In order to ascertain whether a dc machine is lap wound or wave wound on the basis of visual observations of the armature, one should observe the

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(a) connections to the brushes.
(b) connections to the commutator.
(c) connections to the field winding.
(d) direction of end connections.

Answer: (d) direction of end connections.

Explanation:

CONCEPT: Armature Windings in DC Machines

  1. The armature winding of a DC machine can be of two main types:
    • Lap winding
    • Wave winding
  2. The fundamental difference between these windings lies in how the coil ends are connected to the commutator segments.

Lap Winding

  1. In lap winding:
    • The end of each coil is connected to an adjacent commutator segment.
  2. The winding appears to lap back on itself.
  3. Number of parallel paths:



(where  = number of poles)
6. Used for:

  • Low voltage
  • High current DC machines

Wave Winding

  1. In wave winding:
    • The coil ends are connected to commutator segments that are widely separated (approximately two pole pitches apart).
  2. The winding progresses in a wave-like series path around the armature.
  3. Number of parallel paths:



  1. Used for:
  • High voltage
  • Low current DC machines

Key Visual Identification

  1. By observing the direction and span of coil-end connections on the commutator, one can clearly identify:
  • Adjacent segments → Lap winding
  • Distant segments → Wave winding

Why Other Options Are Incorrect

(a) Connections to the brushes ❌

  1. Brush arrangement may differ:
  • Lap winding → number of brushes ≈ number of poles
  • Wave winding → usually two brushes
  1. However, brush connections do not define the winding type.

(b) Connections to the commutator ❌ (too general)

  1. Both lap and wave windings connect to the commutator.
  2. The specific pattern of connections, not merely the presence of connections, is what distinguishes them.

(c) Connections to the field winding ❌

  1. Field winding connections (series, shunt, compound) affect machine characteristics, not the armature winding type.

Equalizer Rings (Additional Important Concept)

  1. In lap winding, parallel paths are equal in number to poles.
  2. Ideally, all paths carry equal current.
  3. In practice:
  • Slight air-gap variations
  • Unequal pole flux
    cause unequal induced EMF in parallel paths.
  1. This leads to:
  • Circulating currents
  • Brush overloading
  • Sparking at the commutator

Function of Equalizer Rings

  1. Equalizer rings are:
  • Low-resistance copper conductors
  • Connected between points of equal potential in the armature
  1. They:
  • Provide an alternative path for circulating currents
  • Prevent these currents from flowing through brushes
  1. Result:
  • Uniform current distribution
  • Reduced sparking
  • Improved commutation

FINAL CONCLUSION

  1. Lap and wave windings are distinguished by the direction and span of coil-end connections on the commutator.
  2. Equalizer rings are essential in lap windings to:
  • Avoid unequal current distribution
  • Reduce sparking at brushes

Correct Answer

✅ Option (d): direction of end connections.

✔️ Key One-Line Exam Statement

Lap and wave windings differ in the way coil ends are connected to the commutator; equalizer rings are used in lap windings to prevent circulating currents.

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