Electrical control system

The electrical control system, often referred to as the secondary control loop of electrical equipment, plays a crucial role in managing and regulating the operation of various devices. Each type of equipment has its own specific control loop, and the methods used for controlling high-voltage and low-voltage equipment differ significantly.

Key Functions

To ensure the reliable and safe operation of primary equipment, several auxiliary electrical components are required. These components work together to perform specific control functions, forming what is known as a control loop or secondary loop. The main features of these systems include:

(1) Automatic Control. High-voltage and high-current switching devices are typically large in size. They are usually operated using an automated system, especially when a fault occurs, where the switch must automatically disconnect the circuit. This requires the use of automatic control devices to manage power supply operations effectively.

(2) Protective Function. During operation, electrical equipment and lines may experience malfunctions, causing current or voltage to exceed safe limits. To address this, a set of detection systems is needed to identify fault signals and automatically adjust the equipment or line, such as by disconnecting or activating protection devices.

(3) Monitoring Function. Since electricity is invisible, it's impossible to determine whether a device is energized just by looking at it. Therefore, various audio-visual signals—like lights and alarms—are used to monitor the status of the primary equipment.

(4) Measurement Function. While lighting and sound signals can indicate the presence or absence of power, they do not provide quantitative data. For detailed information about the operational state of the equipment, measuring instruments are used to measure parameters like voltage, current, frequency, and power levels.

In modern applications, traditional control components, switches, and monitoring devices have largely been replaced by computer-based control systems and electronic circuits. However, they are still used in smaller devices and local control circuits. This foundation is essential for implementing microcomputer-based automation and control systems.

System Components

A typical control circuit consists of several key parts:

(1) Power Supply Circuit. This part is powered by AC voltages such as 380V or 220V, depending on the application.

(2) Protection Circuit. The protection (auxiliary) circuit operates on single-phase 220V, 36V, or DC 220V, 24V. It includes protective elements like fuses, thermal relays, voltage loss coils, rectifiers, and voltage regulators to safeguard against short circuits, overloads, and voltage loss.

(3) Signal Circuit. This circuit provides real-time feedback on the normal or abnormal conditions of the equipment and lines through indicators such as colored lights or audible alarms.

(4) Automatic and Manual Circuits. Most electrical systems include automatic controls to improve efficiency. However, manual controls are also necessary during installation, testing, and emergency situations. A combination switch or transfer switch is used to switch between automatic and manual modes.

(5) Braking and Parking Circuit. This circuit cuts off the power supply and applies braking mechanisms to stop the motor quickly. Common methods include energy braking, reverse power braking, and regenerative braking.

(6) Self-locking and Interlocking. Once a start button is pressed, the circuit remains energized, allowing the equipment to continue operating. This is called a self-locking circuit. In contrast, interlocking ensures that only one device can be activated at a time, preventing conflicts. For example, the normally open contacts of two contactors are connected in series in each other’s coil circuits to prevent simultaneous activation.

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