# Identifying windings of a single phase induction motor

How can I identify the main and auxiliary windings of a single phase induction motor which has only three terminals (1.Red, 2. Yellow & 3. Black) out of the device? How to connect capacitor and power supply to it?

• How do you know it is what you think it is? May 17, 2015 at 17:46
• Also, how would you know the correct value capacitor should it be what you think it is? May 17, 2015 at 20:25

Tricky solution:

Measure the winding resistances. Auxiliary windings are often thinner and therefore have a higher resistance (even if it's just a little bit higher).

Once you know which terminals are used for each of the two windings, you can connect them to the power supply and the capacitor as shown in the following picture:

The picture was taken from the book "R. Fischer: Elektrische Maschinen, 15.Auflage, Hanser Verlag München"

Cleaner solution

Apply a (small) DC voltage step to a test circuit, consisting of a resistor and one of the windings in serial combination. Capture the current with an oscilloscope. Measure the time constant of the system and calculate the inductance L of the winding. Repeat this process for the second winding. The winding with the higher inductance value should be the main winding.

• while the auxiliary winding may be thinner there are fewer windings. So is this an accurate method of measurement? May 17, 2015 at 18:09
• I can't confirm that this is a completely accurate method, but it was the first thing that came to my mind when I saw the question. Nevertheless, although there will be fewer windings the auxiliary winding should have a noticeably higher resistance. According to the things I learned (and also according to the book) the difference should be enough. Still it's possible that I'm wrong. May 17, 2015 at 18:13
• I guess the other (more expensive) test could be to use an LCR meter and measure the inductance of the two windings. May 17, 2015 at 18:15
• I think that this would be the cleanest solution, but therefore one needs to have a LCR meter. May 17, 2015 at 18:17
• One other method would be to have a look at the (current) step response of each winding + a resistor. The time constant could be used to calculate the inductance L of each winding. However, therefore one needs to have an oscilloscope. May 17, 2015 at 18:20
1. If the direction of rotaion of motor is shown or known then ther is a solution except exact value of capactitor ( however the norms says.. loking at shaft end motor should generally rotate in clock-wise direction. I am not sure of your motor).
2. connect a capacitor as suggested by Simon and connect supply between common terminal and capacitor ( winding) termnal and if motor runs in opposite direction then the shift the connection to other treminal of capacitor . Now the motor will rotate in the specified direction. the motor terminal which is connected directly to supply is main winding and the one connected through capacitor is aux. winding. vtingole

first of all identify common terminal by checking resistance. and connect the capacitor b/w main and aux terminal. and connect the terminal of common and main or aux as per required direction of rotation.

The aux winding should have the greatest resistance as this is what puts it out of phase with main winding to give an apparent rotating magnetic field. This is achieved by using thinner wire or alternately resistance wire.

• Welcome to EE.SE. "The aux winding should have the greatest resistance as this is what puts it out of phase with main winding ...". This is incorrect. It is the combination of auxiliary winding capacitance, inductance and resistance that causes the phase-shift. See the answer that was accepted six months ago. Nov 10, 2017 at 22:58