A coil when placed in a slot, the distance between two coil sides of a coil is equal to the distance between the center of one pole to the center of an adjacent pole (i.e., pole-pitch) is known as 'Full Pitch'. The coil span of a full pitch coil is always 180° electrical.
Similarly, if the distance between two coils sides of a coil placed in the slot is less than the pole pitch (i.e., less than 180° electrical), it is known as 'Short Pitch Coil or Chorded Coil' as shown in the below figure.
In practice, short-pitch coils are more preferred because of the following advantages,- By using a short pitch coil copper is saved at the end connections.
- The effects of distorting harmonics are less compared to full pitch coil, thus improving the waveform of the induced emf i.e., more sinusoidal.
- The high-frequency harmonics are reduced. Since the hysteresis and eddy current loss depends upon frequency. Hence the losses are decreased, thereby increasing efficiency.
Effect of Short Pitch Coil on Induced EMF :
When the coil is full pitched the net emf induced in the coil is the arithmetic sum of the emfs induced in two coil sides under adjacent poles. The magnitude of the induced emf in the two coil sides is the same without any phase difference as shown below.
But in the case of a short pitch coil, the coil is short pitched by angle α either by one or more slots. Hence the two coil sides of a coil under two adjacent opposite poles are not in identical positions as shown below.
Though the magnitude of the induced emfs in the coil side under two adjacent poles remains the same. There will be time-phase difference between the emfs induced in the two coil sides. Therefore, instead of arithmetic sum, the phasor sum of emfs induced in the coil sides gives the net emf of the coil. Also, the amount of emf induced will be lesser compared to the full-pitched coil as shown in the below phasor diagram.
From the above, we conclude that,- In a full pitch coil, the coil span will be 180° electrical (equal to pole pitch).
- In a short pitch coil, the coil span will be (180 - α)° electrical (α = angle by which coils are short-pitched and always less than pole pitch).
Coil Span Factor or Pitch Factor :
The ratio of net emf induced in the coil when it is short-pitched to the net emf induced in the coil when it is full-pitched is defined as the 'Coil Span Factor or Pitch Factor or Chording Factor'. In general, it is the factor (Kp or Kc) by which reduction in induced emf in the short pitch coil compared to the full-pitch coil.
It can be also defined as the ratio of the vector sum of the emfs induced per-coil to the arithmetic sum of the emfs induced per-coil. It is always less than unity (Kc < 1).
Let,- Es = EMF induced in each coil side.
- α = Angle (electrical degree) by which the coil is short-pitched.
- E = Vector sum of the emfs induced = 2 Es cos α/2
