Distributed Winding in Alternator Explained

In electrical machines like alternators, the arrangement of coils in the stator slots plays a crucial role in determining the quality of the generated electromotive force (EMF). One such arrangement is the distributed winding, which is commonly employed in alternators for various benefits, especially regarding the waveform of the induced EMF.

Distributed Winding Configuration

A distributed winding is an arrangement where the coils of each phase are not placed in a single slot (as in concentrated winding) but are spread out over several adjacent slots under each pole. This spreading out of the coils is done to achieve specific performance characteristics in the alternator.

• For a multi-phase machine like an alternator, each phase winding is divided into several coil groups.
• These coil groups are then placed in different slots, distributed over a certain electrical angle under each pole.

Harmonics in Alternator Armature EMF

The voltage or electromotive force (EMF) induced in the armature winding of an alternator is ideally expected to be a pure sinusoidal waveform. However, due to various factors such as the non-sinusoidal distribution of the magnetic flux in the air gap and the slotting effects, the actual induced EMF often contains unwanted components called harmonics. These harmonics are integer multiples of the fundamental frequency (e.g., 3rd harmonic, 5th harmonic, 7th harmonic, etc.).

The presence of harmonics in the armature EMF is undesirable because it can lead to:

• Distortion of the output voltage waveform.
• Increased losses in the alternator and connected equipment.
• Radio frequency interference (RFI) and electromagnetic interference (EMI).
• Unnecessary heating and reduced efficiency.

Distributed Winding and Harmonics Reduction

The primary advantage of using a distributed winding in an alternator is its effectiveness in reducing harmonics from armature EMF. This reduction is achieved through the principle of the "distribution factor" or "breadth factor" (\(K_d\)).

When coils are distributed over several slots, the EMFs induced in these coils are slightly out of phase with each other. The resultant EMF of the entire phase winding is the phasor sum of these individual coil EMFs. For the fundamental frequency, these EMFs add up almost in phase, leading to a large resultant voltage.

However, for harmonic frequencies, the phase displacement between the EMFs induced in individual coils becomes much larger. This larger phase displacement causes the phasor sum of the harmonic EMFs to be significantly smaller, effectively canceling out or greatly reducing the amplitudes of these harmonic components.

The distribution factor for the \(n\)-th harmonic is given by:

\(K_{dn} = \frac{\sin\left(\frac{m n \beta}{2}\right)}{m \sin\left(\frac{n \beta}{2}\right)}\)

Where:

• \(m\) is the number of slots per pole per phase.
• \(n\) is the order of the harmonic (e.g., 1 for fundamental, 3 for third harmonic).
• \(\beta\) is the angular displacement between adjacent slots in electrical degrees, calculated as \(\beta = \frac{180^\circ}{\text{slots per pole}}\).

For the fundamental frequency (\(n=1\)), \(K_{d1}\) is close to 1 but slightly less than 1. For higher harmonics (\(n>1\)), \(K_{dn}\) becomes significantly smaller than 1, and for certain harmonics, it can even be zero, meaning those harmonics are completely eliminated.

This inherent property of distributed winding makes it a very effective method for suppressing unwanted harmonic components in the generated EMF, leading to a nearly sinusoidal output voltage waveform.

Benefits of Harmonics Reduction by Distributed Winding

By effectively reducing harmonics from armature EMF, distributed windings contribute to several operational advantages for the alternator:

• Improved Waveform Quality: The output voltage waveform becomes closer to a pure sine wave, which is ideal for most electrical loads.
• Lower Eddy Current Losses: Reduced harmonics lead to less fluctuating magnetic fields, thereby reducing eddy current losses in the core.
• Reduced Noise and Vibration: Harmonic currents can cause mechanical vibrations and acoustic noise. Their reduction leads to quieter operation.
• Increased Efficiency: With reduced losses, the overall efficiency of the alternator improves.
• Better Voltage Regulation: A smoother EMF waveform helps in maintaining stable voltage output under varying load conditions.

Alternator Winding: A Summary on Harmonics

In summary, the strategic placement of coils in a distributed winding configuration in an alternator is a fundamental design choice specifically engineered to reduce harmonics from armature EMF. This ensures a higher quality, nearly sinusoidal output voltage, which is essential for efficient and reliable power generation.