Short circuiting is caused in the armature winding due to

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(a) failure of insulation between two turns of a coil.
(b) failure of insulation between two commutator bars.
(c) grounding of two or more turns of the same coil.
(d) any of the above.

Answer: (d) any of the above.

Explanation:

CONCEPT: Armature Winding in DC Machines

  1. The armature winding consists of insulated copper coils placed in slots on the armature core.
  2. It carries current and produces a magnetic field that interacts with the field poles to develop torque.
  3. Proper insulation is essential to:
    • Maintain the designed current path
    • Prevent abnormal current flow
    • Avoid overheating and damage

EXPLANATION: Causes of Short Circuit

1. Failure of Insulation Between Two Turns

  1. When insulation between adjacent turns breaks down:
    • Current flows through a shorter path
    • Some turns are bypassed
  2. This results in an internal short circuit within the coil.

2. Failure of Insulation Between Commutator Bars

  1. Each commutator bar is connected to an armature coil.
  2. If insulation between bars fails:
    • Current bypasses coil sections
    • Adjacent coils become short-circuited
  3. This produces severe sparking and heating.

3. Grounding of Two or More Turns

  1. If insulation fails at two points and both touch the armature core or shaft:
    • A path is formed through the ground
    • Current flows abnormally
  2. This also results in a short circuit.

Why Other Options Are Incorrect

(a), (b), and (c) Individually ❌

  1. Each option is a valid cause of short circuit.
  2. However, taken alone, each represents only one possible cause.
  3. The question asks for all possible causes, so individual options are incomplete.

FINAL CONCLUSION

  1. A short circuit in armature winding can be caused by:
  • Turn-to-turn insulation failure
  • Commutator bar insulation failure
  • Grounding of turns
  1. Therefore, the correct answer is:

Correct Answer

✅ Option (d): All of the above

Additional Important Definitions

Turn

  1. Two conductors joined at one end by an end connector form a turn.

Coil

  1. Several turns connected in series form a coil.

Armature Winding

  1. An armature winding is a set of insulated copper coils placed in armature slots that:
  • Carry current
  • Produce magnetic field
  • Enable torque generation

✔️ Key One-Line Exam Statement

Short circuits in DC armature windings are caused by insulation failure between turns, between commutator bars, or by grounding of coils.

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