Commutation and Brushes Exam Questions
with Correct Answers (VerifiedAnswers) Plus
Rationales 2026 Q&A Instant Download PDF
Practice Exam
1. What is the primary purpose of commutation in a DC
machine?
A. To increase the mechanical speed of the rotor
B. To convert AC induced in the armature conductors into
unidirectional current at the external circuit
C. To increase the field winding resistance
D. To reduce the magnetic flux to zero
Correct answer: B. To convert AC induced in the armature
conductors into unidirectional current at the external circuit
Rationale: In a DC machine, the emf induced in individual
armature conductors is inherently alternating as the conductors
move under successive magnetic poles. The commutator and
brushes mechanically switch the connections of the armature
coils so that the current delivered to the external circuit remains
unidirectional. Effective commutation is therefore essential for
obtaining useful DC output from a generator and maintaining
the proper current reversal in the armature of a motor.
,2. Which component performs the mechanical switching
required for commutation in a conventional DC machine?
A. Field pole
B. Armature core
C. Commutator
D. Yoke
Correct answer: C. Commutator
Rationale: The commutator consists of insulated copper
segments connected to the armature coils. As the armature
rotates, the brushes maintain contact with these segments and
progressively switch the connections of the coils. This
mechanical switching reverses the current in the appropriate
coil at approximately the correct position, allowing the external
current or torque-producing current to remain properly
directed.
3. What is the main function of brushes in a DC machine?
A. To produce the main magnetic field
B. To provide electrical contact between the stationary circuit
and rotating commutator
C. To cool the armature
D. To support the rotor mechanically
,Correct answer: B. To provide electrical contact between the
stationary circuit and rotating commutator
Rationale: Brushes form the electrical interface between the
stationary external circuit and the rotating armature. They are
normally made from carbon, graphite, or related materials and
press against the commutator surface. Besides carrying current,
they must maintain reliable contact while allowing the
commutator to rotate, making brush condition, pressure,
alignment, and surface condition important factors in machine
performance.
4. Which material is most commonly associated with
conventional DC machine brushes?
A. Pure steel
B. Carbon or graphite
C. Glass
D. Aluminum oxide
Correct answer: B. Carbon or graphite
Rationale: Carbon and graphite are widely used because they
provide a useful combination of electrical conductivity, relatively
low friction, self-lubricating characteristics, and acceptable
wear behavior. Brush composition can be selected according to
machine voltage, current, speed, and commutation
, requirements. Some applications use electrographitic, resin-
bonded, copper-graphite, or other specialized brush grades.
5. What is sparking at the brushes usually an indication of?
A. Perfect commutation
B. A possible commutation or contact problem
C. Normal operation under every condition
D. Complete loss of magnetic flux
Correct answer: B. A possible commutation or contact
problem
Rationale: Small amounts of visible sparking may occur under
some operating conditions, but excessive or persistent sparking
is generally undesirable. It can result from incorrect brush
position, poor brush contact, excessive brush pressure, a
damaged or contaminated commutator, overload, armature
reaction, open or shorted coils, or unsuitable brush grade.
Prolonged sparking can damage both brushes and commutator
segments.
6. What is meant by commutation of an armature coil?
A. Increasing the coil's resistance permanently
B. Reversing the current direction in the coil at the appropriate
time
with Correct Answers (VerifiedAnswers) Plus
Rationales 2026 Q&A Instant Download PDF
Practice Exam
1. What is the primary purpose of commutation in a DC
machine?
A. To increase the mechanical speed of the rotor
B. To convert AC induced in the armature conductors into
unidirectional current at the external circuit
C. To increase the field winding resistance
D. To reduce the magnetic flux to zero
Correct answer: B. To convert AC induced in the armature
conductors into unidirectional current at the external circuit
Rationale: In a DC machine, the emf induced in individual
armature conductors is inherently alternating as the conductors
move under successive magnetic poles. The commutator and
brushes mechanically switch the connections of the armature
coils so that the current delivered to the external circuit remains
unidirectional. Effective commutation is therefore essential for
obtaining useful DC output from a generator and maintaining
the proper current reversal in the armature of a motor.
,2. Which component performs the mechanical switching
required for commutation in a conventional DC machine?
A. Field pole
B. Armature core
C. Commutator
D. Yoke
Correct answer: C. Commutator
Rationale: The commutator consists of insulated copper
segments connected to the armature coils. As the armature
rotates, the brushes maintain contact with these segments and
progressively switch the connections of the coils. This
mechanical switching reverses the current in the appropriate
coil at approximately the correct position, allowing the external
current or torque-producing current to remain properly
directed.
3. What is the main function of brushes in a DC machine?
A. To produce the main magnetic field
B. To provide electrical contact between the stationary circuit
and rotating commutator
C. To cool the armature
D. To support the rotor mechanically
,Correct answer: B. To provide electrical contact between the
stationary circuit and rotating commutator
Rationale: Brushes form the electrical interface between the
stationary external circuit and the rotating armature. They are
normally made from carbon, graphite, or related materials and
press against the commutator surface. Besides carrying current,
they must maintain reliable contact while allowing the
commutator to rotate, making brush condition, pressure,
alignment, and surface condition important factors in machine
performance.
4. Which material is most commonly associated with
conventional DC machine brushes?
A. Pure steel
B. Carbon or graphite
C. Glass
D. Aluminum oxide
Correct answer: B. Carbon or graphite
Rationale: Carbon and graphite are widely used because they
provide a useful combination of electrical conductivity, relatively
low friction, self-lubricating characteristics, and acceptable
wear behavior. Brush composition can be selected according to
machine voltage, current, speed, and commutation
, requirements. Some applications use electrographitic, resin-
bonded, copper-graphite, or other specialized brush grades.
5. What is sparking at the brushes usually an indication of?
A. Perfect commutation
B. A possible commutation or contact problem
C. Normal operation under every condition
D. Complete loss of magnetic flux
Correct answer: B. A possible commutation or contact
problem
Rationale: Small amounts of visible sparking may occur under
some operating conditions, but excessive or persistent sparking
is generally undesirable. It can result from incorrect brush
position, poor brush contact, excessive brush pressure, a
damaged or contaminated commutator, overload, armature
reaction, open or shorted coils, or unsuitable brush grade.
Prolonged sparking can damage both brushes and commutator
segments.
6. What is meant by commutation of an armature coil?
A. Increasing the coil's resistance permanently
B. Reversing the current direction in the coil at the appropriate
time
