What is Magnetic contactor?

Have you ever used a magnetic contactor in your application? This was first developed by Hein Moller. This unique device has been used in switching applications for many years. Due to its huge market size, magnetic contactors are still the most important members in control applications. In this article, I will explain all the technical details of this device based on my research.

1. What is a magnetic contactor?

A magnetic contactor is an electromechanical switch used in applications that require a circuit “on and off” process, such as starting motors, heaters, and lighting applications. Through the switch contacts, the magnetic contactor takes care of transferring energy from one place to another.

2. The working principle of magnetic contactor

The working principle of the magnetic contactor is that the main contact is closed due to the coil being energized, and the main contact is disconnected due to the de-energization of the coil.

The control circuit is composed of electromagnetic and spring system. The current through the coil activates the magnetism, and the two magnetism approaches each other. This movement shuts off two magnetic forces. Therefore, the contact is closed and the spring provides power to the contact. When the control current is disconnected, the contact is disconnected.

This basic principle has been used for more than 100 years, and no new alternative methods have emerged so far. You can also watch the video below to better understand how the magnetic contactor works.

3. Application of magnetic contactor

Generally, magnetic contactors are used to start and stop the motor. The most commonly used type is a three-pole contactor suitable for use in a three-phase system. In addition to motor applications, there are also applications in heaters, lighting, DC switching and transmission applications.

4. How to connect the magnetic contactor?

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

Connect the main power contacts to L1, L2, and L3. The auxiliary contact input is NO and NC. Assistive contacts provide positioning information to a remote device (such as opening or closing information) about the contactor.

When tightening the product screws, try to tighten them according to the torque value written on the product. If you apply too much force, the screws may wear out, and an electric arc will occur after power on.

5. The figure below is “Magnetic Contactor Wiring Diagram”

Structure of magnetic contactor

The structure of the magnetic contactor is not complicated. The following are the components of the device.

  • Terminal bar.
  • Moving and fixed contacts.
  • Coil terminal.
  • Arc parachute.
  • Shaded coils.


If the magnetic contactor is selected correctly and used under nominal conditions, it will not malfunction. The most common faults are contact wear and coil burnout.

If excessive current passes through the power contacts, they will heat up and stick to each other. Magnetic contactors are not protective devices like circuit breakers. Tripping when overcurrent passes through the circuit breaker. However, if an overcurrent flows through the magnetic contactor, the main contactor will stick.

Likewise, if the voltage applied to the coil terminals is nominal, the coil burns out. Therefore, the power contacts and coils must be input with nominal voltage and current values.

Protection devices such as overload relays and fuses must be used with contactors to better protect the system.

6. Selection parameters of magnetic contactor

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

  • Type of load. (Motor, heating, lighting, HVAC)
  • Rated current/load power.
  • Operating voltage.
  • Control voltage.
  • The number of poles.
  • Availability of internal auxiliary contacts.

7. Frequently asked questions about magnetic contactors

What happens to the contactor when it is short-circuited?

If there is a protective device in the circuit, the contactor is protected because the protective device will trip. Otherwise, contactor failures may occur, such as contactor bonding or coil burnout.

8. What do the numbers on the contactor mean? What is the 13 14 21 22 on the contactor?

13 and 14 are the terminals of the normally open auxiliary contacts. The terminals of the normally closed auxiliary contact are 21 and 22.

What are A1 and A2 on the contactor?

A1 and A2 are the coil terminals of the contactor. A1 is the positive electrode and A2 is the negative electrode.

What caused the contactor to fail or jam?

  • Overcurrent flows through the contacts.
  • Apply low or high voltage to the coil.
  • Dust, corrosion or vibration in the environment.
  • Wrong product selection.
  • Shortcircuits are caused by the electromotive force.
  • Transients and voltage fluctuations.
  • Ambient temperature.

Why use contactors for motors?

The motor contactor allows you to turn the motor on and off remotely. You can use the contactor to start and stop the motor.

What does Fla mean on a contactor?

Full Load Amps (F.L.A) represents the designed output current of the motor under the rated horsepower. If you see FLA on the contactor, it means the ampere of the motor that can be connected to the contactor.

How do you know that a contactor is broken?

Buzzing and vibrato during contact, and overheating of the body are all signs of a bad contactor.

Can I change the 30 amp contactor to a 40 amp one? or higher?

The contactor can be replaced with a higher version. There is a cost associated with this option.


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