What is Electrical contactor and how is it useful for power system

What is Electrical contactor

A contactor is a large relay which can enable the heavy load to be switched on and off with the very small operating current. A contactor is an electromechanical switching device and it is operated remotely and they are capable of making carrying and breaking currents under normal conditions and during operating overload currents. It is placed nearest to the load centers it has high service reliability in terms of mechanical and electrical.

image

It is an electrically controlled switch which is used for switching an electrical power circuit and they are similar to a relay except the contactors has higher current ratings it is a vessel were two streams interact, it is controlled by a circuit which a lower power than a switched circuit. Contactors come in many forms with varying capacities and features. The contactor is not intended to interrupt short circuit current. A contactor range from those having a breaking current of several amperes to thousands of amperes and the size of the contactor ranges from a device which is small enough to pick up with one hand and to a large device which is almost a meter on a side.

Contactors are used to control motors, lighting, heating, capacitor banks, thermal evaporators and other electrical loads.

Components of contactor

  • Electromagnet system
  • Contact and arc quenching system
  • Housing

Contacts are the current-carrying part of the contactor and it includes auxiliary contacts, power contacts, and contact springs. The driving force to close the contact is provided by the electromagnet (or coil). An auxiliary contact reduces coil current after contactor closes, a large amount of current is needed to close the contactor initially than it’s required to keep its closed and such circuits can save a great amount of power

a contactor will have a coil input which may be AC or DC depending on the contactor design and the coil could be energized at the same voltage as a motor. The contactor may be controlled with a lower coil voltage

What is the difference between a relay and a contactor

Contactors are designed to be directly connected to high current load device were as relays are used for the lower capacity and they are normally used for normally closed and normally open applications. Devices switching more than 15 amperes or in circuits which are rated more than few kilowatts are called as contactors. Contactors have features to control and suppress arc produced when interrupting heavy motor currents and relays doesn’t have these features.

Operating principle

image

Contactors are designed for high current loads, when current passes through an electromagnet a magnetic field is produced and it will attract the moving core of the contactor. initially, Electromagnet coil draws more current until its inductance increases when the metal core enters to the coil and the moving contact is propelled by the moving core the electromagnet force holds the moving and fixed contacts together and when the contactor coil is de-energized gravity or spring returns the electro-magnetic core to its initial position and opens the contacts

What are the problems that can be caused in a contactor

  • Over-heating of contacts
  • Chattering
  • Humming
  • Coil burning
  • Welding of contacts
  • Contact life

Contactors problems can be caused by

  • Arc chamber interlock
  • Mechanical interlocking of contactors
  • AC –DC coil conversion
  • Coil, main contacts replacement

If we don’t use proper contact protection then the degradation of contact will happen due to the occurrence of the electrical current arc. This could cause severe damage electric arc occurs between the two contact points and the heat which is developed by the electric arc can cause the metal on the contact to migrate with current

How can we do the maintenance of a contactor

  • Contactor must not be operated without arc-chamber
  • Clean contacts with CRC 2-26
  • Replace contact in pairs
  • Ensure correct tightening torque
  • Genuine spare parts must be used
  • Cable/busbar must be properly connected at the contactor terminal

vacuum contactor

vacuum contactors use vacuum bottle encapsulated contacts to suppress the arc and this arc suppression allows the contacts to be smaller and it will only use less space compared to the air break contacts at high currents. Vacuum contactors are used for dirty applications such as mining, vacuum contactors are only used for AC system in AC system vacuum contacts can prevent the restrike of arc across the open contacts and so that vacuum contactor is very efficient

Applications of Electric contactor

Lightning control – contactors are used to provide central control for large lightning installation such as an office or retail buildings. Latching contactors are used to reduce power consumption in contactor coils

Magnetic starter – A magnetic starter can provide power to an electric motor and it uses contactor in it, which will provide power cutoff, under-voltage, and overload protection.