A. Commutation problem
B. Overloading
C. Regulation
D. None of the above
Answer: A. Commutation problem
Explanation:
CONCEPT: DC Machine and Commutation
A DC machine (generator or motor) uses a commutator and brushes.
The commutator converts the internally generated AC in the armature into DC at the terminals.
Proper commutation requires:
Adequate insulation between commutator segments
Spark-free current reversal under the brushes
Why High Voltage Is a Problem
Commutation Problem
To generate or handle high DC voltage:
A large number of commutator segments is required
Each segment must be insulated (usually by mica)
This leads to:
Very large and heavy commutator
Increased mechanical stress
Excessive sparking and arcing
Rapid brush and commutator wear
As voltage increases, sparkless commutation becomes extremely difficult, making the machine:
Bulky
Unreliable
High-maintenance
Why Other Options Are Incorrect
B. Overloading ❌
Overloading can occur in many machines but is not the primary reason DC machines cannot be used for high voltage.
Overloading can be managed by design and protection.
C. Regulation ❌
Voltage regulation is a performance issue, not a fundamental limitation.
It does not prevent high-voltage operation by itself.
D. None of the above ❌
Since commutation problem is a definite and well-known limitation, this option is incorrect.
FINAL ANSWER
✅ Correct option: A — Commutation problem
Additional Exam Notes: Applications of DC Motors
DC Series Motor
Very high starting torque
Used in:
Electric traction (railways)
Cranes and hoists
Rolling mills
Mine hoists
DC Shunt Motor
Almost constant speed
Used in:
Lathes
Fans and pumps
Machine tools
Printing presses
DC Cumulative Compound Motor
High starting torque + fairly constant speed
Used in:
Elevators
Compressors
Conveyors
Door lifts
DC Differentially Compound Motor
Low starting torque
Dangerously high speed at no load
Hence, rarely used
Important Note
Applications of DC machines are limited mainly due to commutation problems, especially at:
High voltage
High speed
Large ratings
✔️ Key One-Line Exam Statement
DC machines are unsuitable for high-voltage applications mainly due to commutation problems associated with the commutator and brushes.