Introduction: A Journey into the World of AC Contactors
Greetings, esteemed readers! If you’ve ever been curious about the workings of Alternating Current (AC) contactors, this is the perfect place to start. In this guide, we’ll explore the technical details, functions, and applications of AC contactors. Let’s dive into the world of electrical control!
What is an AC contactor
An AC contactor is an electrical device that connects or disconnects AC or DC circuits carrying high power loads. It mainly controls electric equipment such as motors and transformers. You can operate it remotely, and it supports frequent switching with reliable performance. It also includes functions like undervoltage release and zero-voltage protection.
Contactor electrical graphics and text symbols shown in Figure 1.
Depending on the current type, contactors are divided into two types:
AC contactors (CJ)
DC contactors (CZ)
Despite the current difference, both types share similar structures and principles.
Structure of AC contactor
An AC contactor consists of several main parts: the electromagnetic system, the contact system, and the arc extinguishing device.
The structure is shown in Figure 2.
(1) Electromagnetic system
The electromagnetic system moves the contacts to open or close the circuit. It includes a static iron core, coil, and armature. The iron core and armature are E-shaped and made of stacked silicon steel sheets to reduce eddy currents and heat loss.
When the coil is energized, the magnetic field pulls the armature, overcoming spring resistance. This action causes vibrations and noise due to alternating current zero-crossing.
To reduce this, engineers add a short-circuit ring (also called a damping or demagnetization ring). This ring creates a phase difference in magnetic flux, reducing vibrations and noise significantly.
(2) Contact System
Contacts carry the electrical load. They come in three types: point, line, and surface contact. Based on their function, they are classified into:
Main contacts: Handle high current in the main circuit (normally open)
Auxiliary contacts: Handle control circuits (normally open and normally closed)
When the contactor is idle:
Normally open (NO) contacts are disconnected
Normally closed (NC) contacts are connected
(3) Arc extinguishing device
During disconnection, arcs form between contacts due to high current. These arcs can burn the contacts and delay disconnection, potentially causing damage.
For small capacity (<10A), double-break electrodynamic extinguishing is used. For larger capacities, semi-enclosed arc grills handle arc interruption effectively.
The role of AC contactors
The AC contactor functions like a gatekeeper. It allows or blocks AC flow based on control signals. This precise control helps maintain system stability. Common applications include:
Motor control
Industrial equipment
Electric heaters
Lighting systems
The working principle of AC contactor
When the coil is energized, the iron core generates magnetic force. This pulls the armature, which closes the main contacts. At the same time, auxiliary contacts switch—normally closed ones open and normally open ones close.
When power to the coil stops, the magnetic field disappears. The spring pushes the armature back, and the contacts return to their default positions.
AC Contactor Rating Explained
Understanding the parameters of AC contactors is paramount in their appropriate selection for a given application.
Parameters such as voltage rating, current capacity, and contactor size play pivotal roles in ensuring optimal performance within a designated electrical system. Selecting a contactor that aligns with these parameters is critical for longevity and efficiency.
(1) Rated voltage
The rated voltage on the nameplate of the contactor refers to the rated voltage of the main contact, and the rated voltage of the main contact should be selected to be greater than or equal to the rated voltage of the load circuit.
(2) Rated current
The rated current on the nameplate of the contactor refers to the rated current of the main contact, and the rated current of the selected main contact should be greater than or equal to the rated current of the motor.
(3) Rated voltage of the attracting coil
The coil voltage must match the control circuit voltage. Typical values:
AC: 36V, 110V, 220V, 380V
DC: 24V, 48V, 220V, 440V
(4) Rated operating frequency
The rated operating frequency of the contactor is the number of operations permitted per hour of the contactor
What are a1 and a2 on an AC contactor?
When selecting a contactor, consider these five factors:
(1) Types of contactors
Use AC contactors for AC equipment and DC contactors for DC equipment. If you mostly use AC loads, you may still use AC contactors for small DC loads, with adjusted ratings.
(2) Rated voltage of contactor
The contactor’s voltage should match or exceed the load circuit’s voltage.
