What is contactor ?

The magnetic contactor is an electrical switches (electrical controlled ) which is used for switching electrical power circuit.

It work on  basic electromagnetic principles.It is not a protection device. It cannot used as a protection devices like circuit breaker etc..

The basic purpose of magnetic contactor in a circuit is switching or it is simply called switching device. This switching of high power and frequent operation can done with the help of magnetic contactor.

With the help of  magnetic contactor we can operate motor of any rating from a distance and control with the help of single switches. Different range of magnetic contactor are available in the market.

Magnetic Contactor and Relay

Magnetic contactor is a switching device like a relay but it has the higher current capacity whereas relay has low current capacity. Magnetic contactor is the robust version of relay.

Magnetic contactor can handle high power but Relay handles small power for its weak contacts, It operate by an external signal or power  like relay. On the other hand circuit breaker is not made for frequent operation but its operate by itself and send some signal to the external circuit

Relay can operated by below 1 ampere but magnetic contactor can used in over 1KA power circuits. Contactor made for frequent switching operation and its may be switched on & off 1000 times in a day. Hence it is mechanically robust and strong to withstand such switching and it is  capable of handling high power.

Magnetic Contactor  symbole

Construction

A contactor has three component

1-Power coil   2- Auxiliary coil    3-spring mechanism

The power coil carry high current and auxiliary contactor receives signal and which the help of that signal it make the contactor open or close or send the status of the contactor(on or off) to external systems like PLC, SCDA. The spring mechanism  is used to provides mechanical force to make the contact on or off.

Enclosures are made with  insulating materials like Bakelite, Nylon 6, and thermosetting plastics to protect and insulate the contacts and  also to provide protection against personnel touching the contacts and human hazards.

Magnetic blowouts are use blowout coils to lengthen and move the electric arc which is produce  to switching due to high voltage.

magnetic contactor structure

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These  magnetic blowouts are especially useful in DC power circuits because AC arcs have periods of low current during which the arc can be extinguished with relative ease but DC arcs have continuous high current, so  blowing them out requires  than an AC arc of the same current.

Sometimes an economizer circuit is also used to reduce the power required to keep it closed as an auxiliary contact reduces coil current after the contactor closes.

Sometime amount of power is required to initially close a contactor than is required to keep it closed.Econimizer circuit save amount of power  due to which it allow the energized coil to stay cooler. Economizer circuits are always applied on direct-current  ( DC ) contactor coils and also used on large alternating current contactor coils.

A basic contactor will have a coil input .The coil may be energized at the same voltage as that the  motor which is controlling by  contactor or may be separately controlled with a lower coil voltage bit it is better suited to control by programmable controller or PLC and lower-voltage pilot devices.

Operating Principle

When current passes through the electromagnet or in a coil  a magnetic field is produced which attracts the moving core of the contactor. The electromagnet coil draws more current initially until its  inductance increase when the metal core enters the coil. The force developed by the electromagnet connect the moving and fixed contacts together. When the contactor coil is de-energized or supply is off, gravity or a spring returns the electromagnet core to its initial position and due to which contact open

For contactors energized with alternating current ( AC ),  a very  small part of the core is surrounded with a shading coil which is used to  slightly delays the magnetic flux in the core. This  effect is to average out the alternating pull of the magnetic field and  due to which it prevent the core from buzzing at twice line frequency

Because arcing and damage occurs due to  contacts opening or closing, contactors are designed to open and close very rapidlly there is often an internal tipping point mechanism to ensure rapid action

Due to Rapid closing it will lead to increase contact bounce which causes additional unwanted open-close cycles. One solution is to have bifurcated contacts  due to which its minimize the contact bounce  : two contacts designed to close simultaneously :but bounce at different times so the circuit will not be entirely  disconnected and cause an arc.

Arc suppression

Without proper contact protection the occurrence of electrical current arc causes significant degradation of the contact which causes damage. An electrical arc produced between the two contact points (electrodes) when they transition from an open to a closed (make arc) or  closed to an open (break arc). The break arc is which is more energetic and therefore more destructive

Most motor control contactors at low voltages which is less then 600 volt are air break contactors. Modern medium-voltage AC motor controllers control with the help of  vacuum contactors. High voltage DC contactors which is greater than 600 volt are still base on air within specially designed arc-chutes to break the arc energy produced during the transition of contactors.

Rating

 Contactors are rated by designed load current per contact  or pole,maximum fault withstand current, duty cycle, design life expectancy, voltage, and coil voltage.

Application

1-Light control

 Contactor are used normally to provide central control for residential building or commercial building lighting installation .To reduce power consumption in the contactor coil a latch contactor is used which have two operating coils. One coil which is momentarily energized, closes the power circuit poles or contacts, which are then mechanically held closed the second coil opens the contacts.

2-Magnetic Starter

A magnetic starter is a device which is designed to provide power to electric motors. It  basic component is contactor .This also also providing power-cutoff, under-voltage, and overload protection. Automation is base on the magnetic contactor and due to magnetic contactor we can design automatic start delta starter ,direct online starter ,Light automatic on and off ,Automatic changeover switch ,Elevator control panel automation ,etc See the video where lighting is control by magnetic contactor below

Automatic changeover switch by using contactor drawing

Automatic changeover switch by using contactor drawing

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