All About Automation: Contactor Cotrol

It is the old Method for Control.
It is Called " Classic Control " or " Relay Control ", but it is prefered to call it " Contactor Control " as every type of control will be a classic for what come after it. and it is not the relay that is used in control it is something work on the same relay's principle of operation. this device is called " Contactor " and from it that name come.

This type of control uses some components:
Contactors - relays - Timers .
and these all components are all arranged in a Control Panel

Contactors

General overview on Contactors

It consist of :
1- Electromagnetic Coil.
2- Spring.

3- Contacts (3&4).

Principle of Operation:
The contactor is operated by the control circuit.
There is an electrical signal - its value and type depend on contactor - this electrical signal energize the coil to generate electromagnetic force which overcome the force of spring and attract the contacts to be a short circuit.
The spring keep the contacts in it is normal status.

This figure shows the contacts while coil energized and be in closed contacts.

and here is a wiring diagram for both de-energized and energized cases:

Types of contactors :
1- Solenoid type.
2- Clapper type.

1- Solenoid Type Contactor:
In this type the movable contacts are attached to the movable core of a magnet. When the
electromagnet coil is energized, the movable core is pulled to the stationary core, thus closing
the contacts.

For better understanding, the contacts have been shown mounted in vertical plane though actually the contacts are in horizontal plane.

The position of plunger i.e., movable core shown in the figure is for the coil in de-energized state. When the coil is energised, plunger moves up, moving contacts mounted on plunger also moves up and closes the normally open contacts. At the same time normally closed contacts open. When the coil is de-energised contacts are broken and they come back to their normal position by the pull of gravity.

Note:
- Mounting of contacts in horizontal plane reduces the size of the contactor.

- This Type of contactors is used for small ratings.

for high ratings, there is a type which i called : Contactor with E-type magnetic core .

note that The pole face of the magnet of both are provided with shading coil. This creates an out of phase flux to hold the magnet closed during the zero points of alternating current thus preventing
chatter of the contactor.
and For contactors of higher ratings where chattering noise is to be avoided, dc coil is used.

each contactor provide its main contacts plus its auxiliary points.

Symbols used for contactor :

- Contactor represent the coil which is energized from control circuit.
- Normally Open contacts (NO) when energized become Closed Contacts .
- Normally Closed contacts (NC) become open.

here is a picture for it :

2- Clapper Type Contactor :
In this type, the movable contacts are mounted on a hinged movable
armature. The hinged armature when pulled by magnetic core moves the movable contact in more or less in the horizontal direction to make contact with the stationary contact mounted on the vertical
back-plate of the contactor.

It Consist of:

1- A magnet:
       consist of a round solid core on which the coil is mounted and a bent piece on which the moving armature is hinged. All parts of the magnetic circuit are made from a soft steel having high permeability.
2- A movable contact:
       It is not attached to the armature but is held in place by a spring and bolt.
3- A Spring:
       It exerts a force on the movable contact. The amount of force to be applied on the moving contact can be varied with the help of a bolt on which the spring is fixed.
4- Arcing horns:
       They are provided on both the fixed and movable contacts. Arc shifts towards the arcing horns during interruption thus saving the main contacts from damage.
5- A blow out coil:
        provided to extinguish the arc. It is a circular strip having an insulated solid core inside. Current flows from fixed contact through the circular strip to the outlet terminal.

Operation :
when the coil is energized , it generates an electromagnetic force that attract the hinged armature so that the moving contact move toward the fixed contact and then current flows from incoming terminal to outlet terminal.
when the coil become de-energized then the spring will exerts its force on the movable contact to return it to its normal position.

and here it is areal picture:

In the previous part we discussed the contactors, now we will move to :

Relays

A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and most have double throw (changeover) switch contacts as shown in the diagram.
Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical.

The animated picture shows a working relay with its coil and switch contacts

The function of relays:
The literary meaning of the word relay is to transmit information.
also in control circuits, it do the same ....so it sense or accept information from some sensing device
and feed it into control circuit at proper level. The sensing devices used in conjunction with
relays are known as pilot devices.
These Pilot Devices can sense or detect variables like current, voltage, overload, frequency, temperature, pressure and many others. - it will be discussed later -