DC Drives Exam Questions with Correct
Answers (VerifiedAnswers) Plus Rationales
2026 Q&A Instant Download PDF
1. What is the primary purpose of a DC drive?
A. To convert DC electrical energy directly into heat
B. To maintain a fixed mechanical speed regardless of load
C. To control the speed, torque, and direction of a DC motor
D. To increase the motor's field resistance permanently
Rationale: A DC drive regulates the electrical quantities supplied
to a DC motor so that its speed, torque, acceleration,
deceleration, and direction can be controlled. Depending on the
drive configuration, armature voltage, field current, or both can
be varied. This controllability is one of the major advantages of
DC motor drives.
2. Which equation most directly describes the steady-state
speed of a separately excited DC motor?
A. (N \propto \frac{\Phi}{V_a})
B. (N \propto \frac{V_a-I_aR_a}{\Phi})
C. (N \propto V_a\Phi)
D. (N \propto I_aR_a\Phi)
Rationale: The back EMF is approximately (E_a=V_a-I_aR_a),
while motor speed is proportional to back EMF divided by field
,flux. Therefore, speed is approximately proportional to ((V_a-
I_aR_a)/\Phi). This relationship forms the basis of armature-
voltage and field-weakening speed control.
3. In a separately excited DC motor, electromagnetic torque is
approximately proportional to:
A. Armature resistance only
B. Speed squared
C. Field flux multiplied by armature current
D. Armature voltage divided by speed
Rationale: The electromagnetic torque is (T_e=K\Phi I_a). With
constant field flux, torque is directly proportional to armature
current. This is why current control is commonly used as the
inner control loop in high-performance DC drives.
4. Which speed-control method is normally used for operation
below base speed?
A. Field weakening
B. Increasing armature resistance indefinitely
C. Armature-voltage control
D. Increasing field flux beyond its rated value
Rationale: Below base speed, the motor normally operates with
approximately rated field flux while the armature voltage is
varied. Increasing armature voltage raises speed while
,maintaining approximately constant flux. This region is
commonly associated with constant-torque operation.
5. What is the principal effect of field weakening in a DC motor
drive?
A. It increases the available torque indefinitely
B. It reduces speed below zero
C. It permits operation above base speed while reducing
torque capability
D. It eliminates armature current
Rationale: Since speed is approximately inversely proportional
to flux, reducing field flux permits the motor to operate at
speeds above base speed. However, torque is proportional to
flux and armature current, so reduced flux lowers the torque
capability for a given armature current.
6. What is meant by base speed in a DC drive?
A. The minimum speed at zero load
B. The speed corresponding approximately to rated armature
voltage and rated field flux
C. The maximum possible speed during plugging
D. The speed at which armature current is zero
Rationale: Base speed generally marks the transition between
armature-voltage control and field-weakening control. Below
base speed, rated flux is normally maintained and armature
, voltage is varied. Above base speed, armature voltage is
generally limited near its rated value while field flux is reduced.
7. A four-quadrant DC drive is capable of:
A. Forward motoring only
B. Forward and reverse motoring only
C. Motoring and braking in only one direction
D. Forward motoring, forward braking, reverse motoring, and
reverse braking
Rationale: Four-quadrant operation allows both speed and
torque to have either polarity. This provides forward and reverse
motoring as well as regenerative or braking operation in both
directions. Such operation is especially useful in traction,
elevators, hoists, and reversing industrial machinery.
8. In the first quadrant of the torque-speed plane, a DC motor
operates with:
A. Positive speed and negative torque
B. Negative speed and positive torque
C. Negative speed and negative torque
D. Positive speed and positive torque
Rationale: First-quadrant operation corresponds to forward
motoring. Both speed and electromagnetic torque have positive
signs, so mechanical power is delivered by the motor to the
load.
Answers (VerifiedAnswers) Plus Rationales
2026 Q&A Instant Download PDF
1. What is the primary purpose of a DC drive?
A. To convert DC electrical energy directly into heat
B. To maintain a fixed mechanical speed regardless of load
C. To control the speed, torque, and direction of a DC motor
D. To increase the motor's field resistance permanently
Rationale: A DC drive regulates the electrical quantities supplied
to a DC motor so that its speed, torque, acceleration,
deceleration, and direction can be controlled. Depending on the
drive configuration, armature voltage, field current, or both can
be varied. This controllability is one of the major advantages of
DC motor drives.
2. Which equation most directly describes the steady-state
speed of a separately excited DC motor?
A. (N \propto \frac{\Phi}{V_a})
B. (N \propto \frac{V_a-I_aR_a}{\Phi})
C. (N \propto V_a\Phi)
D. (N \propto I_aR_a\Phi)
Rationale: The back EMF is approximately (E_a=V_a-I_aR_a),
while motor speed is proportional to back EMF divided by field
,flux. Therefore, speed is approximately proportional to ((V_a-
I_aR_a)/\Phi). This relationship forms the basis of armature-
voltage and field-weakening speed control.
3. In a separately excited DC motor, electromagnetic torque is
approximately proportional to:
A. Armature resistance only
B. Speed squared
C. Field flux multiplied by armature current
D. Armature voltage divided by speed
Rationale: The electromagnetic torque is (T_e=K\Phi I_a). With
constant field flux, torque is directly proportional to armature
current. This is why current control is commonly used as the
inner control loop in high-performance DC drives.
4. Which speed-control method is normally used for operation
below base speed?
A. Field weakening
B. Increasing armature resistance indefinitely
C. Armature-voltage control
D. Increasing field flux beyond its rated value
Rationale: Below base speed, the motor normally operates with
approximately rated field flux while the armature voltage is
varied. Increasing armature voltage raises speed while
,maintaining approximately constant flux. This region is
commonly associated with constant-torque operation.
5. What is the principal effect of field weakening in a DC motor
drive?
A. It increases the available torque indefinitely
B. It reduces speed below zero
C. It permits operation above base speed while reducing
torque capability
D. It eliminates armature current
Rationale: Since speed is approximately inversely proportional
to flux, reducing field flux permits the motor to operate at
speeds above base speed. However, torque is proportional to
flux and armature current, so reduced flux lowers the torque
capability for a given armature current.
6. What is meant by base speed in a DC drive?
A. The minimum speed at zero load
B. The speed corresponding approximately to rated armature
voltage and rated field flux
C. The maximum possible speed during plugging
D. The speed at which armature current is zero
Rationale: Base speed generally marks the transition between
armature-voltage control and field-weakening control. Below
base speed, rated flux is normally maintained and armature
, voltage is varied. Above base speed, armature voltage is
generally limited near its rated value while field flux is reduced.
7. A four-quadrant DC drive is capable of:
A. Forward motoring only
B. Forward and reverse motoring only
C. Motoring and braking in only one direction
D. Forward motoring, forward braking, reverse motoring, and
reverse braking
Rationale: Four-quadrant operation allows both speed and
torque to have either polarity. This provides forward and reverse
motoring as well as regenerative or braking operation in both
directions. Such operation is especially useful in traction,
elevators, hoists, and reversing industrial machinery.
8. In the first quadrant of the torque-speed plane, a DC motor
operates with:
A. Positive speed and negative torque
B. Negative speed and positive torque
C. Negative speed and negative torque
D. Positive speed and positive torque
Rationale: First-quadrant operation corresponds to forward
motoring. Both speed and electromagnetic torque have positive
signs, so mechanical power is delivered by the motor to the
load.
