(a) results in reduced leakage reactance
(b) results in reduced axial length of the machine
(c) makes it stiffer
(d) all of the above
Answer: (a) results in reduced leakage reactance
Explanation:
CONCEPT: Fractional Pitch (Short-Pitch) Winding
Coil Pitch / Coil Span
Coil Pitch (or Coil Span) is defined as:
👉 The angular distance between the two sides of a coil, expressed in electrical degrees.
It is measured in terms of:
- Slots, or
- Electrical degrees
Types of Coil Pitch
| Type | Coil Span |
|---|---|
| Full Pitch | Equal to pole pitch = 180° electrical |
| Short / Fractional Pitch | Less than pole pitch (< 180° electrical) |
Chording (Short Pitch) Angle
The difference between full pitch and short pitch is called:
👉 Chording angle (ξ)
ξ = 180° − Actual coil span
DEFINITION
When the angular distance between the two sides of a coil is less than one pole pitch, the coil is called a fractional pitch (short-pitch) coil, and the winding is called a fractional pitch winding.
MAIN OBJECTIVE OF FRACTIONAL PITCH WINDING ⭐
Fractional pitch winding is mainly used to:
✔ Reduce harmonics
✔ Improve EMF waveform
✔ Reduce leakage reactance
✔ Save copper
ADVANTAGES OF FRACTIONAL PITCH WINDING ⭐
1. Reduction of Harmonics
- Short-pitching reduces certain harmonic components in EMF.
- Especially reduces:
- 3rd harmonics
- 5th harmonics
- Higher-order harmonics
Result:
👉 Less waveform distortion.
2. Improved EMF Waveform
Due to harmonic reduction, generated EMF becomes:- More sinusoidal
- Smoother
Result:
👉 Better voltage quality.
3. Saving of Copper
- End connections are shorter.
- Coil sides are closer.
- Less copper is required.
Result:
👉 Lower cost and lower copper loss.
4. Reduction in Leakage Reactance
- Shorter end turns reduce leakage flux.
Less leakage flux means:
- Lower leakage reactance
- Better voltage regulation
Result:
👉 Improved machine performance.
5. Reduction in Copper Losses
- Less copper → Less resistance.
- Lower resistance → Less I²R loss.
Result:
👉 Higher efficiency.
6. Reduced Eddy Current and Hysteresis Loss
- Improved flux distribution.
- Less distortion in magnetic field.
Result:
👉 Lower iron losses.
7. Increased Mechanical Strength
- Shorter end turns make winding compact.
- Winding becomes stiffer and stronger.
Result:
👉 Better mechanical stability.
8. Reduced End Turns and Core Length
- Shorter end turns reduce overhang.
- Helps in reducing flux leakage.
- Allows compact design.
Result:
👉 Better magnetic performance.
IMPORTANT NOTE ❗
❌ Fractional pitch winding is NOT mainly used to reduce machine size.
Its main purpose is:
✔ Electrical performance improvement
✔ Harmonic reduction
✔ Efficiency improvement
DISADVANTAGE (Minor) ❌
Slight Reduction in Generated EMF
- In short-pitch coil, EMFs in two sides are not exactly 180° apart.
- Hence, they do not add fully.
Result:
👉 Slight reduction in voltage.
But this is acceptable due to other major benefits.
COMPARISON: FULL PITCH vs FRACTIONAL PITCH
| Feature | Full Pitch | Fractional Pitch |
|---|---|---|
| Coil Span | 180° | < 180° |
| Harmonics | More | Less |
| Copper | More | Less |
| Leakage Reactance | High | Low |
| EMF | Maximum | Slightly Reduced |
| Waveform | Distorted | Better |
USES OF FRACTIONAL PITCH WINDING ⭐
Fractional pitch winding is commonly used in:
✔ Alternators
✔ Induction motors
✔ Synchronous machines
✔ Large AC generators
Where:
- Good waveform
- Low harmonics
- High efficiency
are required.
FINAL CONCLUSION ⭐
Fractional pitch winding is used to reduce harmonics, improve EMF waveform, save copper, reduce leakage reactance, and improve efficiency, making it superior to full-pitch winding in most practical machines.
KEY ONE-LINE EXAM STATEMENT ⭐
Fractional pitch winding is used to reduce harmonics, improve EMF waveform, save copper, and reduce leakage reactance.