CONTACTOR

What is Contactor ?

A contactor is an electromechanical control device that is used to make or break the connection between the load and power supply. The use of a contactor is similar to the relay. But the device is used for higher current carrying application is known as a contactor and the device used for lower current applications is known as Relay.

A contactor has several contacts as per the application and load.

Generally, these contacts are normally open (NO) contact. And hence the load is shut off when the coil of the contactor is de-energized. But the contactor can design for both normally open and normally close applications.

The most common application of contactor is in the starter that used to turn ON and OFF

the equipment like motor, transformer, etc. A contactor’s applications include

controlling electric motors, thermal evaporators, lighting, capacitor banks ,

heating, and other electrical loads.

Construction of Contactor :

A contactor has three main parts;
• Coil or Electromagnet
• Contacts
• Enclosure or Frame

Coil or Electromagnet :

The coil is wound on electromagnetic core and behaves as an electromagnet. Generally, it has two parts, one is a fixed part and the second is a movable part. A spring is connected between both parts. Hence, there is a spring return arrangement. A rod is connected with the moving part. This rode is also known as an armature. When a force of coil is more than the force of spring, both contacts are connected and when the force of spring is more than the force of the coil, both contacts are extracted with each other.A very small amount of current will flow through the spring from the supply or external control circuit to excite the core of the electromagnet. For AC applications, the electromagnetic core is made up of laminated soft iron to reduce the eddy current. For DC applications, there is no issue of eddy current, the core is made up of solid steel.

Contacts :

This is the only component from which the entire load current will flow. Hence, it is a very important component of the contactor. The contacts are classified as power contact, auxiliary contact, and contact spring. There are two types of power contact; stationary contact and movable contact.

The material used for the contacts has stable arc resistance and high welding resistance. These materials must withstand mechanical stress, erosion, and arc.

Encloser or Frame :

The encloser is used to protect the internal parts of the contactor. It is made up

of plastic, nylon 6, ceramic, or Bakelite. It provides housing to the electromagnet

and contacts. The encloser is used to insulate the contacts and provide

protection from the dust, oil, weather, and other explosion hazards. It avoids

direct touching of contact when it is powered.

Working of Contactor :

An electromagnetic field is produced when the electromagnetic coil is energized. As we have seen in the construction, the moving contact of the contactor is

connected with the armature (metallic rod) of an electromagnet.

When an electromagnetic field is produced, the armature experiences the force

and pulls towards the fixed contact. The force produced by the coil is more than

the force of the spring. Both contacts remain in this position until the coil is not

de-energized.Once the coil is de-energized, the electromagnetic force is zero and the

armature pulls back due to the force of spring. And return in the normal

condition (OFF position). The contactors are designed for the rapid ON-OFF

operation.

The input of the contactor coil may be AC or DC or in some cases, the universal

coil is used as an electromagnetic coil. The universal coils operate on AC and DC both. A small amount of power loss occurs in the contacts and an economizer

circuit is used to reduce this loss.

Application of Contactor :

The contactor is used in the following applications.

• The most common application of contactor is in the motor starter. It is used with overload and short circuit protection for the industrial motor.

• The contactors are used for the automation of lights for industrial, commercial, and residential lighting applications. For this type of application, latch type relay is used. In this type of relay, two coils are used. One for open contact and second for close contact.

• Single pole contactors are used to operate 12VDC load in the vehicle.

• The use of contactors with the circuit breaker assures the safety of the operation of load in industries. And in such an application, it is used to fast switching of a load.

• It is used in mercury relay and mercury-wetted relay.

• Two-pole (3-wire, 1-phase) contactors are used to operate 240VAC load like air conditioner.