Pitch Factor or Coil Span Factor in Alternator



Coil span or coil pitch is defined as the distance between the two sides of a coil.

Pole pitch is defined as the angular distance between the central line of one pole to the central line of the next pole. A pole pitch is always 180° electrical regardless of the number of poles on the machine.

  • When a coil is having a span of 180° electrical, it is called as a full-pitch coil.
  • When the coil is having a span less than 180° electrical, it is known as a short-pitch coil or fractional-pitch coil.

The fractional-pitch coil is also known as chorded coil. A stator winding using fractional pitch coil is called as a chorded winding.

If the span of the coil is reduced by an angle of electrical, then the coil span will be (180 − α) electrical degrees.

In case of a full-pitch coil, the two coil sides span a distance exactly equal to the pole pitch of 180° electrical. Consequently, the voltage generated in a full pitch coil is such that the coil-side voltages are in phase.

Let EC1 and EC2 are the voltages generated in the coil sides and EC is the resultant coil voltage. Then,

$$\mathrm{E_{C} \:=\: E_{C1} \:+\: E_{C2}}$$

Pitch Factor or Coil Span Factor in Alternator1

Since EC1 and EC2 are in phase, the resultant coil voltage is equal to their arithmetic sum (Refer the figure of full-pitch coil).

$$\mathrm{∴ \: E_{C} \:=\: E_{C1} \:+\: E_{C2}}$$

If the coil span of a single coil is less than the pole pitch of 180° electrical, i.e., the case of short-pitch coil, the voltages generated in each coil side are not in phase. The resultant coil voltage $E_{C}$ is equal to the phasor sum of EC1 and EC2.

Pitch Factor or Coil Span Factor in Alternator2

If the coil span is decreased by an angle of electrical, the coil span is (180 − α) electrical degree. The voltages generated EC1 and EC2 in the two coil sides will be out of phase with respect to each other by an angle of a° electrical. The phasor sum of EC1 and EC2 is EC = AC (Refer the figure of short pitch coil).

The coil span-factor or pitch factor is defined as the ratio of the voltage generated in the short-pitch coil to the voltage generated in the full-pitch coil. The coil span factor is also known as chording factor.

$$\mathrm{\text{Coil Span Factor, } \:k_{C} \:=\:\frac{\text{Voltage generated with short - pitch coil}}{\text{Voltage generated with full pitch coil}}}$$

Refer the phasor diagram,

$$\mathrm{k_{C} \:=\: \frac{\text{Phasor sum of voltages of two coil sides}}{\text{Arithmetic sum of voltages of two coil sides}} \:=\: \frac{AC}{2AB}}$$

$$\mathrm{\Rightarrow \: k_{C} \:=\: \frac{2AD}{2AB} \:=\: \cos\left(\frac{\alpha}{2}\right)}$$

For a full pitch coil,

$$\mathrm{\alpha \:=\: 0°;\:\: \cos \frac{\alpha}{2} \:=\: 1;\:\: or \:\: k_{C} \:=\: 1}$$

For a short pitch coil,

$$\mathrm{k_{C} \:\lt\: 1}$$

Advantages of Chording

The advantages of the chorded winding are as follows −

  • There is a saving in the conductor material because the chording shortens the ends of the winding.
  • The chording or short-pitching reduces the effects of distorting harmonics and hence the waveform of the generated voltage is improved and making it approach a sine wave.
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