The concept of low-voltage electrical appliances
Low-voltage electrical appliances are components or devices that can manually or automatically connect or disconnect circuits according to external signals and requirements, in order to achieve switching, control, protection, detection, transformation, and regulation of circuits or non-electrical objects. Control electrical appliances can be divided into two main categories based on their operating voltage, with AC 1200V and DC 1500V as the dividing line: high-voltage control electrical appliances and low-voltage control electrical appliances. In general, low-voltage electrical appliances can be divided into two main categories: distribution electrical appliances and control electrical appliances, and are the basic components of complete sets of electrical equipment. In industry, agriculture, transportation, national defense, and other sectors of electricity consumption, low-voltage power supply is widely used; therefore, the quality of electrical components directly affects the reliability of low-voltage power supply systems.
Classification of low-voltage electrical appliances
Low-voltage electrical appliances refer to various electrical appliances that operate at AC voltages below 1200V and DC voltages below 1500V. Low-voltage electrical appliances are mostly used in production machinery. There are many types of low-voltage electrical appliances, which can be classified in different ways according to their structure, application, and the object they control.
1. Based on their purpose and the object they control, low-voltage electrical appliances can be divided into distribution electrical appliances and control electrical appliances.
Electrical appliances used for the transmission and distribution of electrical energy are called low-voltage distribution appliances. These appliances include knife switches, transfer switches, air circuit breakers, and fuses. Electrical appliances used in various control circuits and control systems are called control appliances. These appliances include contactors, starters, and various control relays.
2. Based on different operating methods, low-voltage electrical appliances can be divided into automatic electrical appliances and manual electrical appliances.
Electrical appliances that automatically perform actions such as connecting, disconnecting, starting, reversing, and stopping through changes in their own parameters or external signals (such as electricity, magnetism, light, heat, etc.) are called automatic electrical appliances. Commonly used automatic electrical appliances include contactors and relays.
Electrical appliances that are operated manually to perform actions such as connecting, disconnecting, starting, reversing, and stopping are called manual electrical appliances. Commonly used manual electrical appliances include knife switches, changeover switches, and master control appliances.
3. Based on their working principle, electrical appliances can be divided into electromagnetic electrical appliances and non-electrical quantity control electrical appliances.
Electromagnetic electrical appliances are those that operate based on the principle of electromagnetic induction, such as contactors and various electromagnetic relays. Non-electrical quantity control electrical appliances operate based on external force or changes in some non-electrical quantity, such as limit switches and speed relays.
Basic structure of low voltage electrical appliances
Electromagnetic low-voltage electrical appliances generally have two main components: the sensing part—the electromagnetic mechanism and the actuating part—the contact system.
1. Electromagnetic mechanism
The main function of an electromagnetic mechanism is to convert electromagnetic energy into mechanical energy, which drives the contacts to move, thereby completing the function of connecting or disconnecting the circuit.
An electromagnetic mechanism consists of three basic parts: an attraction coil, an iron core, and an armature. Common electromagnetic mechanisms, as shown in the figure, can be divided into three types.
2. DC electromagnets and AC electromagnets
Electromagnets can be classified into DC electromagnets and AC electromagnets based on the nature of the current flowing through the attraction coil.
Since a DC electromagnet is powered by direct current, its core does not heat up; only the coil heats up. Therefore, the coil is in contact with the core to facilitate heat dissipation. The coil is made into a tall, thin, frameless shape to improve its own heat dissipation. The core and armature are made of mild steel and engineering pure iron.
Because alternating current flows through an AC electromagnet, hysteresis and eddy current losses exist in the iron core, causing both the coil and the iron core to heat up. Therefore, the attraction coil of an AC electromagnet has a frame to isolate the iron core from the coil, and the coil is made short, thick, and stout to facilitate heat dissipation for both the iron core and the coil. The iron core is made of stacked silicon steel sheets to reduce eddy currents.
When direct current flows through the coil, the air gap magnetic induction intensity remains constant, and the electromagnetic attraction force of the DC electromagnet is a constant value. When alternating current flows through the coil, the magnetic induction intensity becomes an alternating variable, and the electromagnetic attraction force F of the AC electromagnet varies between 0 (minimum) and Fm (maximum), as shown in the figure below. Within one cycle, when the instantaneous value of the electromagnetic attraction force is greater than the reaction force, the armature is attracted; when the instantaneous value of the electromagnetic attraction force is less than the reaction force, the armature is released. Therefore, for each cycle of power supply voltage change, the electromagnet attracts and releases twice, causing severe vibration and noise in the electromagnetic mechanism, thus preventing it from working properly.
To eliminate the vibration and noise generated by the AC electromagnet, a small groove is made on the end face of the iron core, and a copper short-circuit ring is embedded in the groove, as shown in the figure below.
3. Contact system
Contacts are the actuators of electrical appliances, serving to connect and disconnect circuits.
There are two main structural forms of contacts: bridge contacts and finger contacts, as shown in the figure below.
4. Arc extinguishing device
When a circuit is broken in the atmosphere, the presence of an electric field causes a large number of electrons to escape from the contact surface, thus generating an electric arc. Once generated, the arc produces a large amount of heat. The presence of the arc not only erodes the metal surface of the contacts, reducing the lifespan of the electrical appliance, but also prolongs the circuit breaking time, so the arc must be extinguished quickly.
The role of low-voltage electrical appliances
Control functions, such as the elevator's up and down movement, automatic switching between fast and slow speeds, and automatic stopping at floors.
In terms of regulation, low-voltage electrical appliances can adjust some electrical and non-electrical quantities to meet user requirements, such as adjusting the throttle of a diesel engine, regulating room temperature and humidity, and automatically adjusting illuminance.
It has a protective function, which can automatically protect equipment, the environment, and people according to the characteristics of the equipment, such as overheat protection of motors, short circuit protection of power grid, and leakage protection.
Indicative functions: Utilizing the control and protection functions of low-voltage electrical appliances, the operating status of equipment and the working condition of electrical circuits are detected, such as insulation monitoring and protection tripping indication.
What are some commonly used low-voltage electrical appliances?
1. Knife switch: also known as a knife switch or disconnect switch, it is the simplest and most widely used low-voltage electrical appliance in manual electrical control.
2. Fuse: Also known as a circuit breaker, the IEC 127 standard defines it as a "fuse-link". It is an electrical component installed in a circuit to ensure the safe operation of the circuit. Fuses are widely used in high and low voltage power distribution and control systems, as well as electrical equipment, serving as protectors against short circuits and overcurrents, and are one of the most commonly used protective devices.
3. Low-voltage circuit breaker (formerly known as automatic switch): A low-voltage circuit breaker is a switching device that can connect and disconnect normal load current and overload current, as well as short-circuit current. In addition to its control function, low-voltage circuit breakers also have certain protection functions in the circuit, such as overload, short circuit, undervoltage, and leakage protection.
4. Contactor: A contactor is an electrical device used in industrial electricity that uses the magnetic field generated by the current flowing through a coil to close the contacts, thereby controlling the load.
5. Relay: A relay is an electrical control device. It has an interactive relationship between the control system (also known as the input circuit) and the controlled system (also known as the output circuit). It is commonly used in automated control circuits. In fact, it is an "automatic switch" that uses a small current to control a large current. Therefore, it plays a role in automatic adjustment, safety protection, and circuit switching in circuits.
6. Control electrical appliances: These are switching electrical appliances used to connect and disconnect control circuits to issue commands or to perform program control on the production process.
