Winding factor


In power engineering, winding factor is a technique that is employed to improve the rms generated voltage or electromotive force in a three-phase AC electrical machine so that the output voltage and hence torque does not have any harmonics in it which may reduce efficiency. This is because the armature winding of each phase is distributed in a number of slots. Since the EMF induced in different slots are not in phase, their phasor sum is less than their numerical sum. This reduction factor is called distribution factor Kd. Another factor that can reduce the winding factor is when the slot pitch is smaller than the pole pitch, called pitch factor Kp.
The winding factor can be calculated as

Kw = Kd * Kp

To calculate the winding factor,

Kw=Kd X Kp ----

Where

Kd=~1 motors are balanced systems.

Kp=Cos ----

where a is the value of by which the coil is short-pitched

To calculate a, one needs to know and hence calculate the coil pitch and pole pitch of the motor.

Coil pitch is 2pi/number of slots. It needs to be in electrical degrees so the answer has to be divided by 2.

Pole pitch is 2pi/number of poles. Again, divide the answer by 2 to get elec deg.
Then a=pole pitch - coil pitch.

And one can say that the winding is short pitched by a.
Putting the value of a in gives the Kp which will go in to get the winding distribution factor.

Example:

For a 3-phase 6 slot 4 pole non-overlapping winding motor:

Coil pitch=2pi/6 = pi/3 and 2pi/3 g
Pole pitch =2pi/
4=pi/2 & pi

Most of 3phase motors have winding factor values between 0.85 and 0.95.
The winding factor can help to improve the Harmonic Contents in the generated EMF of machine.