as its power source.
(3) Signal Circuit. This circuit provides visual or audible feedback about the system's status, using signals like colored lights or different sounds to indicate normal or abnormal conditions.
(4) Automatic and Manual Circuits. Most systems include both automatic and manual controls. While automatic links improve efficiency, manual controls are essential during installation, testing, and emergencies. A switch or selector switch is used to toggle between the two modes.
(5) Braking and Stopping Circuit. This circuit ensures that motors stop quickly when needed. Methods include energy braking, reverse power braking, and regenerative braking, all designed to halt motor movement safely and efficiently.
(6) Self-Locking and Interlocking Circuits. Once a start button is pressed, the circuit should remain energized until manually turned off. This is known as a self-locking circuit, often achieved through contactor contacts. Interlocking circuits prevent multiple devices from being activated simultaneously, ensuring safety and reliability by requiring one device to be powered off before another can be turned on.
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