BCIT Electrical DC Machines Level 3
Exam Questions With Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A | Instant Download Pdf
1. What is the primary function of the field winding in a DC machine?
A. To mechanically support the armature
B. To produce the main magnetic field
C. To convert AC into mechanical energy
D. To reduce brush friction
Answer: B. To produce the main magnetic field
The field winding establishes the stationary magnetic field in which the
armature conductors operate. The interaction between this field and
armature current produces torque in a motor or induced voltage in a
generator.
2. Which component of a conventional DC machine performs
mechanical rectification?
A. Slip rings
B. Field poles
C. Commutator
D. Armature core
,Answer: C. Commutator
The commutator reverses the direction of current in individual armature
coils at the appropriate time. This mechanical switching allows a DC
generator to provide unidirectional output and allows a DC motor to
maintain unidirectional torque.
3. In a DC motor, the electromagnetic torque is primarily produced by
the interaction between:
A. Resistance and capacitance
B. Armature current and magnetic flux
C. Voltage and armature resistance only
D. Frequency and inductance
Answer: B. Armature current and magnetic flux
Motor torque is proportional to the product of magnetic flux and
armature current. Increasing either the effective flux or armature
current generally increases electromagnetic torque, provided the
machine remains within its operating limits.
4. What is the purpose of laminating the armature core?
A. To increase mechanical friction
B. To reduce eddy-current losses
C. To increase brush voltage drop
D. To eliminate copper losses
Answer: B. To reduce eddy-current losses
,The armature core is made from electrically insulated laminations
rather than one solid piece of steel. This construction restricts circulating
eddy currents and significantly reduces associated power losses and
heating.
5. Which type of DC motor normally provides the highest starting
torque?
A. Separately excited motor
B. Shunt motor
C. Series motor
D. Permanent-magnet motor
Answer: C. Series motor
A DC series motor has its field winding connected in series with the
armature, so the field flux increases substantially with armature current
at startup. This produces very high starting torque, making series
motors suitable for applications such as traction and lifting.
6. Why should a DC series motor generally not be operated without a
mechanical load?
A. Its field current becomes zero immediately
B. Its speed can rise to a dangerously high value
C. Its armature resistance becomes infinite
D. Its commutator stops rotating
Answer: B. Its speed can rise to a dangerously high value
, When the series motor operates with little or no load, armature current
decreases and the field flux becomes weak. Because motor speed tends
to increase as flux decreases, the motor can accelerate to a destructive
overspeed condition.
7. What is the back electromotive force (back EMF) of a DC motor?
A. Voltage generated by the armature that opposes the applied voltage
B. Voltage generated only by the field winding
C. Voltage measured across an open switch
D. Voltage produced by the commutator when stationary
Answer: A. Voltage generated by the armature that opposes the
applied voltage
As the motor rotates, its armature conductors cut magnetic flux and
generate an induced EMF. By Lenz's law, this back EMF opposes the
applied supply voltage and limits armature current during normal
operation.
8. At the instant a DC motor is started from rest, its back EMF is
approximately:
A. Equal to the supply voltage
B. Half the supply voltage
C. Zero
D. Greater than the supply voltage
Answer: C. Zero
Exam Questions With Correct Answers
(Verified Answers) Plus Rationales 2026
Q&A | Instant Download Pdf
1. What is the primary function of the field winding in a DC machine?
A. To mechanically support the armature
B. To produce the main magnetic field
C. To convert AC into mechanical energy
D. To reduce brush friction
Answer: B. To produce the main magnetic field
The field winding establishes the stationary magnetic field in which the
armature conductors operate. The interaction between this field and
armature current produces torque in a motor or induced voltage in a
generator.
2. Which component of a conventional DC machine performs
mechanical rectification?
A. Slip rings
B. Field poles
C. Commutator
D. Armature core
,Answer: C. Commutator
The commutator reverses the direction of current in individual armature
coils at the appropriate time. This mechanical switching allows a DC
generator to provide unidirectional output and allows a DC motor to
maintain unidirectional torque.
3. In a DC motor, the electromagnetic torque is primarily produced by
the interaction between:
A. Resistance and capacitance
B. Armature current and magnetic flux
C. Voltage and armature resistance only
D. Frequency and inductance
Answer: B. Armature current and magnetic flux
Motor torque is proportional to the product of magnetic flux and
armature current. Increasing either the effective flux or armature
current generally increases electromagnetic torque, provided the
machine remains within its operating limits.
4. What is the purpose of laminating the armature core?
A. To increase mechanical friction
B. To reduce eddy-current losses
C. To increase brush voltage drop
D. To eliminate copper losses
Answer: B. To reduce eddy-current losses
,The armature core is made from electrically insulated laminations
rather than one solid piece of steel. This construction restricts circulating
eddy currents and significantly reduces associated power losses and
heating.
5. Which type of DC motor normally provides the highest starting
torque?
A. Separately excited motor
B. Shunt motor
C. Series motor
D. Permanent-magnet motor
Answer: C. Series motor
A DC series motor has its field winding connected in series with the
armature, so the field flux increases substantially with armature current
at startup. This produces very high starting torque, making series
motors suitable for applications such as traction and lifting.
6. Why should a DC series motor generally not be operated without a
mechanical load?
A. Its field current becomes zero immediately
B. Its speed can rise to a dangerously high value
C. Its armature resistance becomes infinite
D. Its commutator stops rotating
Answer: B. Its speed can rise to a dangerously high value
, When the series motor operates with little or no load, armature current
decreases and the field flux becomes weak. Because motor speed tends
to increase as flux decreases, the motor can accelerate to a destructive
overspeed condition.
7. What is the back electromotive force (back EMF) of a DC motor?
A. Voltage generated by the armature that opposes the applied voltage
B. Voltage generated only by the field winding
C. Voltage measured across an open switch
D. Voltage produced by the commutator when stationary
Answer: A. Voltage generated by the armature that opposes the
applied voltage
As the motor rotates, its armature conductors cut magnetic flux and
generate an induced EMF. By Lenz's law, this back EMF opposes the
applied supply voltage and limits armature current during normal
operation.
8. At the instant a DC motor is started from rest, its back EMF is
approximately:
A. Equal to the supply voltage
B. Half the supply voltage
C. Zero
D. Greater than the supply voltage
Answer: C. Zero
