Armature Reaction Exam Questions with
Correct Answers (VerifiedAnswers) Plus
Rationales 2026 Q&A Instant Download PDF
1. What is armature reaction in a DC machine?
A. The heating effect produced by the armature winding
B. The mechanical reaction between the armature and field
poles
C. The effect of armature-produced magnetomotive force on
the main field flux
D. The voltage drop caused by armature resistance
Rationale: Armature reaction refers to the influence of the
magnetic field produced by the armature current on the main
magnetic field established by the field poles. This interaction
changes the distribution and, depending on operating
conditions and machine design, the magnitude of the resultant
air-gap flux. It can cause flux distortion, changes in generated
emf, and difficulties in commutation.
2. The magnetic field produced by armature current is
commonly called:
,A. Main field
B. Residual field
C. Leakage field
D. Armature field
Rationale: Current flowing through the armature conductors
produces its own magnetomotive force (MMF) and magnetic
field. This field is called the armature field. Its interaction with
the main pole field is the fundamental cause of armature
reaction.
3. In a DC generator, the armature reaction primarily causes:
A. Increase in mechanical losses only
B. Elimination of brush voltage
C. Distortion of the main magnetic field and possible reduction
of useful flux
D. