BCIT DC Machines Level 3 Midterm 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
generator?
A. To increase field resistance
B. To provide mechanical support for the armature
C. To convert the internally generated alternating voltage into
unidirectional output voltage
D. To reduce the magnetic flux produced by the field poles
The voltage induced in individual armature coils is inherently
alternating as the coils rotate through the magnetic field. The
commutator mechanically switches the connections of the
rotating coils to the external circuit at the appropriate instant,
producing a unidirectional voltage at the generator terminals.
This is why the commutator is essential to the operation of a
conventional DC generator.
,2. Which component of a DC machine provides the stationary
magnetic field?
A. Armature core
B. Commutator
C. Interpoles
D. Field poles and their windings
The field poles establish the main magnetic field through which
the armature conductors rotate. Depending on the machine
design, the field may be produced by permanent magnets or,
more commonly in industrial DC machines, by energized field
windings. The armature then interacts with this field to produce
either electromagnetic torque or generated EMF.
3. According to Faraday's law, an EMF is induced in a
conductor when:
A. Its resistance becomes zero
B. Its temperature increases
C. There is a change in magnetic flux linkage
D. The conductor is stationary in a constant magnetic field
Faraday's law states that the magnitude of induced EMF is
proportional to the rate of change of magnetic flux linkage. In a
DC generator, rotating armature conductors continuously cut
magnetic flux, causing an EMF to be induced. Simply having a
,magnetic field present is not sufficient; there must be relative
motion or another change in flux linkage.
4. Which rule is commonly used to determine the direction of
induced current in a generator?
A. Fleming's left-hand rule
B. Lenz's mechanical rule
C. Fleming's right-hand rule
D. Right-hand screw rule
Fleming's right-hand rule is used for generator action. With the
thumb representing conductor motion, the first finger
representing magnetic-field direction, and the second finger
representing induced current direction, the rule establishes the
relationship among these three quantities. Fleming's left-hand
rule, in contrast, is associated with motor force.
5. What happens to generated EMF if the speed of a DC
generator increases while flux remains constant?
A. It decreases
B. It remains unchanged
C. It increases
D. It becomes zero
, The generated EMF of a DC generator is proportional to
magnetic flux and rotational speed. The fundamental
relationship is (E_g = k\Phi N), where (k) incorporates machine
constants, (\Phi) is flux per pole, and (N) is speed. Therefore,
with constant flux, increasing speed directly increases
generated EMF.
6. Which equation represents the general generated EMF
relationship for a DC generator?
A. (E_g = I_aR_a)
B. (E_g = V/R)
C. (E_g = \frac{P\Phi ZN}{60A})
D. (E_g = VI)
The DC-generator EMF equation is (E_g = P\Phi ZN/(60A)),
where (P) is the number of poles, (\Phi) is flux per pole in
webers, (Z) is the total number of armature conductors, (N) is
speed in revolutions per minute, and (A) is the number of
parallel paths. The equation demonstrates why generated
voltage depends on machine construction, flux, and speed.
7. In a simplex lap winding, the number of parallel paths is
generally:
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
generator?
A. To increase field resistance
B. To provide mechanical support for the armature
C. To convert the internally generated alternating voltage into
unidirectional output voltage
D. To reduce the magnetic flux produced by the field poles
The voltage induced in individual armature coils is inherently
alternating as the coils rotate through the magnetic field. The
commutator mechanically switches the connections of the
rotating coils to the external circuit at the appropriate instant,
producing a unidirectional voltage at the generator terminals.
This is why the commutator is essential to the operation of a
conventional DC generator.
,2. Which component of a DC machine provides the stationary
magnetic field?
A. Armature core
B. Commutator
C. Interpoles
D. Field poles and their windings
The field poles establish the main magnetic field through which
the armature conductors rotate. Depending on the machine
design, the field may be produced by permanent magnets or,
more commonly in industrial DC machines, by energized field
windings. The armature then interacts with this field to produce
either electromagnetic torque or generated EMF.
3. According to Faraday's law, an EMF is induced in a
conductor when:
A. Its resistance becomes zero
B. Its temperature increases
C. There is a change in magnetic flux linkage
D. The conductor is stationary in a constant magnetic field
Faraday's law states that the magnitude of induced EMF is
proportional to the rate of change of magnetic flux linkage. In a
DC generator, rotating armature conductors continuously cut
magnetic flux, causing an EMF to be induced. Simply having a
,magnetic field present is not sufficient; there must be relative
motion or another change in flux linkage.
4. Which rule is commonly used to determine the direction of
induced current in a generator?
A. Fleming's left-hand rule
B. Lenz's mechanical rule
C. Fleming's right-hand rule
D. Right-hand screw rule
Fleming's right-hand rule is used for generator action. With the
thumb representing conductor motion, the first finger
representing magnetic-field direction, and the second finger
representing induced current direction, the rule establishes the
relationship among these three quantities. Fleming's left-hand
rule, in contrast, is associated with motor force.
5. What happens to generated EMF if the speed of a DC
generator increases while flux remains constant?
A. It decreases
B. It remains unchanged
C. It increases
D. It becomes zero
, The generated EMF of a DC generator is proportional to
magnetic flux and rotational speed. The fundamental
relationship is (E_g = k\Phi N), where (k) incorporates machine
constants, (\Phi) is flux per pole, and (N) is speed. Therefore,
with constant flux, increasing speed directly increases
generated EMF.
6. Which equation represents the general generated EMF
relationship for a DC generator?
A. (E_g = I_aR_a)
B. (E_g = V/R)
C. (E_g = \frac{P\Phi ZN}{60A})
D. (E_g = VI)
The DC-generator EMF equation is (E_g = P\Phi ZN/(60A)),
where (P) is the number of poles, (\Phi) is flux per pole in
webers, (Z) is the total number of armature conductors, (N) is
speed in revolutions per minute, and (A) is the number of
parallel paths. The equation demonstrates why generated
voltage depends on machine construction, flux, and speed.
7. In a simplex lap winding, the number of parallel paths is
generally:
