What is is Auxiliary Contacts and it's working in Contactor?

In Electrical Power wiring we use different types of relays and contactor/motor starters and these have some contacts which we know with names of main contacts and coil contacts but this is not an end we have also other contacts which we known with the name of auxiliary contacts. These contacts are available in every contactor but these auxiliary contacts in different types just lick Normally close and normally open which can set or fit on the contactor/motor stater and person can use it for different types of works. But the reason of this post to learn that what is auxiliary contacts and what is the work of auxiliary contacts on a motor starter.
So let's start it step by step.

What is Auxiliary Contact and A Contacts on Contactor?

Auxiliary means helper or we can say these are helper contacts in magnetic contact or relay which help us to do our work easily. And one thing more without these contacts we can not control anything by using a magnetic contactor.
So these are also important contacts lick main contacts. One thing more these contacts are available in every motor starter but in Small and low ampere (load) contactor we have these contacts only on one side but in big and high loaded motor starter we have these contacts on both sides. In these contacts, one is normally close (NC) and another one is normally open contacts (NO). Which we use for different purposes.

What is the work or Auxiliary contacts on a motor starter?

These points are very important in the motor starter and without these contacts a contactor is incomplete. As I was written in my above words that in these contacts in one is normally close and 2nd is normally open circuit.
These contacts are directly proposal to the setting coil and when the magnetic contactor coil energizes and set, the normally close contacts become open contacts and normally open contacts change to the close contacts.  From this change, we can control many things which can see it in my last published post about controlling three phase motor using contactor. So get a look at these posts.

Also, read below:
How to wire a three-phase induction motor with contact?
How to wire a phase failure relay for 3 phase appliances?
What is MC?
MC / motor stater installation for single phase motor

You can also get an idea about the working principle of auxiliary contact s from the above post links however let's get a review of the working of these contacts.

For example

  • Without these contacts, we can't do a proper connection of a 3 phase motor or three phase appliances.
  • By using these A-C (A cont...) we can do the star delta starter connection for three phase 6 wire motor.
  • By using these A-C we can control two contactors with one another, i.e if we have 2 contactors which we install in our main board and both are installed for different machines. But if we want that if a machine starts then the other one does not start then we can easily use these contacts for that type of work.

The above example is not too close to understand completely, however, you can get an idea and if you try and think about that, I hope you will do it. However, everything, when we start learning, is not completely learned in a short time and it's required some time.
Further, in our post, we will use these contacts in the circuit and IN SHA ALLAH you will learn if you gave it a short time.

Now let's get a view of these Auxiliary contact s, in the below image i showed these contacts in animation - type look + images from which you can learn it easily.
Now one thing more in every Auxiliary contacts we have two types of contacts in which NC and NO. But in the contactor, we have moveable contacts when circuit or A-C in the normal state the NC will NC and NO will be NO but when connector energize or these contacts are set by using any force the NC contacts will change to NO (normally open) and NO will be changed to NC (normally close). You can get an idea from the below image plus the diagram.

auxiliary contacts diagram

I hope after the above image + diagram of auxiliary contacts, know you understood completely, however as I told in my above words that further we will publish the diagram in which we use these contacts and you will completely understand how to use these Auxiliary contacts.

Sikandar Haidar

Electricalonline4u.com is a hub of electrical and electronics tutorials and easy diagrams.


  1. Hello,I have a 5hp , 380 volt ,3 phase motor that bends reinforcing bars.it had 3 contactor's wired to it ,one energised when power was put on and the other two were for toward and reverse with the overload connected in the bottom.its set at 9 amp overload.I purchased this motor in auction and all the wires are missing and the contactor's are not working.I want to replace all the contactor's and over load,the machine has 3 buttons ,one for stop the other two are toward and reverse respectively.no pilot lights or indicators are connected.can u kindly assist me with a proper wiring diagram for this motor please.the motor is working as I connected directly to the supply and it turns.

    1. Dear i designed a diagram about this and it is save in my system and IN SHA ALLAH soon i will published it.

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