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Electrical Contactors

A contactor is an electrically controlled switch used for switching power circuit, similar to a relay except for higher ratings. Contactors have multiple contacts, usually open, so that the load is shut off once the coil is de-energized. The most common use of contactors is control of electric motors where they are used in conjunction with an overload relay or an MPCB to reduce the load of starting current on the motor.

The three contacts of a contactor switch the respective phases of the incoming 3 phase power. There is also an auxiliary contact with a much lower current rating but is actuated  by the same armature as power contacts. A contactor may have multiple such normally open (NO) or normally close (NC) auxiliary contacts.

The contacts are controlled by the electromagnet. These electromagnets give the initial force to the contacts and make them closed. Both these contacts and electromagnet are enclosed in a frame which is usually made of insulating materials. The usually used insulating materials are Nylon 6, thermosetting plastics and so on. They are useful, as they completely insulate the contacts and help in preventing the touch of contacts.

When current is passed through the contactor, the electromagnet starts to build up, producing a magnetic field. Thus the core of the contactor starts to wind up. This process helps in energizing the moving contact. Thus the moving and fixed contacts make a short circuit. Thus the current is passed through them to the next circuit. The armature coil brings in high current in the initial position. This reduces as soon as the metal core enters the coil. When the current is stopped, the coil gets de-energized and thus the contacts get open circuited.

The contactors generally contain overload heaters which are connected to each phase going to the motor. Each heater element is a low resistance strip of metal which gets heated as the motor draws current. Once the element gets past its critical heating point, the normally closed switch contact would open, thereby de-energising the coil and shutting the load off to the motor. This high heating generally happens due to high flowing current. The shutting of the contactor thus prevents the motor from getting burnt due to high amperage flowing through the circuit. The heating element is designed in such a way to mimic the power(I2R) characteristics of the motor. This helps in shutting the load at the appropriate load.

A number of companies provide various types of contactors depending upon the load and the cost to the customer. Whereas L&T provides the MNX series, ML series and MCX series of contactors, Siemens provides the 3TF series and 3RT series of contactors. Schneider has the globally used telemechanique series of LC1D  contactors and LC1F contactors and the Indian specific LC1E series of contactors. GE provides the CL and CK series of contactors while ABB provides the A series and the newly launched AF series of contactors to cater to the huge demands of the industry.

You can download the various switchgear pricelists and catalogs for the different companies (L&T, Schneider, Siemens, GE, ABB etc) for your reference and budgetery cost estimates.

Contactor Ratings

Ratings of a contactor are given according to the pole of the contactor. It also depends on factors like fault withstand current, coil voltage and so on. According to their rating, contactors are classified into the following.

  • AC1 – Non-inductive rows
  • AC2 – Contactors for starting of slip-ring motors
  • AC3 – Starting of squirrel-cage motors and switching off only after the motor is up to speed.
  • AC4 – Starting of squirrel-cage motors with inching and plugging duty.
  • AC11 – Auxiliary control circuits