The dummy coils in dc machines are useful to

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(a) increase the efficiency.
(b) improve the commutation.
(c) reduce the cost of the machine.
(d) maintain mechanical balance of armature.

Answer: (d) maintain mechanical balance of armature.

Explanation:

CONCEPT: Dummy Coils

  1. Dummy coils are used mainly in wave winding of DC machine armatures.
  2. Wave winding requires a specific number of coils/conductors to satisfy winding pitch conditions.
  3. Sometimes, the available armature slots (from standard armature punchings) do not exactly match the number required for proper wave winding.

Why Dummy Coils Are Needed

  1. When the number of available slots is more than required:
    • Some slots cannot be used for active armature winding.
  2. Leaving slots empty would cause:
    • Uneven mass distribution
    • Mechanical unbalance
    • Vibrations during rotation
  3. To avoid this, dummy coils are placed in the unused slots.

Construction of Dummy Coils

  1. Dummy coils are:
    • Exactly similar to normal armature coils in shape and size
    • But their ends are cut short and taped
  2. They are not connected to the commutator bars.
  3. Hence, no current flows through dummy coils.

Effect on Machine Performance

  1. Since dummy coils are not electrically connected:
    • They do not affect EMF
    • They do not affect current
    • They do not affect efficiency
  2. Their sole purpose is to ensure mechanical balance of the armature.

Why Other Options Are Incorrect

(a) Increase efficiency ❌

  1. Dummy coils carry no current and do not influence electrical output.
  2. Hence, they do not increase efficiency.

(b) Improve commutation ❌

  1. Commutation depends on:
    • Brush position
    • Interpoles
    • Armature reaction
  2. Dummy coils have no role in commutation.

(c) Reduce cost ❌

  1. Dummy coils require:
    • Extra copper
    • Additional labor
  2. Hence, they increase, not reduce, manufacturing cost.

FINAL CONCLUSION

  1. Dummy coils are used to:
    • Fill unused slots
    • Ensure uniform mass distribution
    • Prevent vibration and bearing wear
  2. Therefore, their purpose is purely mechanical, not electrical.

Correct Answer

✅ Option (d): Maintain mechanical balance of the armature

✔️ Key One-Line Exam Statement

Dummy coils are used in DC machines to maintain mechanical balance of the armature without affecting electrical performance.

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