DC Motor Torque and Back EMF Exam
Questions with Correct Answers (Verified
Answers) Plus Rationales 2026 Q&A Instant
Download PDF
1. What is the primary purpose of back EMF in a DC motor?
A. To increase the applied armature voltage
B. To reduce the magnetic flux produced by the field winding
C. To oppose the applied armature voltage and limit armature
current
D. To reverse the direction of armature rotation
Rationale: As the armature of a DC motor rotates within the
magnetic field, an electromotive force is induced in its
conductors. By Lenz's law, this induced voltage opposes the
applied armature voltage, so it is called back EMF. The
relationship is approximately (E_b = V - I_aR_a). Because the
back EMF increases as the motor accelerates, it naturally limits
the armature current during normal operation.
2. Which equation represents the back EMF of a DC motor?
,A. (E_b = V + I_aR_a)
B. (E_b = I_aR_a - V)
C. (E_b = V/I_a)
D. (E_b = V - I_aR_a)
Rationale: Applying Kirchhoff's voltage law to the motor
armature gives (V = E_b + I_aR_a). Rearranging produces (E_b =
V-I_aR_a). This equation shows that the applied voltage is
divided between the back EMF and the armature resistance
drop.
3. What happens to the armature current when a DC motor
is first switched on?
A. It is approximately zero
B. It is very high
C. It immediately reaches its rated value
D. It becomes negative
Rationale: At starting, the armature is stationary, so no
rotational EMF is induced and the back EMF is essentially zero.
Consequently, the starting current is approximately (I_a=V/R_a).
Since armature resistance is usually very small, this current can
be several times the rated current. A starter or electronic
current-limiting method is therefore commonly required.
, 4. Why is a starter traditionally used with a DC shunt
motor?
A. To increase field resistance
B. To eliminate mechanical losses
C. To limit the excessive starting armature current
D. To increase the supply frequency
Rationale: At standstill, the motor has essentially no back EMF.
Directly applying the rated supply voltage would therefore
cause a very large armature current because armature
resistance is small. A starter introduces temporary resistance
into the armature circuit and gradually removes it as the motor
develops speed and back EMF.
5. The electromagnetic torque of a DC motor is
approximately proportional to which quantity?
A. (V/I_a) only
B. (R_a/I_a)
C. (\Phi I_a)
D. (V\Phi/I_a)
Rationale: The electromagnetic torque relationship for a DC
motor is (T_e=K\Phi I_a), where (K) depends on the motor
construction, (\Phi) is flux per pole, and (I_a) is armature
, current. Therefore, for a given motor and constant flux, torque
is directly proportional to armature current.
6. What is the SI unit of motor torque?
A. Watt
B. Volt
C. Ampere
D. Newton-metre
Rationale: Torque is the rotational equivalent of force and is
measured in newton-metres (N·m). Although mechanical power
can be calculated from torque and angular speed, torque itself
is not measured in watts. The distinction is important when
analyzing motor performance.
7. For a DC motor with constant field flux, doubling
armature current will approximately:
A. Halve the torque
B. Leave torque unchanged
C. Double the electromagnetic torque
D. Quadruple the back EMF
Rationale: With constant flux, (T_e=K\Phi I_a) reduces to
(T_e\propto I_a). Therefore, doubling the armature current
approximately doubles electromagnetic torque. This
Questions with Correct Answers (Verified
Answers) Plus Rationales 2026 Q&A Instant
Download PDF
1. What is the primary purpose of back EMF in a DC motor?
A. To increase the applied armature voltage
B. To reduce the magnetic flux produced by the field winding
C. To oppose the applied armature voltage and limit armature
current
D. To reverse the direction of armature rotation
Rationale: As the armature of a DC motor rotates within the
magnetic field, an electromotive force is induced in its
conductors. By Lenz's law, this induced voltage opposes the
applied armature voltage, so it is called back EMF. The
relationship is approximately (E_b = V - I_aR_a). Because the
back EMF increases as the motor accelerates, it naturally limits
the armature current during normal operation.
2. Which equation represents the back EMF of a DC motor?
,A. (E_b = V + I_aR_a)
B. (E_b = I_aR_a - V)
C. (E_b = V/I_a)
D. (E_b = V - I_aR_a)
Rationale: Applying Kirchhoff's voltage law to the motor
armature gives (V = E_b + I_aR_a). Rearranging produces (E_b =
V-I_aR_a). This equation shows that the applied voltage is
divided between the back EMF and the armature resistance
drop.
3. What happens to the armature current when a DC motor
is first switched on?
A. It is approximately zero
B. It is very high
C. It immediately reaches its rated value
D. It becomes negative
Rationale: At starting, the armature is stationary, so no
rotational EMF is induced and the back EMF is essentially zero.
Consequently, the starting current is approximately (I_a=V/R_a).
Since armature resistance is usually very small, this current can
be several times the rated current. A starter or electronic
current-limiting method is therefore commonly required.
, 4. Why is a starter traditionally used with a DC shunt
motor?
A. To increase field resistance
B. To eliminate mechanical losses
C. To limit the excessive starting armature current
D. To increase the supply frequency
Rationale: At standstill, the motor has essentially no back EMF.
Directly applying the rated supply voltage would therefore
cause a very large armature current because armature
resistance is small. A starter introduces temporary resistance
into the armature circuit and gradually removes it as the motor
develops speed and back EMF.
5. The electromagnetic torque of a DC motor is
approximately proportional to which quantity?
A. (V/I_a) only
B. (R_a/I_a)
C. (\Phi I_a)
D. (V\Phi/I_a)
Rationale: The electromagnetic torque relationship for a DC
motor is (T_e=K\Phi I_a), where (K) depends on the motor
construction, (\Phi) is flux per pole, and (I_a) is armature
, current. Therefore, for a given motor and constant flux, torque
is directly proportional to armature current.
6. What is the SI unit of motor torque?
A. Watt
B. Volt
C. Ampere
D. Newton-metre
Rationale: Torque is the rotational equivalent of force and is
measured in newton-metres (N·m). Although mechanical power
can be calculated from torque and angular speed, torque itself
is not measured in watts. The distinction is important when
analyzing motor performance.
7. For a DC motor with constant field flux, doubling
armature current will approximately:
A. Halve the torque
B. Leave torque unchanged
C. Double the electromagnetic torque
D. Quadruple the back EMF
Rationale: With constant flux, (T_e=K\Phi I_a) reduces to
(T_e\propto I_a). Therefore, doubling the armature current
approximately doubles electromagnetic torque. This
