BCIT Electrical DC Machines Level 3 Exam
Questions with Correct Answers
(VerifiedAnswers) plus Rationales 2026 Q&A
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1. What is the primary function of the commutator in a DC
machine?
A. To increase the magnetic flux
B. To reduce armature resistance
C. To mechanically rectify the armature voltage/current
D. To control field current
Rationale: In a DC generator, the voltage induced in the rotating
armature conductors is inherently alternating. The commutator
and brushes mechanically rectify this internally generated AC
into a unidirectional DC output at the terminals. In a DC motor,
the commutator performs the complementary function of
reversing current in the armature conductors at the appropriate
point so that electromagnetic torque remains in the same
rotational direction.
,2. Which component of a DC machine carries the armature
conductors?
A. Yoke
B. Pole shoe
C. Field frame
D. Armature core
Rationale: The armature core is the rotating magnetic structure
that contains slots into which the armature conductors are
placed. It is normally constructed from laminated electrical steel
to reduce eddy-current losses. The yoke and pole structure
establish and support the magnetic circuit, but they do not
normally contain the rotating armature winding.
3. Why is the armature core of a DC machine laminated?
A. To increase mechanical strength
B. To increase brush pressure
C. To reduce eddy-current losses
D. To increase field current
Rationale: Changing magnetic flux induces circulating currents
in conductive iron. These eddy currents cause unwanted heating
and power loss. Laminating the armature core into thin
electrically insulated sheets increases the resistance of the
possible circulating-current paths, substantially reducing eddy-
,current losses while allowing the core to provide a low-
reluctance magnetic path.
4. In a DC motor, what produces the electromagnetic torque?
A. Field resistance
B. Bearing friction
C. Interaction between armature current and magnetic flux
D. Armature resistance alone
Rationale: Electromagnetic torque is produced when current-
carrying armature conductors are placed in the magnetic field
produced by the field system. In simplified form, motor torque is
proportional to magnetic flux multiplied by armature current.
Therefore, both the field strength and armature current have
major effects on the torque-producing capability of the
machine.
5. What happens to the back EMF of a DC motor as its speed
increases?
A. It decreases to zero
B. It remains constant regardless of speed
C. It increases
D. It becomes equal to the supply voltage plus the armature
drop
, Rationale: Back EMF is approximately proportional to the
product of flux and speed. As the motor accelerates, its
generated back EMF increases. This back EMF opposes the
applied voltage and limits the armature current. This is why a
DC motor normally draws a much larger current during starting
than during normal running.
6. Why does a DC motor draw high current during starting?
A. The field winding is short-circuited
B. The commutator produces excessive voltage
C. Back EMF is initially very low
D. The armature resistance becomes infinite
Rationale: At standstill, motor speed is zero, so the generated
back EMF is essentially zero. The armature current is therefore
determined mainly by the applied voltage and the very low
armature resistance. Without some form of current limiting, the
starting current can be several times the rated current and can
damage the armature winding, commutator, or mechanical
components.
7. Which equation best represents armature current in a basic
DC motor?
Questions with Correct Answers
(VerifiedAnswers) plus Rationales 2026 Q&A
Instant Download PDF
1. What is the primary function of the commutator in a DC
machine?
A. To increase the magnetic flux
B. To reduce armature resistance
C. To mechanically rectify the armature voltage/current
D. To control field current
Rationale: In a DC generator, the voltage induced in the rotating
armature conductors is inherently alternating. The commutator
and brushes mechanically rectify this internally generated AC
into a unidirectional DC output at the terminals. In a DC motor,
the commutator performs the complementary function of
reversing current in the armature conductors at the appropriate
point so that electromagnetic torque remains in the same
rotational direction.
,2. Which component of a DC machine carries the armature
conductors?
A. Yoke
B. Pole shoe
C. Field frame
D. Armature core
Rationale: The armature core is the rotating magnetic structure
that contains slots into which the armature conductors are
placed. It is normally constructed from laminated electrical steel
to reduce eddy-current losses. The yoke and pole structure
establish and support the magnetic circuit, but they do not
normally contain the rotating armature winding.
3. Why is the armature core of a DC machine laminated?
A. To increase mechanical strength
B. To increase brush pressure
C. To reduce eddy-current losses
D. To increase field current
Rationale: Changing magnetic flux induces circulating currents
in conductive iron. These eddy currents cause unwanted heating
and power loss. Laminating the armature core into thin
electrically insulated sheets increases the resistance of the
possible circulating-current paths, substantially reducing eddy-
,current losses while allowing the core to provide a low-
reluctance magnetic path.
4. In a DC motor, what produces the electromagnetic torque?
A. Field resistance
B. Bearing friction
C. Interaction between armature current and magnetic flux
D. Armature resistance alone
Rationale: Electromagnetic torque is produced when current-
carrying armature conductors are placed in the magnetic field
produced by the field system. In simplified form, motor torque is
proportional to magnetic flux multiplied by armature current.
Therefore, both the field strength and armature current have
major effects on the torque-producing capability of the
machine.
5. What happens to the back EMF of a DC motor as its speed
increases?
A. It decreases to zero
B. It remains constant regardless of speed
C. It increases
D. It becomes equal to the supply voltage plus the armature
drop
, Rationale: Back EMF is approximately proportional to the
product of flux and speed. As the motor accelerates, its
generated back EMF increases. This back EMF opposes the
applied voltage and limits the armature current. This is why a
DC motor normally draws a much larger current during starting
than during normal running.
6. Why does a DC motor draw high current during starting?
A. The field winding is short-circuited
B. The commutator produces excessive voltage
C. Back EMF is initially very low
D. The armature resistance becomes infinite
Rationale: At standstill, motor speed is zero, so the generated
back EMF is essentially zero. The armature current is therefore
determined mainly by the applied voltage and the very low
armature resistance. Without some form of current limiting, the
starting current can be several times the rated current and can
damage the armature winding, commutator, or mechanical
components.
7. Which equation best represents armature current in a basic
DC motor?
