I. Classification of Ac Contactors
Ac Contactors can be classified in various ways, with different classification criteria corresponding to different product types. The selection can be made based on the voltage level, structural characteristics, arc-extinguishing method, and other requirements of the actual application scenario.
(1) Classification by Rated Voltage of Main Contacts
This is the most commonly used classification method, directly related to the applicable circuit range of the Contactor, mainly including the following categories:
Low-Voltage Ac Contactors: The rated voltage of the main contacts is usually 1000V or below. They are widely used in low-voltage power distribution systems in industrial and civil fields, such as machine tool control, household appliances (control of air conditioners and refrigerator compressors), and building automation equipment. Common rated voltage levels are 220V, 380V, and 660V. These Contactors have a small size and compact structure, adapting to the safe operation requirements of low-voltage circuits.
Medium and High-Voltage Ac Contactors: The rated voltage of the main contacts exceeds 1000V. They are mostly used in the control of power systems and large industrial equipment (such as high-voltage motors and transformers). Common rated voltage levels include 10kV and 35kV. Such contactors need to have stronger insulation performance and arc-extinguishing capacity. Structurally, they are equipped with additional high-voltage insulation layers and dedicated arc-extinguishing chambers, and are usually used in conjunction with high-voltage circuit breakers and disconnectors to ensure the safe connection and disconnection of high-voltage circuits.
(2) Classification by Arc-Extinguishing Method
When an AC contactor disconnects a circuit, an arc is generated between the contacts. If the arc cannot be extinguished in a timely manner, it may burn the contacts and cause faults. Therefore, the arc-extinguishing method is one of the core performance indicators of a contactor, mainly including the following types:
Air Arc-Extinguishing Contactors: These contactors rely on air as the arc-extinguishing medium. When the contacts separate, the air flow generated or the contact structure (such as grid-type or longitudinal slot-type) stretches and cools the arc to extinguish it. They are suitable for low-voltage, small-current circuits (e.g., rated current below 100A), with a simple structure and low cost, and are commonly used in small motor control circuits.
Vacuum Arc-Extinguishing Contactors: The contacts of these contactors are sealed in a vacuum arc-extinguishing chamber. They use the high insulation and arc-extinguishing capabilities of a vacuum to quickly extinguish the arc. They have excellent arc-extinguishing effects, minimal contact wear, a long service life, and are not affected by the external environment (such as humidity and dust). They are suitable for high-voltage, large-current scenarios, such as the control of large motors in metallurgy and mining industries.
SF₆ Arc-Extinguishing Contactors: These contactors use sulfur hexafluoride (SF₆) gas as the arc-extinguishing medium. SF₆ gas has excellent insulation and arc-extinguishing capabilities, with fast arc-extinguishing speed, no open flame, and no noise. However, since SF₆ is a potent greenhouse gas, its application is restricted by environmental protection policies. It is mainly used in special high-voltage equipment (such as some GIS combined electrical appliances) that have extremely high requirements for arc-extinguishing performance.
(3) Classification by Structural Form
Based on the assembly method and structural characteristics of the contactor, it can be divided into the following types:
Open-Type AC Contactors: The core components such as contacts and coils are exposed to the air, without a closed housing. They are only suitable for dry, clean environments free of dust and corrosive gases, such as inside indoor low-voltage control cabinets. Their advantages include good heat dissipation and convenient maintenance, while the disadvantage is weak protection capability, making them vulnerable to environmental influences.
Protected AC Contactors: These contactors are equipped with a semi-closed housing, which can effectively prevent dust and water droplets from splashing in. The protection level is usually IP20 or IP30, and they are suitable for environments with slight dust or humidity, such as motor control circuits in workshops. Heat dissipation holes are reserved on the housing to balance protection and heat dissipation.
Enclosed AC Contactors: These contactors adopt a fully enclosed housing, with a protection level of IP54 or above. They can resist the erosion of dust, rainwater, and corrosive gases, and are suitable for harsh environments such as outdoor areas, metallurgical plants, and chemical plants. Some enclosed contactors are also equipped with built-in cooling fans or heat sinks to solve the heat dissipation problem of the closed structure.
(4) Classification by Operation Method
According to the driving method of the contactor, it can be divided into the following types:
Electromagnetic AC Contactors: These contactors rely on electromagnetic force to drive the contacts to act. They are the most widely used type at present. With a simple structure, reliable operation, and low cost, they can realize the connection and disconnection of contacts by controlling the energization and de-energization of the coil, adapting to most automatic control scenarios.
Pneumatic AC Contactors: These contactors use compressed air to drive the contacts to act, and are suitable for high-voltage, large-current circuits (e.g., rated current above 1000A). Due to the large and stable pneumatic driving force, they can effectively reduce the impact when the contacts are connected or disconnected, and have no electromagnetic interference. They are suitable for industrial scenarios with extremely high reliability requirements (such as the control of high-voltage switches in power systems).
