(a) 73
(b) 72
(c) 71
(d) 70
Answer: (b) 72
Explanation:
CONCEPT: Commutator in a DC Machine
- A commutator is
a mechanical rectifier used in DC machines.
- Its
main functions are:
- To collect
current from the armature conductors
- To convert
the internally induced AC in the armature into unidirectional
(DC) current in the external circuit
- The
commutator is:
- Cylindrical
in shape
- Made
of wedge-shaped, high-conductivity copper segments
- Segments
are insulated from each other by mica
- Each
commutator segment is connected to the armature winding using:
- Copper
lugs / risers
Relationship Between Coils, Slots, and Segments
- In a
DC armature:
- Each
armature coil corresponds to one commutator segment
- Therefore:
- This
relationship is valid:
- For lap
winding
- For wave
winding
- For any
number of poles
Given Data
- Number
of armature slots = 72
- In
standard DC machines:
- Number
of armature coils = Number of slots
- Hence:
Explanation
- Since:
- Number
of commutator segments = Number of armature coils
- And:
- Number
of armature coils = 72
- The number
of poles (6) does not affect the number of
commutator segments.
- Poles
influence:
- EMF
- Parallel
paths
- Torque
- Speed
but not the number of commutator segments.
FINAL CONCLUSION
- The
number of commutator segments required is:
- Hence, if
72 coils (or slots) are used, the DC machine must have 72
commutator segments.
✔️ Key One-Line Exam Statement
In a DC machine, the number of commutator segments is
always equal to the number of armature coils (or slots).