未命名的设计 (42)

 

Low Voltage Circuit Breakers

Low voltage circuit breakers, also known as automatic switches, are electrical devices that serve as manual switches and can automatically provide under-voltage, over-voltage, overload, and short-circuit protection. They are used for distributing electrical energy, infrequently starting asynchronous motors, and protecting power lines and motors. In cases of severe overload, short circuit, or under-voltage, they can automatically disconnect the circuit. Their functionality is akin to a combination of fuse switches and thermal overload relays. After interrupting fault currents, they generally do not require the replacement of components and have thus gained widespread application. The structure and operating principle of low voltage circuit breakers consist of operating mechanisms, contacts, protective devices (various trip units), and arc extinguishing systems. The main contacts of low voltage circuit breakers are closed either manually or by motorized closing. Once closed, the free-trip mechanism locks the main contacts in the closed position. The coil of the overcurrent trip unit and the thermal element of the thermal trip unit are in series with the main circuit, while the coil of the undervoltage trip unit is parallel to the power source.

Dc Breaker

In the event of a short circuit or severe overload, the armature of the overcurrent trip unit is attracted, activating the free-trip mechanism and opening the main contacts of the main circuit. In the case of overload, the thermal element heats up, causing the bimetallic strip to bend and activate the free-trip mechanism. When undervoltage occurs, the undervoltage trip unit’s armature is released, also triggering the free-trip mechanism. The shunt trip unit is used for remote control; during normal operation, its coil is de-energized. When remote control is required, pressing the start button energizes the coil, moving the armature, which activates the free-trip mechanism and opens the main contacts.

Isolator Switches

Isolating Switches

Isolating switches are the most used high-voltage switchgear. Although their working principle and structure are relatively simple, their large usage volume and high reliability requirements significantly impact the design, establishment, and safe operation of substations and power plants. The main characteristic of knife switches is their lack of arc extinguishing capability; they can only open or close circuits without load current. Their main functions are:

  1. After opening, they establish a reliable insulating gap, separating the equipment or lines needing maintenance from the power source with a clear disconnection point, ensuring the safety of maintenance personnel and equipment.
  2. They are used to switch lines as needed during operation.
  3. They can open or close small currents in lines, such as charging currents of bushings, busbars, connectors, short cables, the capacitive current of voltage equalizing capacitors, circulating currents during double busbar switching, and the excitation current of voltage transformers.
  4. Depending on the specific type of structure, they can open or close the no-load excitation current of transformers with certain capacities. Outdoor knife switches, based on their insulating post structure, can be single-column, double-column, or triple-column. Single-column knife switches use the vertical space directly under the overhead busbar as an electrical insulator for the break, saving land area, reducing lead wires, and providing clear visibility of the open and closed states. In ultra-high voltage transmission scenarios, the use of single-column knife switches in substations significantly saves land area. In low voltage equipment, they are mainly used in residential and building low voltage terminal distribution systems. Their main functions include load breaking and connecting line isolation.

Contactors

Contactors

The working principle of DC contactors is as follows: when the contactor coil is energized, the current generates a magnetic field, attracting the movable iron core with electromagnetic force and actuating the contacts: normally closed contacts open, and normally open contacts close, the two being interlinked. When the coil is de-energized, the electromagnetic force disappears, and the armature is released under the action of the release spring, restoring the contacts: normally open contacts open, and normally closed contacts close. The working principle of AC contactors is similar, with the difference being that AC contactors are powered by AC sources, while DC contactors are powered by DC sources. Additionally, as DC contactors are supplied with direct current, which lacks instantaneous values and zero-crossing points, the effective value is equal at any moment. Thus, there is no need for a short-circuit ring on the armature of a DC contactor to prevent the reduced attraction force at zero-crossing points, which could cause excessive noise and vibration in the contactor.

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Thermal Relays

Thermal relays, also known as thermocouples, operate when the load current passes through the heating element (a type of alloy resistor that generates and dissipates heat when current flows through it), heating the nearby expansion element. The expansion element is made of two metal strips with different expansion properties, welded along their entire surface, known as a bimetallic strip. The lower metal strip of the bimetallic strip has a larger expansion coefficient. When the current exceeds a specific level, the heat from the heating element causes the bimetallic strip to bend upwards, thereby actuating the mechanism, opening the contacts in the control circuit, which in turn causes the main contacts of the contactor to open, cutting off the load circuit.

Universal Changeover Switch

A universal changeover switch is a multi-position, multi-stage, control multi-circuit master command electrical device. When the operating handle is turned, it drives the internal cam to rotate, causing the contacts to close or open in a predetermined sequence. Universal changeover switches are primarily used for switching various control circuits, phase-measuring control of voltmeters and ammeters, switching distribution device lines, remote control, etc. They can also be used for directly controlling the starting, speed regulation, and reversing of small-capacity motors. Common products include the LW5 and LW6 series. The LW5 series can control motors of up to 5.5 kW, while the LW6 series is limited to controlling motors of up to 2.2 kW.

For reversible operation control, reverse starting is only allowed after the motor has stopped. The LW5 series of universal changeover switches are divided into self-resetting and self-locking types based on the operating mode of the handle. The self-resetting type refers to the handle automatically returning to its original position after being moved to a certain position and released; the self-locking type refers to the handle staying in the placed position without automatically returning. The operating positions of the handle of a universal changeover switch are indicated by angles.

Different models of universal changeover switches have different contacts, as shown in figure 2 of the circuit diagram. However, as the state of the contacts is related to the position of the operating handle, besides showing the contact symbols in the circuit diagram, the relationship between the handle position and the state of the contacts should also be illustrated. In the figure, when the universal changeover switch is turned to the left by 45°, contacts 1-2, 3-4, 5-6 close, and contact 7-8 opens; at 0°, only contact 5-6 closes, and at right 45°, contact 7-8 closes, with the rest open.

Control Buttons Control buttons are a type of simple structure, widely used master command electrical device, used to momentarily connect or open small current circuits manually.

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Intermediate Relays

Intermediate relays are used in relay protection and automatic control systems to increase the number and capacity of contacts. They are used to transmit intermediate signals in control circuits. The structure and principle of intermediate relays are basically the same as AC contactors. The main difference is that the main contacts of contactors can carry large currents, while the contacts of intermediate relays can only carry small currents. Therefore, they are only used in control circuits. Generally, they do not have main contacts due to their limited overload capacity. They use auxiliary contacts, which are numerous. The new national standard defines intermediate relays as K, while the old standard was KA. Generally, they are powered by DC sources, with a few using AC power.