# Coil Span Factor

The **Coil Span Factor** or **Pitch Factor** K_{C} is defined as the ratio of the voltage generated in the short pitch coil to the voltage generated in the full pitch coil. The distance between he two sides of a coil is called the** Coil Span** or **Coil Pitch Factor**. It is also known as **Chording Factor**

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 is always 180 electrical degrees, regardless of the number of poles on the machine. A coil having a span equal to 180⁰ electrical is called a **full pitch coi**l as shown in the figure below.

A Coil having a span less than 180⁰ electrical is called a **short pitch coil** or fractional pitch coil. It is also called a Chorded coil. The short pitch coil factor is shown in the figure below.

A stator winding using fractional pitch coil is called a chorded winding. If the span of the coil is reduced by an angle α electrical degrees, the coil span will be (180 – α) electrical degrees.

In case of a full pitch coil, the distance between the two sides of the coil is exactly equal to the pole pitch of 180⁰ electrical. As a result, the voltage in a full pitch coil is such that the voltage of each side of the coil is in phase.

Let E_{C1} and E_{C2} be the voltages generated in the coil sides, and E_{C} is the resultant coil voltage.

Then the equation is given as shown below.

Since E_{C1} and E_{C2} are in phase, the resultant coil voltage E_{C} is equal to their arithmetic sum.

Therefore,

If the coil span of a single coil is less than the pole pitch of 180⁰ electrical, the voltages generated on each coil side are not in phase. The resultant coil voltage E_{C }is equal to the phasor sum of E_{C1} and E_{C2}

If the coil span is reduced by an angle α electrical degrees, the coil span is (180 – α) electrical degrees. The voltage generated E_{C1} and E_{C2} in the two coil sides will be out of phase with respect to each other by an angle α electrical degrees. The phasor sum of E_{C1} and E_{C2} is E_{C}, which is equal to AC as shown in the phasor diagram above.

The coil span factor is represented as

For full pitch coil, the value of α will be 0⁰, cos α/2 = 1 and **K _{C} = 1.**

For a short pitch coil** K _{C} < 1.**

### Advantages of Short Pitch Coil or Chording

- It shortens the ends of the winding and, therefore, there is a saving in the conductor’s material.
- It Reduces the effects of distorting harmonics and thus the waveform of the generated voltage is improved and making it a sine wave.