Winding factor

In power engineering, winding factor is a technique that is employed to improve the rms generated voltage or electromotive force (EMF) 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 K_d. Another factor that can reduce the winding factor is when the slot pitch is smaller than the pole pitch, called pitch factor K_p.

The winding factor can be calculated as
K_w = K_d * K_p

To calculate the winding factor,
K_w=K_d X K_p ----(1)
Where
K_d=~1 (point here to keep in mind is that all(three phase) motors are balanced systems.
K_p=Cos(a/2) ----(2)
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 (2) gives the K_p which will go in (1) 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 (mech) and 2pi/3 (elec)g Pole pitch =2pi/ 4=pi/2(mech) & pi (elec)

Most of 3phase motors have winding factor values between 0.85 and 0.95.

The winding factor (along with some other factors like winding skew) can help to improve the Harmonic Contents in the generated EMF of machine.

See also

• Electrical machine

References

• Saadat, Hadi. 2004. Power Systems Analysis. 2nd Ed. McGraw Hill. International Edition.
• "Emetor winding editor", December 18, 2010, Emetor AB