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 -

Choosing a relay:
You need to consider several features when choosing a relay:

1. Physical size and pin arrangement .
2. Coil voltage .
3. Coil resistance .
   

   Relay coil current   =	supply voltage
   	coil resistance

4. Switch ratings (voltage and current) .
5. Switch contact arrangement (SPDT, DPDT etc) .

Some of the important type of relays are discussed as follows:

1- Voltage Relays:
       This is just a small contactor which changes its contact positions from normally open to close and normally close to open when a proper voltage is applied across its coil. These relays come with as many normally open and close contacts as required.

2- D.C. Series Current Relay:
        This relay changes its contact position in response to current change in its coil. The relay coil is connected in series with the circuit in which current change is to be sensed. The armature of the relay is light thereby making it very fast in action.
Some terms used in connection with a current relay are:
Pull in current. It is the minimum value of current to close or pull in the relay armature.
Drop out current. It is the value of current below which the relay no longer remains
closed after having being pulled in.
Differential current. It is the difference in value of pull in and drop out currents.
For example if a relay is energized or pulled in at 5 amps and drops out at 3 amps, then pull out current is 5 amps, drop out current is 3 amps and the differential current is 2 amps. For heavier current applications, a current transformer is used and its output is applied to the current relay coil.

3- Frequency Responsive Relay:       
       This type of relay changes its contact position when the frequency of applied voltage to the relay coil falls below a pre-determined value. The relays are designed differently for different applications. In one type of construction, voltage is applied to the relay coil with a capacitor in series through a potentiometer resistor.

Operation:
    The inductance of the relay coil and capacitor form a series resonant circuit through which a high current flows for a certain frequency range. The band width of the frequency range can be varied by changing the tapping of the potentiometer resistor. When the frequency
falls below the resonant frequency range, current through the relay coil falls to a low value
and the relay drops and its contacts open.

     

Timers

 It is called " Time Delay Relay".
The relay contacts change over their position after a pre set delay from the time of energisation or de-energisation of the relay coil.
Timers can be :
- ON-delay type : the contacts change over after a pre-set delay after energization.
- OFF-delay type : the contacts change over after a pre-set delay from the instant of de-energization.

Symbols for Timers:

Timers can also be classified as :

- Cyclic Timers : a timer continues to repeat its sequence of operation till the supply is switched off, it is called a cycle timer. The change over contacts would continually switch ON and OFF at intervals as long as operating voltage is on.

- non-cyclic timers : operate only for one timing cycle when they are switched on. The change over contacts get switched over after the set time and stay in that state till they are reset either manually or by a knob or by switching ON or OFF an electrical signal.

Note:
           Majority of applications require ON-delay, non-cyclic, non-manual reset type timers.

There are four different types of timers utilizing different operating principles :

- thermal timers.

- pneumatic timers.

- electronic timers.

- motor driven (synchronous) timers.



1- Thermal Timers:         A type of analog timer that uses heat generated from an electric current to bend a bi-metal strip, which closes a set of normally open contacts. The time it takes to heat the bi-metal strip produces the timing delay.
These are available with a limited delay range of 0–20 sec.
The timing error can be very large of the order of ± 10 to 30%. By timing error it is meant that the time variation (plus or minus) between successive timing operations on the same timer represented as a percentage of maximum range of the timer.

Note : Due to higher timing error thermal timers are used only in star-delta starters where such error does not matter much.

2-Pneumatic Timers:
consists of :
-It consists of solenoid coil, a plunger assembly, and an air bleed unit consisting of an air    chamber divided into upper chamber and lower chamber by a diaphragm.
- The diaphragm is attached to an operating rod and disc D.
- There is a provision of air bleed (air flow) from upper chamber to lower chamber through a needle valve.

Operation:

When the solenoid coil is energised the plunger is attracted and thus pressure on the
disc (D) and the operating rod is released. Now the diaphragm is free. Air from upper chamber leaks to the lower chamber through the needle valve. The rate of air flow can be adjusted by the screw of the needle valve. As pressure of air in the lower chamber starts increasing the diaphragm moves up slowly at a speed which depends upon the rate of air flow through the needle valve. The diaphragm in turn moves the operating rod and the disc upward. The diaphragm will move up till air pressure in both the chambers is equal. An actuating lever attached to the disc, which moves upwards, actuates the timer contacts.
The delay or time for actuation can be varied by adjustment of the needle valve. Operating
rod, disc and diaphragm remain in up position as long as the coil is energised and therefore relay contacts remains closed.

3-Motor Driven Timers:
consists of:
  - A small motor, usually a synchronous motor.
  - Gear arrangement reduces the speed of motor to a desired low value.

To Be Cont.......