DC Motor Speed Control Exam Questions with
Correct Answers (VerifiedAnswers) Plus
Rationales 2026 Q&A Instant Download PDF
Question 1
What is the fundamental relationship used to determine the
speed of a separately excited DC motor?
A. Speed is proportional only to armature resistance
B. Speed is proportional to armature current only
C. Speed is approximately proportional to the ratio of
armature voltage to flux
D. Speed is proportional to the square of armature voltage
Rationale: The approximate DC motor speed equation is (N
\propto (V_a-I_aR_a)/\Phi). Therefore, increasing armature
voltage tends to increase speed, while increasing field flux tends
to reduce speed for a given armature voltage. This relationship
forms the basis of the two major methods of DC motor speed
control: armature-voltage control and field-flux control.
Question 2
,Which equation most directly represents the back EMF of a DC
motor?
A. (E_b=I_aR_a)
B. (E_b=V_a+I_aR_a)
C. (E_b=V_a-I_aR_a)
D. (E_b=V_a/I_a)
Rationale: Applying Kirchhoff's voltage law to the armature
circuit gives (V_a=E_b+I_aR_a), so (E_b=V_a-I_aR_a). Back EMF
opposes the applied armature voltage and rises as motor speed
increases. This is an important self-regulating feature of a DC
motor.
Question 3
A DC motor is operating at constant flux. What happens to its
speed if the armature voltage is increased?
A. Speed decreases
B. Speed remains unchanged
C. Speed becomes zero
D. Speed increases
Rationale: With flux held constant, motor speed is
approximately proportional to the effective armature voltage.
Increasing (V_a) increases back EMF, and the motor accelerates
,until its back EMF reaches a new steady-state value. This is the
principle behind armature-voltage speed control.
Question 4
Which speed-control method is normally used to obtain speeds
below the base speed of a separately excited DC motor?
A. Field weakening only
B. Mechanical braking
C. Armature-voltage control
D. Increasing field resistance
Rationale: Below base speed, the motor normally operates with
approximately rated field flux. Reducing the armature voltage
reduces back EMF and therefore reduces speed. Armature-
voltage control provides good torque capability because the
field can remain at its rated value.
Question 5
Which speed-control method is commonly used for operation
above the base speed of a DC motor?
A. Increasing armature resistance
B. Increasing armature voltage above its rated value indefinitely
, C. Field weakening
D. Increasing mechanical load
Rationale: Once rated armature voltage has been reached,
further speed increase can be obtained by reducing the field
flux. Since speed is approximately inversely proportional to flux,
weakening the field allows the motor to run faster. However,
torque capability decreases because electromagnetic torque is
proportional to flux and armature current.
Question 6
What is the primary disadvantage of armature-resistance speed
control?
A. It increases motor efficiency
B. It eliminates heat production
C. It produces constant flux automatically
D. It wastes power in the external resistance
Rationale: Adding resistance in series with the armature
produces a voltage drop (I_aR). The electrical power associated
with this drop is dissipated as heat. Consequently, the method
has poor efficiency, especially at reduced speeds where
substantial current may still be required.
Question 7
Correct Answers (VerifiedAnswers) Plus
Rationales 2026 Q&A Instant Download PDF
Question 1
What is the fundamental relationship used to determine the
speed of a separately excited DC motor?
A. Speed is proportional only to armature resistance
B. Speed is proportional to armature current only
C. Speed is approximately proportional to the ratio of
armature voltage to flux
D. Speed is proportional to the square of armature voltage
Rationale: The approximate DC motor speed equation is (N
\propto (V_a-I_aR_a)/\Phi). Therefore, increasing armature
voltage tends to increase speed, while increasing field flux tends
to reduce speed for a given armature voltage. This relationship
forms the basis of the two major methods of DC motor speed
control: armature-voltage control and field-flux control.
Question 2
,Which equation most directly represents the back EMF of a DC
motor?
A. (E_b=I_aR_a)
B. (E_b=V_a+I_aR_a)
C. (E_b=V_a-I_aR_a)
D. (E_b=V_a/I_a)
Rationale: Applying Kirchhoff's voltage law to the armature
circuit gives (V_a=E_b+I_aR_a), so (E_b=V_a-I_aR_a). Back EMF
opposes the applied armature voltage and rises as motor speed
increases. This is an important self-regulating feature of a DC
motor.
Question 3
A DC motor is operating at constant flux. What happens to its
speed if the armature voltage is increased?
A. Speed decreases
B. Speed remains unchanged
C. Speed becomes zero
D. Speed increases
Rationale: With flux held constant, motor speed is
approximately proportional to the effective armature voltage.
Increasing (V_a) increases back EMF, and the motor accelerates
,until its back EMF reaches a new steady-state value. This is the
principle behind armature-voltage speed control.
Question 4
Which speed-control method is normally used to obtain speeds
below the base speed of a separately excited DC motor?
A. Field weakening only
B. Mechanical braking
C. Armature-voltage control
D. Increasing field resistance
Rationale: Below base speed, the motor normally operates with
approximately rated field flux. Reducing the armature voltage
reduces back EMF and therefore reduces speed. Armature-
voltage control provides good torque capability because the
field can remain at its rated value.
Question 5
Which speed-control method is commonly used for operation
above the base speed of a DC motor?
A. Increasing armature resistance
B. Increasing armature voltage above its rated value indefinitely
, C. Field weakening
D. Increasing mechanical load
Rationale: Once rated armature voltage has been reached,
further speed increase can be obtained by reducing the field
flux. Since speed is approximately inversely proportional to flux,
weakening the field allows the motor to run faster. However,
torque capability decreases because electromagnetic torque is
proportional to flux and armature current.
Question 6
What is the primary disadvantage of armature-resistance speed
control?
A. It increases motor efficiency
B. It eliminates heat production
C. It produces constant flux automatically
D. It wastes power in the external resistance
Rationale: Adding resistance in series with the armature
produces a voltage drop (I_aR). The electrical power associated
with this drop is dissipated as heat. Consequently, the method
has poor efficiency, especially at reduced speeds where
substantial current may still be required.
Question 7
