Coil Pitch or Pole Pitch and Coil Span Factor or Pitch factor

COIL PITCH : The distance between the two sides of a coil is called the coil span or coil pitch.

POLE PITCH: The angular distance between the central line of one pole to the central line of the next pole is called Pole Pitch.

A pole pitch always 180 electrical degrees regardless of the number of poles on the machine.

Full Pitch Coil: A coil having a span equal to 180 electrical degree is called a full pitch coil as show in fig. 

In case of full pitch coil, the two coil sides span a distance exactly equal to the pole pitch of 180 electrical degrees.As result , the voltage generated in full pitch coil is such that the coil side voltages are in phase as show in fig-1. Let Ec1 and Ec2 be the voltages generated in the coil sides and Ec the resultant coil voltage.

           Ec = Ec1 + Ec2

           Ec1 = Ec2 = E     (Say )

Since Ec1 and Ec2 are in phase , the resultant coil voltage Ec is equal to their arithmetic sum.

           

           Ec = Ec1 + Ec2 = 2E

Short Pitch Coil: A coil having a span less than 180 electrical degree is called Short pitch coil or frictional pitch coil. It is also called chorded coil.

A stator winding using frictional pitch coil is called chorded winding. if the span of coil is reduced by an angle α electrical degrees, the coil span will be 

(180-α) electrical degree show in fig-2 

If the coil span of single coil is less than the pole pitch of 180 (elect. degree), the voltage generated in each coil side are not in phase . the resultant Coil voltage Ec  is equal to Phasor sum of  Ec1 and Ec2.

If the coil span is reduced by an angle α electrical degrees , the coil span is (180-α) electrical degrees. the voltage generated Ec1 and Ec2 in the two sides will be out of phase with respect to each other by an angle α electrical degrees as show in fig-2 . the phasor sum of  Ec1 and Ec2 is Ec (=AC). 

Coil Span Factor or Pitch factor: K_c is the defined as the ratio of the voltage generated in short pitch coil to the voltage generated in full pitch coil. the coil span factor is also called chording Factor. 

For full pitch coil, α = 0  cos( α/2 ) = 1 and Kc =1 .  for a short pitch coil Kc < 1.

Advantage of short pitching or chording:

1. Shortens the ends of the winding and therefore there is asaving inthe conductor material.
2. Reduces effects of distorting harmonics , and thus the waveform of the generated voltage is improved and making it approach a sine wave.