II. Working Principle of AC Contactors
The core working logic of an AC contactor is "electromagnetic force driving contact action". Its structure mainly consists of five parts: the electromagnetic system, contact system, arc-extinguishing device, reset spring, and housing. These parts work together to realize the connection and disconnection control of the circuit.
(1) Composition of Core Structure
Electromagnetic System: It includes an AC coil, a static iron core, and a moving iron core. When the coil is energized, an alternating magnetic field is generated, which magnetizes the static iron core and attracts the moving iron core. When the coil is de-energized, the magnetic field disappears, and the moving iron core resets under the action of the reset spring. To reduce the core loss (eddy current and hysteresis loss) caused by the alternating magnetic field, the static iron core and moving iron core are usually made of laminated silicon steel sheets. In addition, a short-circuit ring (magnetic shunt ring) is embedded in the end face of the iron core to prevent the moving iron core from vibrating and making noise due to the alternating magnetic field.
Contact System: It is divided into main contacts and auxiliary contacts. The main contacts have a large capacity (capable of carrying large currents) and are used to connect or disconnect the main circuit (such as the motor power circuit). The auxiliary contacts have a small capacity (usually carrying a current below 5A) and are used in control circuits (such as self-locking and interlocking circuits). The contact material is mostly silver alloy (such as silver-cadmium alloy and silver-nickel alloy) to improve conductivity, wear resistance, and arc resistance. According to the action state, the contacts can also be divided into normally open contacts (closed when the coil is energized, open when de-energized) and normally closed contacts (open when the coil is energized, closed when de-energized).
Arc-Extinguishing Device: As mentioned in the previous classification, it includes air arc-extinguishing chambers, vacuum arc-extinguishing chambers, etc. Its function is to quickly extinguish the arc when the contacts are disconnected, protect the contacts from being burned, and prevent the arc from causing circuit short circuits or safety accidents.
Reset Spring: Installed between the moving iron core and the housing, when the coil is de-energized, the elastic force of the reset spring pushes the moving iron core to reset, restoring the contacts to their initial state (normally open contacts open, normally closed contacts closed).
Housing: It plays a protective and fixing role, protecting the internal components from the external environment, and at the same time assembling the components into a whole to ensure structural stability.
(2) Specific Working Process
Taking the most common electromagnetic AC contactor as an example, its working process can be divided into two stages: "energization and attraction" and "de-energization and reset".
1. Energization and Attraction (Main Circuit Connected)
When the switch in the control circuit (such as a button or relay contact) is closed, the coil of the AC contactor is connected to the rated voltage. An alternating current is generated in the coil, which in turn forms an alternating magnetic field in the static iron core. After the static iron core is magnetized, it generates an electromagnetic force that overcomes the elastic force of the reset spring and attracts the moving iron core to the static iron core.
The movement of the moving iron core drives the contact bracket connected to it to act:
The main contacts (normally open) are closed accordingly, connecting the main circuit, and the load (such as a motor or heater) receives power and starts working;
The auxiliary normally open contacts are closed at the same time, which can realize the "self-locking" function (that is, after the control switch is disconnected, the coil remains energized through the auxiliary normally open contacts, and the main contacts remain closed);
The auxiliary normally closed contacts are disconnected, which can realize the "interlocking" function (such as preventing two contactors from being attracted at the same time to avoid a short circuit in the main circuit).
2. De-Energization and Reset (Main Circuit Disconnected)
When the switch in the control circuit is disconnected, the coil of the contactor is de-energized. The current in the coil disappears, and the magnetic field of the static iron core weakens until it disappears, and the electromagnetic force also disappears. At this time, the elastic force of the reset spring is greater than the electromagnetic force, pushing the moving iron core to reset and driving the contact bracket back to its initial position:
The main contacts (normally open) are disconnected, cutting off the main circuit, and the load stops working;
The auxiliary normally open contacts are disconnected, and the self-locking function is released;
The auxiliary normally closed contacts are closed, providing conditions for the next action or other control circuits.
At the moment when the main contacts are disconnected, an arc is generated between the contacts due to the voltage difference. At this time, the arc-extinguishing device starts to work: in the case of air arc-extinguishing, the grid divides the arc into multiple short arcs, reducing the arc voltage, and at the same time, the air cools to extinguish the arc; in the case of vacuum arc-extinguishing, the vacuum environment prevents the arc from continuing to burn, and the arc is extinguished within microseconds, ensuring the safety of the contacts.
III. Summary
Through reasonable classification design, AC contactors can adapt to different application scenarios from low-voltage to high-voltage and from clean to harsh environments. Their working principle is centered on the core logic of "electromagnetic drive + contact action + arc-extinguishing protection", realizing the safe and frequent control of circuits. In practical applications, it is necessary to select the appropriate type of AC contactor according to the main circuit voltage, current, environmental conditions, and control requirements, and at the same time pay attention to key parameters such as coil voltage, contact capacity, and arc-extinguishing method to ensure the stable operation of the electrical system.
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