Magnetic Contactor: Definition, Working principle, Construction

What is a Magnetic Contactor?

The magnetic contactor is an electromechanical switch employed in applications that require the process of the electric circuit “making and breaking”, such as starter motors, heaters, and lighting applications. The main function of the magnetic contactor is to transmit energy from one point to another by switching the contacts.

Working Principle of a Magnetic Contactor

The magnetic contactor works on the principle of the closing of the main contacts as a result of the coil energization and the opening of the main contacts as a result of the coil de-energization.

The control circuit consists of electromagnetism working with a spring system. Magnetism is activated by the current -flowing through the coil- and the two magnetism come closer to each other. This movement turns off the two magnetism. Therefore, the contacts close, and the springs provide power to the contacts. When the control current is off, the contacts open.

This basic principle has been used for over 100 years and no new alternative has yet been developed. You can also watch the video below to better understand the question of how a magnetic contactor works.

Applications of Magnetic Contactors

Generally, magnetic contactors are used in motor-starting applications to start and stop the motor. The most commonly used type is the 3 pole contactor suitable for use in 3 phase systems. In addition to motor applications, there are also areas of use such as heaters, lighting, DC switching, and transfer applications.

Wiring a Magnetic Contactor

Before connecting the product, check whether the label values ​​are suitable for the system you will connect to. Then find the A1 and A2 terminals. These terminals are coil terminals. When the coil is energized, the main power contacts of the contactor will be closed. Connect phase (+) to A1 and neutral (-) to A2.

Then make the connection of main power contacts to L1, L2, L3. Besides, there are auxiliary contact inputs specified as NO and NC. Auxiliary contacts are used to signal the position information of the contactor, ie open-close information, to remote devices.

While tightening the screws of the product, try to tight it according to the torque values ​​written on the product. If you apply too much force, the screws may be worn, which can create an electrical arc when energized.

Below you can see “The magnetic contactor wiring diagram”

Construction of a Magnetic Contactor

The construction of a magnetic contactor is not complex. The following are the parts of the device.

• Terminal bar.
• Moving and fixed contacts.
• Coil.
• Armature.
• Core.
• Coil terminals.
• Arc chutes.
• Shading coil.

Problems

Magnetic contactors will not fail if they are selected correctly and used under nominal conditions. The most common faults are contact wear and coil burn.

If an excessive current is passed through the power contacts, they will get hot and stick to each other. A magnetic contactor is not a protection device like a circuit breaker. Circuit breakers trip when overcurrent passes through them. However, if overcurrent flows through the magnetic contactors, the main contacts stick.

Likewise, if the voltage applied to the coil terminal is out of the nominal, the coil burns. Therefore, the power contacts and coil must be fed with nominal voltage and current values.

Protection devices such as overload relays and fuses must be used together with the contactor for better system protection.

Selection Parameters of a Magnetic Contactor

The selection of a magnetic contactor is based on the following technical parameters:

• Type of the load. (Motor, Heating, Lighting, HVAC)
• Nominal current/power of the load.
• Operating voltage.
• Control voltage.
• The number of poles.
• Internal auxiliary contact availability.
• Dimensions.

FAQ about Magnetic Contactors

What happens to a contactor during a short circuit?

If there is a protection device in the circuit, the contactor is protected because the protection device will trip. Otherwise, there will be a contactor failure such as contact sticking or coil burn.

What do the numbers on a contactor mean? And what is 13, 14, and 21,22 on a contactor?

13 and 14 for the terminals of normally open auxiliary contacts. 21 and 22 for the terminals of normally close auxiliary contacts.

What are A1 and A2 on a contactor?

A1 and A2 are the coil terminals of a contactor. A1 is positive, A2 is the negative terminal.

What causes a contactor to fail or get stuck?

• Overcurrent flowing through the contacts.
• Low voltage or high voltage is applied to the coil.
• Dust, corrosion, or vibration in the environment.
• Wrong product selection.
• Electrodynamic forces during short circuit.
• Age.
• Transients and voltage fluctuations.
• Ambient temperature.

Why contactor is used in a motor?

A motor contactor allows you to turn on and off motors remotely. You can start and stop an electrical motor with a contactor.

What does Fla mean on a contactor?

Full Load Amps, or F.L.A., represents the amount of current the motor is designed to draw at the rated horsepower. If you see FLA on a contactor it indicates the ampere of the motor that contactor can be connected.

How do you know when a contactor is bad?

Humming and chattering in the contact, and overheating in the body are the indicators of a bad contactor.

Can I replace a 30 amp contactor with a 40 amp? Or higher?

Contactors can be replaced with higher versions. But it would be a more expensive option.