DC Machine Troubleshooting Exam Questions
with Correct Answers (Verified Answers) Plus
Rationales 2026 Q&A Instant Download PDF
Question 1
A DC shunt motor is operating normally but suddenly begins
drawing excessive armature current while its speed decreases
significantly. Which condition is the most likely cause?
A. Open armature circuit
B. Mechanical overload
C. Excessive shunt-field resistance
D. Increased field flux
Rationale: A mechanical overload requires the motor to produce
greater electromagnetic torque. To produce this additional
torque, the armature generally draws more current. The
increased armature current causes a larger voltage drop across
the armature resistance, reducing the effective back EMF and
therefore reducing motor speed. An open armature circuit
would stop normal torque production rather than produce
sustained excessive armature current.
,Question 2
What is the primary purpose of the commutator in a
conventional DC machine?
A. To increase the field resistance
B. To reduce bearing friction
C. To mechanically rectify the armature-generated voltage and
maintain unidirectional torque
D. To provide insulation between the field poles
Rationale: The commutator reverses the connections of the
armature coils as they pass through the neutral zone, allowing
the external output of a generator to remain unidirectional and
allowing a motor to develop continuous torque in one direction.
Proper commutation is therefore fundamental to DC-machine
operation. The commutator is not primarily intended to control
field resistance or mechanical friction.
Question 3
A DC motor produces heavy sparking at the brushes
immediately after the brushes have been replaced. What
should be checked first?
A. Bearing lubrication only
B. Cooling-fan rotation
C. Brush seating, brush grade, brush pressure, and neutral-
,plane position
D. Shaft coupling alignment only
Rationale: Newly installed brushes may not properly conform to
the commutator surface, and incorrect brush grade or pressure
can produce poor commutation and excessive sparking.
Incorrect brush position can also cause the coils to be switched
at the wrong point in their electrical cycle. These items should
be inspected before assuming a major internal winding failure.
Question 4
A DC generator has normal field current but produces very little
terminal voltage. Which fault should be investigated first?
A. Excessive bearing grease
B. Loss of residual magnetism or incorrect field connection
C. Oversized shaft
D. High ambient humidity only
Rationale: A self-excited DC generator depends on residual
magnetism to initiate the voltage buildup process. If residual
magnetism has been lost, or if the field winding is connected so
that its magnetic effect opposes the residual flux, voltage may
fail to build up. Field-circuit continuity, polarity, and residual
magnetism are therefore important early troubleshooting
checks.
, Question 5
What is the most likely effect of an open circuit in the shunt
field winding of a DC shunt motor?
A. The motor develops maximum torque
B. The motor may accelerate to dangerously high speed
C. Armature resistance becomes zero
D. The commutator becomes electrically isolated
Rationale: In a shunt motor, field flux is produced by the shunt
field current. If the field circuit opens, flux can fall dramatically.
Because motor speed is approximately related to back EMF
divided by flux, a substantial reduction in flux can cause the
motor speed to rise sharply. This is particularly dangerous in
lightly loaded machines, where overspeed can cause mechanical
damage.
Question 6
Which measurement is most useful for identifying a shorted
turn in a DC armature winding?
A. Measuring the frame temperature only
B. Measuring bearing clearance
C. Performing appropriate armature resistance, growler, or
with Correct Answers (Verified Answers) Plus
Rationales 2026 Q&A Instant Download PDF
Question 1
A DC shunt motor is operating normally but suddenly begins
drawing excessive armature current while its speed decreases
significantly. Which condition is the most likely cause?
A. Open armature circuit
B. Mechanical overload
C. Excessive shunt-field resistance
D. Increased field flux
Rationale: A mechanical overload requires the motor to produce
greater electromagnetic torque. To produce this additional
torque, the armature generally draws more current. The
increased armature current causes a larger voltage drop across
the armature resistance, reducing the effective back EMF and
therefore reducing motor speed. An open armature circuit
would stop normal torque production rather than produce
sustained excessive armature current.
,Question 2
What is the primary purpose of the commutator in a
conventional DC machine?
A. To increase the field resistance
B. To reduce bearing friction
C. To mechanically rectify the armature-generated voltage and
maintain unidirectional torque
D. To provide insulation between the field poles
Rationale: The commutator reverses the connections of the
armature coils as they pass through the neutral zone, allowing
the external output of a generator to remain unidirectional and
allowing a motor to develop continuous torque in one direction.
Proper commutation is therefore fundamental to DC-machine
operation. The commutator is not primarily intended to control
field resistance or mechanical friction.
Question 3
A DC motor produces heavy sparking at the brushes
immediately after the brushes have been replaced. What
should be checked first?
A. Bearing lubrication only
B. Cooling-fan rotation
C. Brush seating, brush grade, brush pressure, and neutral-
,plane position
D. Shaft coupling alignment only
Rationale: Newly installed brushes may not properly conform to
the commutator surface, and incorrect brush grade or pressure
can produce poor commutation and excessive sparking.
Incorrect brush position can also cause the coils to be switched
at the wrong point in their electrical cycle. These items should
be inspected before assuming a major internal winding failure.
Question 4
A DC generator has normal field current but produces very little
terminal voltage. Which fault should be investigated first?
A. Excessive bearing grease
B. Loss of residual magnetism or incorrect field connection
C. Oversized shaft
D. High ambient humidity only
Rationale: A self-excited DC generator depends on residual
magnetism to initiate the voltage buildup process. If residual
magnetism has been lost, or if the field winding is connected so
that its magnetic effect opposes the residual flux, voltage may
fail to build up. Field-circuit continuity, polarity, and residual
magnetism are therefore important early troubleshooting
checks.
, Question 5
What is the most likely effect of an open circuit in the shunt
field winding of a DC shunt motor?
A. The motor develops maximum torque
B. The motor may accelerate to dangerously high speed
C. Armature resistance becomes zero
D. The commutator becomes electrically isolated
Rationale: In a shunt motor, field flux is produced by the shunt
field current. If the field circuit opens, flux can fall dramatically.
Because motor speed is approximately related to back EMF
divided by flux, a substantial reduction in flux can cause the
motor speed to rise sharply. This is particularly dangerous in
lightly loaded machines, where overspeed can cause mechanical
damage.
Question 6
Which measurement is most useful for identifying a shorted
turn in a DC armature winding?
A. Measuring the frame temperature only
B. Measuring bearing clearance
C. Performing appropriate armature resistance, growler, or
