Definition – A contactor is an electrical device that is used to switch ON/OFF electrical power circuit through its coil. Or A contactor is an electromagnetic switch which is used to control electrical power circuit (switch ON/OFF) . It is an electrically operated switching device.

Construction – A contactor consists of 1) Contacts – Fixed & moving 2) coil & 3) Terminals. All above things are fixed in a housing/enclosure which is made of insulating material. Contactors also provide NO & NC contacts which are used for interlocking & to control electrical circuits.These NO/NC contacts arepotential free contacts (as they are only mechanically connected with contactor). NO/NC contacts are also called auxiliary contacts.

Working principle – Cables/wiring is connected with cable terminals which are connected with two sets of fixed contacts of the contactor which are separated with each other. Contactor coil is connected with power supply which may be 24VDC, 110V AC, 220VAC, 415V AC. When there is no power supply connected

with coil, the contactor remains off & current doesn’t flow through the contactor but when power supply is connected with coil, coil gets energized & it pulls down the moving contacts & moving contact connects two sets of fixed contacts with each other & contactor gets ON & current passes through the contactor. This is how contactor works. (Note – When contactor changes its position from OFF to ON or vice versa, the NO & NC contacts changes its position e.g NO becomes NC & NC becomes NO.)


Broadly contactors are classified into two categories – 1) Power Contactors & 2) Control contactors.

Power Contactors – Power contactors are used as a switching device to switch ON/OFF of electrical power circuits & for starters and provide NO/NC contacts as well.

Control (or Auxiliary) Contactors – These contactors are used to control electrical circuits by using NO/NC contacts. Working principal of Control contactor is same as of power contractor but it has no cable terminals & only provides NO/NC contacts which change their position with ON/OFF position of the contactor.


  • POWER CONTACTORS–Following things to remember while selecting a power contactor –
  • Operating voltage – 415V etc
  • Current carrying capacity – 16A or 20A ………400A etc
  • No of poles – 3P or 4P,
  • Coil voltage – 24V DC, 110V AC, 220V AC, 240V AC, 415V AC etc
  • Number of NO/NC contacts – 1NO+1NC, 2NO+2NC, 4NO+4NC,..
  • Duty Cycle – AC1 or AC2 or AC3 …

Ex– Requirement of Power Contactor – 1no (specification – 150A, 415V AC, 3P, Coil Voltage -220V AC with AC3 duty & 2NO+2NC).

  • CONTROL (or Auxiliary) CONTACTORS – Following things to remember while selecting a control contactor –
  • Coil voltage – 24V DC, 110V AC, 220V AC, 240V AC, 415V AC etc
  • Number of NO/NC contacts – 1NO+1NC, 2NO+2NC, 4NO+4NC,..

Ex – Requirement of Auxiliary Contactor – 1no (specification – Coil Voltage -220V AC with 8NO+8NC).


Add On block are nothing but NO/NC contacts which can be mounted on power or control contactors to get additional NO/NC contacts for a particular contactor.

Selection of Add On Block – As they are mounted on contactors (Power & Control) so we need to mention the type/model of contactor of a particular make for which it is required.

Ex – Add On Block (4NO+4NC) – 2nos are required for a contactor for a particular make.


AC Duties are different categories of utilization of contactors which depends on the following –

  • Type of load to be controlled (Inductive load, lighting load, Motor load, Resistive load, Capacitive load etc),
  • The Operating cycle condition.

These duties are of following types –

  • AC1 – These types of contactors are used for Non-inductive or slightly inductive loads, they are used for Heaters which are resistive loads.
  • AC2 – These type of contactors are used in Starting of slip-ring motors
  • AC3 – They are particularly used for 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. Rapid Start/Stop.
  • AC11 – Auxiliary (control) circuits i.e. they don’t have power contacts.
  • AC6a– Switching of Transformers.
  • AC6b– Switching of Capacitor banks