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Low-voltage electrical concept
A low-voltage electrical appliance is a component or device that can manually or automatically turn the circuit on or off according to external signals and requirements to switch, control, protect, detect, change and adjust the circuit or non-electrical object. According to the working voltage of the control electric appliance, it can be divided into two categories: high-voltage control electric appliance and low-voltage control electric appliance, with the boundary of AC 1200V and DC 1500V. In general, low-voltage electrical appliances can be divided into two major categories of distribution electrical appliances and control electrical appliances, and are the basic components of complete electrical equipment. In the industrial, agricultural, transportation, defense, and people's electricity sectors, most use low-voltage power supply, so the quality of electrical components will directly affect 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 an AC voltage of 1200V and a DC voltage of 1500V or less. Most of the production machinery uses low-voltage electrical appliances. There are many types of low-voltage electrical appliances, which can be classified according to their structure, use and objects.
1. According to the purpose and control object, the low-voltage electrical appliances can be divided into power distribution appliances and control appliances.
Electrical appliances used for the delivery and distribution of electrical energy are referred to as low-voltage distribution appliances, which include knife switches, transfer switches, air circuit breakers, and fuses. Appliances used in various control circuits and control systems are referred to as control electronics, and such appliances include contactors, starters, and various control relays.
2. According to different operation modes, low-voltage electrical appliances can be divided into automatic electrical appliances and manual electrical appliances.
An appliance that automatically completes the functions of turning on, breaking, starting, reversing, and stopping by changing the parameters of the electric appliance itself or external signals (such as electricity, magnetism, light, heat, etc.) is called an automatic electric appliance. Commonly used automatic appliances include contactors, relays, and the like.
An appliance that performs operations such as turning on, breaking, starting, reversing, and stopping by direct human operation is called a manual electric appliance. Commonly used manual appliances include knife switches, transfer switches and main electrical appliances.
3. According to the working principle, it can be divided into electromagnetic appliances and non-electrical control appliances.
Electromagnetic appliances are electrical appliances that work according to the principle of electromagnetic induction, such as contactors and various types of electromagnetic relays. The work of non-electrical control appliances is an electrical appliance that operates by external force or some kind of non-electrical quantity change, such as a travel switch, a speed relay, and the like.
Basic structure of low voltage electrical appliances
Electromagnetic low-voltage electrical appliances have two main components, namely: the sensing part - the electromagnetic mechanism and the actuator - the contact system.
Electromagnetic mechanism
The main function of the electromagnetic mechanism is to convert electromagnetic energy into mechanical energy, and drive the contact action to complete the function of turning on or off the circuit.
The electromagnetic mechanism consists of three basic parts: the attracting coil, the iron core and the armature. The commonly used electromagnetic mechanism can be divided into three forms as shown in the figure.
2. DC electromagnet and AC electromagnet
According to the nature of the current flowing through the attracting coil, the electromagnet can be divided into a direct current electromagnet and an alternating current electromagnet.
Since the DC electromagnet is supplied with direct current, its core does not generate heat, and only the coil is heated. Therefore, the coil is in contact with the iron core to facilitate heat dissipation, and the coil is made of a skeleton-free, high and thin thin and high type to improve the heat dissipation of the coil itself. The core and armature are made of mild steel and engineered pure iron.
Since the AC electromagnet is connected to the AC power, there is hysteresis loss and eddy current loss in the core, and both the coil and the iron core are heated. Therefore, the attracting coil of the AC electromagnet has a skeleton, so that the iron core is isolated from the coil and the coil is made short and thick. Shape to facilitate heat dissipation from the core and coil. The core is superposed with silicon steel sheets to reduce eddy currents.
When the coil is connected with direct current, the air gap magnetic induction strength is constant, and the electromagnetic attraction of the direct current electromagnet is constant. When the coil is connected to alternating current, the magnetic induction is an alternating variable, and the electromagnetic attraction F of the alternating current electromagnet varies between 0 (minimum) and Fm (maximum), and the suction curve is as shown in the following figure. In one cycle, when the instantaneous value of the electromagnetic suction is greater than the reaction force, the armature pulls in; when the instantaneous value of the electromagnetic suction is less than the reaction force, the armature is released. Therefore, every time the power supply voltage changes, the electromagnet is sucked twice and released twice, causing the electromagnetic mechanism to generate severe vibration and noise, and thus cannot work normally.
In order to eliminate the vibration and noise generated by the AC electromagnet, a small groove is formed in the end face of the core, and a copper short-circuit ring is embedded in the groove.
3. Contact system
The contacts are the active part of the appliance and function to turn the circuit on and off.
The contacts are mainly in two structural forms: bridge contacts and finger contacts.
4. Arc extinguishing device
When the circuit is broken in the atmosphere, the presence of an electric field causes a large amount of electrons on the surface of the contact to overflow to generate an arc. Once the arc is generated, a large amount of heat is generated. The presence of the arc both ablates the metal surface of the contact, reduces the service life of the appliance, and prolongs the breaking time of the circuit, 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, fast and slow speed automatic switching and automatic stop layer.
Adjusting function, low-voltage electrical appliances can adjust some power and non-electricity to meet user requirements, such as diesel engine throttle adjustment, room temperature and humidity adjustment, automatic illumination adjustment.
Protection, according to the characteristics of the equipment, automatic protection of equipment, environment, and people, such as motor overheat protection, grid short circuit protection, leakage protection.
Indication function, using the control, protection and other functions of low-voltage electrical appliances to detect the operating conditions of the equipment and the operation of the electrical circuit, such as insulation monitoring, protection of the card instructions.
What are the common low-voltage electrical appliances?
1. Knife switch: Also known as the knife switch or the isolating switch, it is the simplest and widely used low-voltage electric appliance in the hand-controlled electric appliance.
2. Fuse: Also known as a fuse, the IEC127 standard defines it as a "fuse-link". It is an electrical component that is installed in the circuit to ensure the safe operation of the circuit. Fuses are widely used in high and low voltage power distribution systems and control systems, as well as in electrical equipment, as a short circuit and over current protector, is one of the most common protection devices.
3, low-voltage circuit breaker: (formerly known as automatic switch) is a switching device that can not only switch on and off the normal load current and overload current, but also can switch on and off the short-circuit current. In addition to the control function, the low-voltage circuit breaker has certain protection functions, such as overload, short circuit, under voltage and leakage protection.
4. Contactor: refers to an electrical appliance that uses a coil to flow a current to generate a magnetic field in an industrial power to close the contact to achieve a controlled load.
5. Relay: The relay is an electrical control device. It has an interaction between the control system (also known as the input loop) and the controlled system (also known as the output loop). Often used in automated control circuits, it is actually an "automatic switch" that uses small currents to control large current operation. Therefore, it plays the role of automatic adjustment, safety protection and conversion circuit in the circuit.
6. Main electric appliance: It is a switch electric appliance used to connect and disconnect the control circuit to issue commands or to control the production process.
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