Understanding Fractional Pitch Winding

Fractional pitch winding, also known as short-pitch winding or chorded winding, is a type of winding used in AC machines where the coil span is less than a full pole pitch. A full pole pitch corresponds to 180 electrical degrees. In a fractional pitch winding, the coil sides are placed in slots that are less than a pole pitch apart.

This winding technique offers several advantages in the design and operation of AC machines:

• Harmonics Reduction: One of the primary reasons for using fractional pitch winding is to reduce or eliminate specific harmonics in the generated voltage waveform (e.m.f.). By appropriately selecting the coil pitch, the pitch factor for certain harmonics can be made zero, effectively cancelling them out or significantly reducing their magnitude. This results in a smoother output voltage closer to a pure sine wave.
• Copper Saving: Since the coil span is less than a full pitch, the length of the conductors forming the coil sides is shorter. This leads to a reduction in the total amount of copper required for the winding compared to a full-pitch winding for the same number of turns.
• Cost Reduction: The reduction in the amount of copper directly translates into a reduction in the material cost of the winding, which contributes to a lower overall cost of the machine.

What Fractional Pitch Winding Cannot Reduce

Let's consider the given options based on the properties of fractional pitch winding:

• Size of the machine: Fractional pitch winding primarily affects the electrical characteristics and the amount of winding material. It does not directly reduce the overall physical size or dimensions of the machine's stator or rotor. The size of the machine is typically determined by factors like its power rating, magnetic loading, and electrical loading, which are not directly reduced by simply chording the winding.
• Harmonics in the generated e.m.f.: As discussed above, fractional pitch winding is a very effective method for reducing harmonics in the output e.m.f.
• Amount of copper in the winding: Fractional pitch winding reduces the required length of conductors, thus reducing the amount of copper used.
• Cost of the machine: By reducing the copper requirement, the cost of the winding material and therefore the overall cost of the machine is reduced.

Based on this analysis, fractional pitch winding can be used to reduce harmonics, copper, and cost, but it cannot be used to reduce the size of the machine.