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Understanding a Fan Motor

August 28th, 2012 Leave a comment Go to comments
Have you opened up a fan motor before?  It is a simple brush less A/C induction motor, where the armature remains as the stator.  Look at the exploded image of a typical fan motor. The important parts are C: Ball Bearing, D: Stator, E: Armature/Stator, G: Capacitor, H: Connector. 

Also, look at the other image that shows the cross section of a motor.  You would notice that there are 4 wires coming out the motor.  Two pairs of wire, with one pair as the starting coil and the another pair for the running coil.  In general, all single phases induction motors have two coils (starting coil, running coil).  Using a capacitor, an artificial phase difference is created between the fields created by the starting and running coils.  The phase difference triggers movement of the armature.  When the armature reaches a particular speed, using a centrifugal switch, the connection to the starting coil is disconnected and the entire fans runs with just one coil.

Although there are four terminals, you would notice 3 terminals are emitted out, with the starting and running coils connected back-to-back.  Let’s say S1,S2 are the terminals of starting coil and R1,R2 are the terminals of the running coil.  To get 3 terminals out of the fan motor, S2 and R2 are shorted. 

Now, you just see 3 terminals; how do you find which terminal is starting, which one is ending and which one is shorted?  Easy.  Using a simple multimeter, you can find it out.

Let’s say you see terminals A, B, C. Our aim is to find which one of these are S1, R1, S2R2.  Now, let’s measure the resistance between A-B, call it X.  Likewise measure the resistance B-C as Y and A-C as Z.  If you would notice X < Y < Z, you would also notice that Z = Y+X.  Which mean, A-C is action A-B+B-C, that’s why the resistance was additive, also the terminal B is S2R2.  The challenge now is to find what is A and what is C.   As we’d notice A-B=X < B-C=Y, we can confirm that terminal A is starting coil S1 and terminal C is running coil R1.  The reasoning is that; starting coil resistance will be less than running coil resistance.

Hope, this article helped.

  1. Kent Pawar
    November 17th, 2013 at 21:43 | #1

    Excellent explanation! My fan is running in the clockwise direction and I am trying to figure out how to reverse the direction.. unfortunately I do not have a multi-meter to carry out the above test..

  2. Jyothi
    July 18th, 2014 at 11:29 | #2

    I am thinking of installing a BLDC (brushless DC motor) fan called the superfan (manufactured by a Coimbatore based company). The manufacturers claim that it is possible to reduce the power consumption by >50% by using this fan. If this is true than it would be really great. Please follow this link to see the fan:


    I was wondering if you could dissect the BLDC fan like the above excellent work, it would be great. It will help me and others to strip the AC interfacing electronics and connect it directly to a 12V DC battery or a solar PV system (without an inverter).

    warm regards


  3. sudarsun
    July 18th, 2014 at 14:13 | #3


    While checking the site, it seems they offer fans that run of 12/24V DC. So, technically, we don’t need to stripe it down to make it DC operable. Nonetheless, I have made an order to buy the fan. Once I get it I shall make a post on the dissection of the fan and its performance characteristics.

    Sudarsun S.

  4. Jyothi
    July 18th, 2014 at 17:07 | #4

    Very good. Now I will receive some real feedback!!!

    warm regards


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