Electronics

Contactor- Types, Construction, Working, Application, Testing

A contactor is an electrically controlled switch that can switch on/off the electrical circuit. There are numerous types of contactors available. Some of them are the knife blade switch, manual contactor, and electromagnetic (magnetic) contactor. Since the magnetic contactor is the most popular and latest type of contactor, we will be discussing it in this post.

Parts of a Magnetic Contactor

Parts of Magnetic Contactor
Parts of a Magnetic Contactor

The contactors are mainly differentiated into five parts. They are as follows.

  • Coil:- It consists of two terminals which are generally designated with the term A1 and A2.
  • Incoming main contact:- It consists of three terminals which are generally designated with the term L1, L2, and L3.
  • Outgoing main contact:- It consists of three terminals which are generally designated with the term T1, T2, and T3.
  • Auxiliary contact:- It consists of two terminals which are generally designated with the terms 13 and 14. It mostly comes with the NO (normally open) mode. However, NC (normally closed) mode is also available in the market.
  • Plunger:- It moves inside when the coil gets energized.

Construction of Contactor

Contactor Working / Contactor Construction
Construction of 3 Phase Contactor

In the given figure you can see the three incoming main contacts. The main supply is connected to the incoming contacts. The supply can be a three-phase supply with the red, green, and blue wires connected to it. The other end consists of three terminals of outgoing contacts which generally connect to the three-phase loads such as motors.

In between the incoming and the outgoing contacts lies the movable contacts which are connected together with the help of the plunger (armature).  The plunger is connected to the coil with the help of a return spring.  Also, the coil is enclosed within the iron core.

The armature also connects to the auxiliary contacts.

Working Principle of Contactor

When the voltage is applied to the coil, the coil gets magnetized. Thus the coil creates a magnetic field. The magnetic coil attracts the armature towards itself. When the magnetic force overcomes the force exerted by the spring, working in an opposite direction, the armature moves towards the coil. Since the armature is directly connected to the movable contacts, they also move towards the coil. During this process, the movable contacts make direct contact with the fixed contacts. Thus the circuit becomes closed due to which the current can flow from the incoming main contact to the load via outgoing main contact. The auxiliary contacts also undergo the same process.

When the power to the magnetic coil gets cut off, the coil becomes demagnetized. So the force exerted by the spring becomes greater than that of the magnetic force. Thus, the spring pulls the armature towards itself and the contactor returns back to its original state.

Advantages

  • It provides a fast switching operation.
  • It can operate for both AC and DC loads.

Disadvantages

  •  It can damage easily.  The overload, short circuit, etc, can burn the contactor. Similarly, the dust and dirt particles can also alter their conductivity. Similarly, the repeated action of the ON and OFF of the contactor can degrade the elasticity of the return spring. Thus, in that case, it may not be able to move the plunger properly.
  • The continuous movement of the movable contacts can produce an arc at the point of contact. This causes an increase in the temperature of the contact. Thus, the contact may fail. Similarly, some dangerous gases such as mono-oxides may also form within the contactor.
  • Unlike MCB, it can not protect the loads. It only shows the switching action.

Applications

The contactors can be used in various applications. Some of them are as follows.

Testing

Here are some of the ways to check the condition of the three-phase contactors.

  • First of all, make the visual inspection of the unit properly. There can be chances of burning or breakage. If the unit seems ok on visual inspections, then follow the steps shown below.
Testing of Contactor
Testing of Contactor

 

  • Take a multimeter and set it to the continuity mode. Then check the continuity between L1T1, L2T2, L3T3, and NO terminals. There should not be any beep sound from the multimeter. Then press the plunger (armature) manually with your finger. Again take the measurement as stated in the previous sentence. This time, the multimeter should give you a beep sound.
  • If there is an NC terminal, then the result from the multimeter will be exactly the opposite of the result obtained from the NO terminal.
  • Finally, test the coil. For this check the continuity of the coil at points A and A2. If the multimeter gives you a beep sound, then that means, there is no breakage within the coil system. However, if the multimeter does not make a beep sound, then the contactor is faulty.      

Relay Vs Contactor

Some of the differences between the contactor and electromagnetic relay are as follows.

Contactor Relay
Used for switching high voltage applications. Used for switching low voltage applications
The load current is generally more than 10A The load current is generally less than 10A
Bigger in size Smaller in size
It has got a higher switching time. It has got a lower switching time
Generally used in three-phase applications Generally used in single-phase applications
The general rating for the contactor is up to 1000 V. The general rating for the relay is up to 250V.
It is generally designed to operate in Normally Open mode. However, the Normally Closed mode might also be possible. It may be designed to operate in Normally Open and /or Normally Closed mode.

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