DC Motor Construction and Operation — Exam
Questions with Correct Answers
(VerifiedAnswers) Plus Rationales 2026 Q&A
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Question 1
What is the primary function of the armature in a DC motor?
A. To provide mechanical support only
B. To produce the stationary magnetic field
C. To carry current and develop electromagnetic torque
D. To reduce the supply voltage
Rationale: The armature is the rotating electrical part of a DC
motor. Current flows through its conductors while they are
subjected to the magnetic field produced by the field system.
The interaction between armature current and magnetic flux
produces electromagnetic force and consequently torque,
causing the armature to rotate.
Question 2
Which component of a conventional DC motor provides the
stationary magnetic field?
,A. Commutator
B. Armature winding
C. Brushes
D. Field poles and field windings
Rationale: The field poles and their windings establish the main
stationary magnetic field in a conventional DC motor.
Depending on the motor design, the field may be produced by
permanent magnets or electromagnets. In wound-field motors,
current through the field winding creates the required magnetic
flux.
Question 3
What is the main purpose of the commutator in a DC motor?
A. To increase the supply frequency
B. To cool the armature
C. To reverse armature current connections as the armature
rotates
D. To regulate the field resistance
Rationale: The commutator mechanically switches the
connections of the armature coils so that the torque remains in
the same rotational direction. Without commutation, the torque
produced by an armature coil would reverse as its position
,changed relative to the magnetic field, preventing continuous
unidirectional rotation.
Question 4
What material is commonly used for the brushes of a DC
motor?
A. Aluminum only
B. Stainless steel only
C. Carbon or graphite-based materials
D. Plastic
Rationale: Carbon and graphite are widely used for DC motor
brushes because they provide acceptable electrical conductivity
while also having relatively good wear characteristics and
suitable friction against the commutator. Brush material is
selected according to current density, speed, voltage, and
commutation requirements.
Question 5
What is the purpose of the motor frame or yoke?
A. To increase armature resistance
B. To act as the commutator
, C. To provide mechanical support and a magnetic return path
D. To supply current directly to the armature
Rationale: The frame, often called the yoke, serves both
mechanical and magnetic functions. It supports the poles and
other components while providing a low-reluctance path for
magnetic flux. In many conventional DC machines, the frame
also contributes significantly to the overall structural strength of
the motor.
Question 6
Which component physically supports the rotating armature
shaft?
A. Brush holder
B. Pole shoe
C. Commutator segment
D. Bearings
Rationale: Bearings support the motor shaft and allow it to
rotate with minimal friction. They also help maintain accurate
alignment between the rotating armature and stationary
components. Proper bearing condition is essential because worn
or damaged bearings can cause vibration, noise, overheating,
and mechanical losses.
Questions with Correct Answers
(VerifiedAnswers) Plus Rationales 2026 Q&A
Instant Download PDF
Question 1
What is the primary function of the armature in a DC motor?
A. To provide mechanical support only
B. To produce the stationary magnetic field
C. To carry current and develop electromagnetic torque
D. To reduce the supply voltage
Rationale: The armature is the rotating electrical part of a DC
motor. Current flows through its conductors while they are
subjected to the magnetic field produced by the field system.
The interaction between armature current and magnetic flux
produces electromagnetic force and consequently torque,
causing the armature to rotate.
Question 2
Which component of a conventional DC motor provides the
stationary magnetic field?
,A. Commutator
B. Armature winding
C. Brushes
D. Field poles and field windings
Rationale: The field poles and their windings establish the main
stationary magnetic field in a conventional DC motor.
Depending on the motor design, the field may be produced by
permanent magnets or electromagnets. In wound-field motors,
current through the field winding creates the required magnetic
flux.
Question 3
What is the main purpose of the commutator in a DC motor?
A. To increase the supply frequency
B. To cool the armature
C. To reverse armature current connections as the armature
rotates
D. To regulate the field resistance
Rationale: The commutator mechanically switches the
connections of the armature coils so that the torque remains in
the same rotational direction. Without commutation, the torque
produced by an armature coil would reverse as its position
,changed relative to the magnetic field, preventing continuous
unidirectional rotation.
Question 4
What material is commonly used for the brushes of a DC
motor?
A. Aluminum only
B. Stainless steel only
C. Carbon or graphite-based materials
D. Plastic
Rationale: Carbon and graphite are widely used for DC motor
brushes because they provide acceptable electrical conductivity
while also having relatively good wear characteristics and
suitable friction against the commutator. Brush material is
selected according to current density, speed, voltage, and
commutation requirements.
Question 5
What is the purpose of the motor frame or yoke?
A. To increase armature resistance
B. To act as the commutator
, C. To provide mechanical support and a magnetic return path
D. To supply current directly to the armature
Rationale: The frame, often called the yoke, serves both
mechanical and magnetic functions. It supports the poles and
other components while providing a low-reluctance path for
magnetic flux. In many conventional DC machines, the frame
also contributes significantly to the overall structural strength of
the motor.
Question 6
Which component physically supports the rotating armature
shaft?
A. Brush holder
B. Pole shoe
C. Commutator segment
D. Bearings
Rationale: Bearings support the motor shaft and allow it to
rotate with minimal friction. They also help maintain accurate
alignment between the rotating armature and stationary
components. Proper bearing condition is essential because worn
or damaged bearings can cause vibration, noise, overheating,
and mechanical losses.
