(a) is costlier.
(b) is of low resistivity.
(c) is of lower thermal conductivity.
(d) requires large winding space.
Answer: (d) requires large winding space.
Explanation:
CONCEPT: Armature Winding Material
- The
armature winding of a DC machine carries high current.
- Therefore,
the winding material must have:
- Low
electrical resistivity
- Adequate
mechanical strength
- Good
commutation characteristics
- Copper is
commonly used as the armature winding material.
EXPLANATION
- Aluminium
has a higher electrical resistivity than copper.
- For
the same current rating:
- Aluminium
requires a larger cross-sectional area than copper.
- This
leads to:
- Occupation
of more space in armature slots
- Difficulty
in accommodating windings
- Reduced
space for insulation and cooling
- Hence,
aluminium conductors require large winding space, making them
unsuitable for DC armatures.
Additional Practical Issues
- Aluminium
also creates jointing and soldering problems:
- Aluminium
oxide forms on the surface
- Makes
reliable electrical joints difficult
- These
issues further discourage its use in DC armature windings.
Why Other Options Are Incorrect
(a) Is costlier ❌
- Aluminium
is generally cheaper than copper.
(b) Is of low resistivity ❌
- Aluminium
has higher resistivity, not lower, compared to copper.
(c) Has lower thermal conductivity ❌
- Aluminium
actually has good thermal conductivity, often comparable or
better than copper.
- However,
higher resistance causes more heat generation, offsetting this advantage.
FINAL CONCLUSION
- Aluminium
is not used as armature winding wire because:
- Therefore,
the correct answer is:
Correct Answer
✅ Option (d): Requires
large winding space
✔️ Key One-Line Exam Statement
Aluminium is not used for DC armature windings because
its higher resistivity requires larger conductor size, occupying excessive slot
space and causing jointing problems.