(3) Rated current of main contacts of contactor
The main contact rated current of a contactor should always be greater than or equal to the rated current of the motor or load. Since a motor’s rated current depends on its rated power, you can also determine the contactor current based on the motor’s power.
For Three-Phase Motors at 380V
When using a three-phase motor with a rated voltage of 380 V, the rated operating current (line current) of the motor can be estimated using the simplified formula:
Where:
IN is the rated line current (A),
PN is the rated power of the motor (kW).
In this equation, PN is the rated capacity of the motor (kW).
When the rated voltage of three-phase motor is 660V, the rated running line current of the motor.
Let’s say the motor’s rated power (PN) is 4 kW, and the rated voltage (UN) is 380 V.
Using the simplified formula:
IN = PN × 2 A/kW = 4 kW × 2 A/kW = 8 A
This means you should select an AC contactor with a main contact rated current of at least 8 A.
⚠️ Important: When the contactor is used in frequent starting, braking, or forward/reverse switching scenarios, it’s recommended to reduce the rated current by one level or choose the contactor based on half of the motor’s maximum power. This helps prevent overheating and extends service life.
(4) Coil Voltage Selection
The contactor coil voltage must match the voltage of your control circuit. Common coil voltages include:
AC: 36 V, 110 V, 220 V, 380 V
DC: 24 V, 48 V, 220 V, 440 V
Always check your system’s control voltage before selecting the contactor.
(5) Choosing Auxiliary Contacts
The auxiliary contacts of a contactor are used for control signaling rather than carrying the main circuit load. When selecting auxiliary contacts, ensure they meet your control circuit’s voltage, current, and quantity requirements.
If the number or type of auxiliary contacts is insufficient, you can add an intermediate relay to expand the control logic.
AC vs. DC Contactors in Mixed Systems
Use AC contactors for AC loads.
Use DC contactors for DC loads.
In cases where the system mainly handles AC loads but includes small DC loads, you can still use AC contactors, but make sure to adjust the rated current accordingly. Always size the contactor conservatively to ensure safe operation.
How to choose an AC contactor
A1 and A2 on an AC contactor are the two terminals of the contactor coil, through which the coil of the AC contactor is connected to the power supply, so that the coil of the contactor can be energized and the AC contactor can work, and these two terminals are connected to the control circuit.
Difference between AC contactor and DC contactor
Although they perform similar roles, AC and DC contactors differ in several ways:
(1) Different control objects
AC contactors are used to control AC electrical equipment, while DC contactors are used to control DC electrical equipment.
(2) The core structure is different
AC cores use laminated silicon steel and include a short-circuit ring to reduce noise. DC cores use solid iron or steel without short-circuit rings.
(3) Arc extinguishing system is different
AC: Grid extinguishes arcs by breaking them into smaller segments.
DC: Uses magnetic blowout due to the lack of a natural current zero-crossing, making arc extinguishing harder.
(4) Coil turns are different
AC coils: Fewer turns, lower resistance, less heat.
DC coils: More turns, higher resistance, greater heat, longer cylindrical shape for better dissipation.
(5) Different operating frequency
AC contactors: Up to 600 operations/hour
DC contactors: Up to 1,200 operations/hour
Can AC contactor be used for DC
Generally, no. AC coils rely on inductive reactance. When powered by DC, they act as resistive loads, drawing too much current and overheating.
In emergencies, you may use an AC contactor for DC briefly (under 2 hours) and reduce current with a resistor. However, using DC contactors for AC is not recommended.
Where to buy ac contactor
Procuring AC contactors requires partnering with a reputable supplier that specializes in high-quality electrical components.
Derlicn is a trusted Chinese manufacturer committed to delivering advanced electrical solutions to customers worldwide. We offer a comprehensive portfolio of reliable products, including:
AC contactors
Circuit breakers
Voltage regulators
Automatic transfer switches
And other essential electrical equipment
Equipped with modern processing and testing facilities and backed by a team of experienced technical professionals, Derlicn provides expert guidance and responsive support to help clients implement their electrical systems efficiently and reliably.
👉 Explore our full PRODUCTS range to discover more.
If you have specific requirements or can’t find what you’re looking for, feel free to CONTACT US directly—we’re here to help you find the perfect solution.
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