BCIT DC Machines Level 3 Final Exam — Exam
Questions with Correct Answers
(VerifiedAnswers) Plus Rationales 2026 Q&A
Instant Download PDF
1. What is the primary function of the commutator in a
conventional DC machine?
A. To increase the resistance of the armature winding
B. To provide mechanical cooling to the armature
C. To convert the internally generated alternating voltage into a
unidirectional external voltage
D. To regulate the field current automatically
Correct answer: C. To convert the internally generated
alternating voltage into a unidirectional external voltage
The armature conductors of a conventional DC machine
generate alternating emf as they move through the magnetic
field. The commutator mechanically reverses the connections of
the armature coils at appropriate povsitions so that the voltage
delivered at the brushes remains unidirectional. This is why the
commutator is essential to the operation of a traditional DC
,generator and also allows a DC motor to maintain unidirectional
torque.
2. In a DC motor, what principle produces the mechanical force
on an armature conductor?
A. Faraday's law only
B. Lenz's law only
C. The force on a current-carrying conductor in a magnetic field
D. Electrostatic attraction between conductors
Correct answer: C. The force on a current-carrying conductor
in a magnetic field
When current flows through a conductor located in a magnetic
field, the conductor experiences a force. In a DC motor, the
interaction between the armature current and the main
magnetic field produces torque on the armature. The direction
of this force can be determined using an appropriate motor-rule
or by applying the vector relationship between magnetic flux,
current, and conductor length.
3. Which equation represents the approximate
electromagnetic torque of a DC motor?
A. T = V/I
B. T ∝ ΦIa
,C. T ∝ V/Ra only
D. T ∝ Ia/Φ²
Correct answer: B. T ∝ ΦIa
The electromagnetic torque of a DC motor is proportional to the
product of the useful magnetic flux per pole and armature
current. In simplified form, T = kΦIa. This relationship explains
why increasing armature current generally increases motor
torque and why weakening the field can reduce torque
capability while affecting motor speed.
4. What is the main purpose of the interpoles in a DC
machine?
A. To increase bearing lubrication
B. To improve commutation
C. To reduce shaft diameter
D. To increase armature resistance
Correct answer: B. To improve commutation
Interpoles are small auxiliary poles located between the main
poles. They are connected so that their magnetic effect changes
with armature current. Their purpose is to counteract the effects
that cause sparking at the brushes during commutation,
particularly armature reaction in the commutation zone. Proper
, interpole action helps produce smoother current reversal in the
armature coils.
5. What is armature reaction?
A. The heating of the armature caused by friction
B. The effect of armature flux on the main magnetic field
C. The mechanical vibration of the armature
D. The resistance created by the commutator
Correct answer: B. The effect of armature flux on the main
magnetic field
Current flowing in the armature produces its own magnetic
field. This armature-produced flux interacts with the main field
and distorts and, depending on operating conditions, weakens
the effective main magnetic field. This phenomenon is called
armature reaction. It can contribute to poor commutation,
changes in flux distribution, and reduced machine performance.
6. What happens to the speed of a DC shunt motor if the field
flux is reduced significantly while the load remains
approximately constant?
A. Speed generally increases
B. Speed becomes exactly zero
Questions with Correct Answers
(VerifiedAnswers) Plus Rationales 2026 Q&A
Instant Download PDF
1. What is the primary function of the commutator in a
conventional DC machine?
A. To increase the resistance of the armature winding
B. To provide mechanical cooling to the armature
C. To convert the internally generated alternating voltage into a
unidirectional external voltage
D. To regulate the field current automatically
Correct answer: C. To convert the internally generated
alternating voltage into a unidirectional external voltage
The armature conductors of a conventional DC machine
generate alternating emf as they move through the magnetic
field. The commutator mechanically reverses the connections of
the armature coils at appropriate povsitions so that the voltage
delivered at the brushes remains unidirectional. This is why the
commutator is essential to the operation of a traditional DC
,generator and also allows a DC motor to maintain unidirectional
torque.
2. In a DC motor, what principle produces the mechanical force
on an armature conductor?
A. Faraday's law only
B. Lenz's law only
C. The force on a current-carrying conductor in a magnetic field
D. Electrostatic attraction between conductors
Correct answer: C. The force on a current-carrying conductor
in a magnetic field
When current flows through a conductor located in a magnetic
field, the conductor experiences a force. In a DC motor, the
interaction between the armature current and the main
magnetic field produces torque on the armature. The direction
of this force can be determined using an appropriate motor-rule
or by applying the vector relationship between magnetic flux,
current, and conductor length.
3. Which equation represents the approximate
electromagnetic torque of a DC motor?
A. T = V/I
B. T ∝ ΦIa
,C. T ∝ V/Ra only
D. T ∝ Ia/Φ²
Correct answer: B. T ∝ ΦIa
The electromagnetic torque of a DC motor is proportional to the
product of the useful magnetic flux per pole and armature
current. In simplified form, T = kΦIa. This relationship explains
why increasing armature current generally increases motor
torque and why weakening the field can reduce torque
capability while affecting motor speed.
4. What is the main purpose of the interpoles in a DC
machine?
A. To increase bearing lubrication
B. To improve commutation
C. To reduce shaft diameter
D. To increase armature resistance
Correct answer: B. To improve commutation
Interpoles are small auxiliary poles located between the main
poles. They are connected so that their magnetic effect changes
with armature current. Their purpose is to counteract the effects
that cause sparking at the brushes during commutation,
particularly armature reaction in the commutation zone. Proper
, interpole action helps produce smoother current reversal in the
armature coils.
5. What is armature reaction?
A. The heating of the armature caused by friction
B. The effect of armature flux on the main magnetic field
C. The mechanical vibration of the armature
D. The resistance created by the commutator
Correct answer: B. The effect of armature flux on the main
magnetic field
Current flowing in the armature produces its own magnetic
field. This armature-produced flux interacts with the main field
and distorts and, depending on operating conditions, weakens
the effective main magnetic field. This phenomenon is called
armature reaction. It can contribute to poor commutation,
changes in flux distribution, and reduced machine performance.
6. What happens to the speed of a DC shunt motor if the field
flux is reduced significantly while the load remains
approximately constant?
A. Speed generally increases
B. Speed becomes exactly zero
