In a dc machine, fractional pitch winding is employed to

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(a) increase generated voltage.
(b) reduce sparking.
(c) save copper.
(d) improve cooling.
(e) both (b) and (c).

Answer: (e) both (b) and (c).

Explanation:

CONCEPT: Fractional Pitch (Short-Pitch) Winding

  1. Fractional pitch winding, also called short-pitch winding, is one in which:
    • The coil span is less than one pole pitch
  2. Pole pitch corresponds to:



  1. When the angular distance between the two sides of a coil is less than 180° electrical, the coil is called a fractional pitch coil.
  2. An armature winding made up of such coils is known as a fractional pitch winding.

EXPLANATION

  1. In a full-pitch coil:
    • EMFs induced in the two coil sides are exactly 180° apart
    • Their magnitudes add arithmetically
  2. In a fractional pitch coil:
    • Coil sides are less than 180° apart
    • EMFs add vectorially, giving slightly lower resultant EMF
  3. However, this small reduction in EMF is accepted because of important practical advantages.

Advantages of Fractional Pitch Winding

1. Reduction of Sparking

  1. Fractional pitch winding:
    • Reduces certain harmonic components in the generated EMF
    • Improves the commutation process
  2. This results in:
    • Reduced sparking at brushes and commutator
    • Increased brush and commutator life
    • Lower maintenance requirements

2. Saving of Copper

  1. In short-pitch coils:
    • Coil sides are closer together
    • End connections are shorter
  2. Hence:
    • Less copper is required
    • Machine weight and cost are reduced

3. Improved EMF Waveform

  1. Fractional pitch winding:
    • Helps the generated EMF approximate a sinusoidal waveform
    • Reduces or eliminates undesirable harmonics
  2. This improves overall machine performance.

Why Other Options Are Incorrect

(a) Increase generated voltage ❌

  1. Fractional pitch winding slightly reduces generated EMF, since coil EMFs are not exactly 180° apart.

(d) Improve cooling ❌

  1. Cooling depends mainly on:
    • Ventilation
    • Cooling ducts
    • Fan arrangement
  2. Winding pitch does not directly improve cooling, although reduced copper losses may slightly reduce heating.

FINAL CONCLUSION

  1. Fractional pitch winding is used to:
  • Reduce sparking
  • Save copper
  • Improve EMF waveform
  1. Therefore, the correct answer is:

Correct Answer

✅ Option (e): both (b) and (c).

 

✔️ Key One-Line Exam Statement

Fractional pitch winding reduces sparking, saves copper, and improves the EMF waveform at the cost of a small reduction in generated voltage.

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