A. Reduces power loss in armature
B. Reduces power loss in field circuit
C. Converts the induced AC armature voltage into direct voltage
D. It is not necessary
Answer: C. Converts the induced AC armature voltage into direct voltage
Explanation:
CONCEPT: Commutator
A commutator is a mechanical switching device used in DC machines.
It consists of:
A set of copper segments
Insulated from each other (usually by mica)
Each commutator segment is connected to an armature coil.
The commutator is mounted on and keyed to the rotor shaft.
Electrical connection to the armature is made via a brush–commutator arrangement.
Role of the Commutator
In a DC Generator
The EMF induced in the armature conductors is alternating (AC) in nature.
The commutator acts as a mechanical rectifier.
It:
Reverses the connections of the armature coils at the proper instant
Converts the internally generated AC into unidirectional (DC) voltage at the terminals
Hence, the external circuit receives DC output.
In a DC Motor
The commutator:
Supplies current to the armature conductors
Reverses current direction in the coils every half-turn
This ensures that the electromagnetic torque remains unidirectional, allowing continuous rotation.
Why Other Options Are Incorrect
A. Reduces power loss in armature ❌
Armature losses are reduced by:
Laminations (eddy current loss reduction)
The commutator actually introduces:
Brush friction losses
Sparking losses
B. Reduces power loss in field circuit ❌
Field circuit losses depend on:
Field winding resistance
Excitation method
The commutator has no role in reducing field losses.
D. It is not necessary ❌
Without a commutator (or electronic rectification):
A DC generator would deliver AC
A DC motor would not maintain unidirectional torque
Hence, the commutator is essential for DC machines.
FINAL ANSWER
✅ Correct option: C
Converts the induced AC armature voltage into direct voltage
Important Exam Notes
Due to commutator limitations:
DC generators are not usually designed beyond ~650 V
Excessive voltage leads to:
Sparking
Commutation problems
Mechanical wear
DC Machine Construction (Quick Revision)
Main parts:
Stator and Rotor
Yoke
Outer frame; provides mechanical strength and magnetic path.
Poles & Pole Shoes
Carry field windings and spread flux uniformly in the air gap.
Field Winding
Copper coils producing alternate N and S poles.
Armature Core
Laminated silicon steel core to reduce eddy current losses.
Slots house the armature winding.
Armature Winding
Lap or wave winding; double-layer windings commonly used.
Brushes
Made of carbon/graphite; collect or supply current via the commutator.
✔️ Key One-Line Exam Statement
The commutator converts the internally generated AC in the armature into DC at the terminals of a DC machine.