Chording, also known as short pitching, is a technique used in AC machine windings to improve the waveform of the induced voltage by reducing or eliminating specific harmonics. Instead of having the coil span $180^\circ$ (full pitch), it spans a smaller angle, say $\alpha$ degrees less than full pitch.

Eliminating Harmonics with Chording

The presence of harmonics in the induced voltage waveform can cause various issues like increased losses, noise, and waveform distortion. Chording helps to filter out these unwanted harmonics.

The chording factor ($k_p$) for the $n$-th harmonic is given by the formula:

$\text{k}_{\text{pn}} = \cos\left(\frac{\text{n}\alpha}{2}\right)$

where:

• $\text{n}$ is the harmonic number (e.g., 3 for third harmonic, 5 for fifth harmonic)
• $\alpha$ is the chording angle in electrical degrees (the angle by which the coil span is less than $180^\circ$)

To completely eliminate a specific harmonic $n$, the chording factor for that harmonic must be zero. This happens when $\cos\left(\frac{\text{n}\alpha}{2}\right) = 0$.

The first angle for which the cosine is zero is $90^\circ$. So, we set:

$\frac{\text{n}\alpha}{2} = 90^\circ$

Calculating Chording Angle for Fifth Harmonic

The question asks for the chording angle required to eliminate the fifth harmonic. This means the harmonic number we want to eliminate is $n = 5$.

Using the condition for harmonic elimination, $\frac{\text{n}\alpha}{2} = 90^\circ$, and substituting $n=5$, we get:

$\frac{5\alpha}{2} = 90^\circ$

Now, we solve for $\alpha$:

$5\alpha = 90^\circ \times 2$

$5\alpha = 180^\circ$

$\alpha = \frac{180^\circ}{5}$

$\alpha = 36^\circ$

Therefore, a chording angle of $36^\circ$ is required to eliminate the fifth harmonic from the induced voltage waveform.