Fuse relays are commonly used in distribution cabinets
Fuse
definition
Fuse refers to an electrical appliance that melts and disconnects the circuit with the heat generated by itself when the current exceeds the specified value. The fuse is based on the current exceeding the specified value for a period of time, with its own heat to melt the melt, so that the circuit is disconnected; A kind of current protector made by using this principle. Fuses are widely used in high and low voltage power distribution systems and control systems and electrical equipment, as a protector for short circuit and overcurrent, and is one of the most widely used protection devices.
Common species
Plug-in fuses, spiral fuses, closed fuses, fast fuses, self-resetting fuses.
How it works
It is an electrical appliance that uses a metal conductor as a melt in series in the circuit, and when the overload or short-circuit current passes through the melt, it is blown because of its own heating, so as to break the circuit. The fuse has a simple structure and is easy to use, and is widely used as a protection device in power systems, various electrical equipment and household appliances.
Fuse Text Symbol: FU
Fuse Graphic Symbols:
Relays
definition
Relay (English name: relay) is a kind of electrical control device, is when the change of input quantity (excitation quantity) reaches the specified requirements, in the electrical output circuit to make the controlled quantity predetermined step change of a kind of electrical appliance. It has an interactive relationship between the control system (also known as the input loop) and the controlled system (also known as the output loop).
Classification of relay structural characteristics
Electromagnetic Relays , Solid Relays , Intermediate Relays , Reed Relays , Time Relays , High Frequency Relays , Polarization Relays , Other Types of Relays: Such as Photorelays , Acoustic Relays , Thermal Relays , Instrumentation Relays , Hall Effect Relays , Differential Relays etc.
Intermediate relays
definition
Intermediate Relay: Used in relay protection and automatic control systems to increase the number and capacity of contacts. It is used to transmit intermediate signals in control circuits. The structure and principle of the intermediate relay are basically the same as the AC contactor, and the main difference with the contactor is that the main contact of the contactor can pass through a large current, while the contact of the intermediate relay can only pass a small current. Therefore, it can only be used in control circuits. It generally does not have a main contact because the overload capacity is relatively small. Therefore, all of the contacts it uses are auxiliary contacts, and there are a large number of them.
The definition of intermediate relay in the new national standard is K, and the old national standard is KA. Generally, it is powered by DC power supply. A few use AC power.
structure
The principle of intermediate relay is the same as that of AC contactors, which is composed of fixed iron core, moving iron core, spring, moving contact, static contact, coil, terminal block and housing.
function
The intermediate relay is a multi-purpose automatic telecontrol appliance in relay protection and automatic control system. Because the intermediate relay has good electrical isolation, the controller and the controlled party have no connection on the electrical appliance, and the purpose of safety control is achieved.
classify
Static intermediate relays are used in a variety of protection and automatic control lines. This kind of relay is composed of electronic components and precision small relays, etc., and is the preferred product for the replacement of intermediate relays in the power series.
Intermediate Relay Word Symbol: KA(K)
Intermediate Relay Pictogram:
Solid state relays
definition
Solid State Relay (SSR) is a contactless switch composed of microelectronic circuits, discrete electronic devices, and power electronic power devices. The isolation device is used to realize the isolation of the control side and the load side. The input of the solid state relay uses a tiny control signal to directly drive a high-current load.
use
Dedicated solid-state relays can have short-circuit protection, overload protection and overtemperature protection, and the combination of logic-cured packages can realize the intelligent modules required by users and be used directly in the control system.
peculiarity
Solid state relay is a non-contact electronic switch with isolation function, no mechanical contact parts in the switching process, so in addition to having the same function as electromagnetic relay, the solid state relay also has logic circuit compatibility, vibration resistance and mechanical impact resistance, unlimited installation position, has good moisture-proof, mildew-proof and anti-corrosion performance, excellent performance in explosion-proof and ozone pollution prevention, small input power, high sensitivity, small control power,
Good electromagnetic compatibility, low noise and high working frequency.
structure
Solid state relays consist of three parts: an input circuit, an isolation (coupling) and an output circuit.
Electromagnetic relays
How it works
Electromagnetic relay is an electronic control device, it has a control system (also known as the input loop) and the control system (also known as the output loop), usually used in the automatic control circuit, it is actually a kind of "automatic switch" with a smaller current and lower voltage to control a larger current and a higher voltage. Therefore, it plays the role of automatic adjustment, safety protection, and conversion circuit in the circuit.
Selection
(1) The power supply voltage of the control circuit, the maximum current that can be provided;
(2) the voltage and current in the controlled circuit;
(3) How many sets and what forms of contacts are required for the controlled circuit. When selecting relays, the power supply voltage of the general control circuit can be used as the basis for selection.
The control circuit should be able to provide sufficient working current to the relay, otherwise the relay engagement is unstable.
Thermal relays
definition
The working principle of the thermal relay is that the current into the heating element generates heat, so that the bimetal sheet with different expansion coefficients is deformed, when the deformation reaches a certain distance, it pushes the connecting rod to act, so that the control circuit is disconnected, so that the contactor loses power, the main circuit is disconnected, and the overload protection of the motor is realized.
structure
It is composed of a heating element, a bimetal sheet, a contact and a set of transmission and adjustment mechanism. The heating element is a resistance wire with a small resistance value, which is connected in series in the main circuit of the motor to be protected. Bimetal sheets are made by rolling two metal sheets with different coefficients of thermal expansion.
function
It is mainly used for overload protection of asynchronous motors, and its working principle is that after the overload current passes through the thermal element, the bimetal sheet is heated and bent to push the action mechanism to drive the contact action, so as to disconnect the motor control circuit to realize the motor power-off and stop, and play the role of overload protection. In view of the fact that it takes a long time for the heat to be transferred during the heating and bending of the bimetal sheet, therefore, the thermal relay cannot be used as short-circuit protection, but can only be used as the overload protection of the overload protection thermal relay, and the symbol is FR.
Protection features
The phase-off protection function of the thermal relay is provided by a differential amplification mechanism composed of internal and external push rods. When the motor is working normally, the current through the thermal relay heating element is normal, and the inner and outer push rods are moved forward to the appropriate position. When the power supply is broken in one phase and causes the lack of phase, the phase current is zero, the phase bimetal sheet is cooled and reset, so that the inner push rod moves to the right, and the other two phases of the bimetal sheet increases due to the increase of current and the degree of bending increases, so that the outer push rod moves more to the left, due to the differential amplification effect, in a short time after the phase failure fault is pushed to the normally closed contact to be disconnected, so that the AC contactor is released, and the motor is powered off and stopped.
Thermal Relay Text Symbol: FR
Thermal Relay Graphic Symbols:
Thermal relay structure diagram:
END
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