According to the level of its working voltage, the electrical equipment is divided into two categories: high-voltage control appliances and low-voltage control appliances. AC 1200V and below, DC 1500V and below are called low-voltage electrical appliances. The development of low-voltage electrical appliances depends on the development of the national economy and the needs of the development of modern industrial automation, as well as the research and application of new technologies, new processes and new materials. It is currently developing in the direction of high performance, high reliability, miniaturization, digital modeling, modularization, combination and component generalization.
The low-voltage electrical appliance can automatically or manually change the state and parameters of the circuit according to the requirements of the operation signal or the external field signal to realize the control, protection, measurement, indication and adjustment of the circuit or the controlled object. The role of low-voltage electrical appliances is:
(1) Control functions such as up and down movement of the elevator, automatic switching between fast and slow speed, and automatic stop layer.
(2) Protection function According to the characteristics of the equipment, the equipment, the environment, and the human body are automatically protected, such as overheat protection of the motor, short circuit protection of the power grid, and leakage protection.
(3) Measurement function Use the instrument and its corresponding electrical equipment to measure equipment, power grid or other non-electrical parameters, such as current, voltage, power, speed, temperature, humidity, etc.
(4) Regulation function The low-voltage electrical appliance can adjust some of the power and non-electricity to meet the requirements of the user, such as the adjustment of the throttle of the diesel engine, the adjustment of the temperature and humidity of the room, and the automatic adjustment of the illumination.
(5) Indication function The function of control and protection of low-voltage electrical appliances is used to detect the operating conditions of the equipment and the working conditions of the electrical circuit, such as insulation monitoring and protection of the card-off instructions.
(6) The conversion function is switched between the electric equipment or the low-voltage electric appliance and the control circuit are put into operation in time to realize the function switching, such as the manual and automatic conversion of the excitation device, the switching of the power supply and the self-supplied electricity.
Of course, the role of low-voltage electrical appliances is far more than this. With the development of science and technology, new functions and new equipment will continue to emerge. The main types and uses of commonly used low-voltage electrical appliances are shown in Table 1-1.
The requirements for low-voltage distribution electrical equipment are strong arc-extinguishing ability, good breaking capacity, good thermal stability and accurate current limiting. For low-voltage control appliances, it requires reliable operation, high operating frequency, long life and a certain load capacity.
The basic structure of low-voltage electrical appliances:Low-voltage electrical appliances generally have two basic parts: one is the sensing part, which senses the external signal and makes a regular reaction. In the self-controlled electrical appliance, the sensing part is mostly composed of an electromagnetic mechanism, in the controlled electrical appliance, the sense The measuring part is usually an operating handle or the like; the other is an executing part, such as a contact that is turned on or off according to an instruction.
Electromagnetic low-voltage electrical appliances have two main components, namely: the sensing part - the electromagnetic mechanism and the actuator - the contact system.
1 . 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 core and the armature.
The commonly used electromagnetic mechanisms are shown in the following figures and can be divided into three forms.
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 supplied 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 AC, the magnetic induction is an alternating variable, and the electromagnetic attraction F of the AC electromagnet varies from 0 (minimum) to F m (maximum). The suction curve is shown in the figure below. 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 as shown in the following figure.
3 . Contact system
The contacts are the active part of the appliance and function to turn the circuit on and off.
There are two main types of contacts: bridge contacts and finger contacts, as shown in the figure below.
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.
Common arc extinguishing methods:
Electric power arc extinguishing
Magnetic blowout
Metal grid arc extinguishing
Ring Type Connecting Terminals
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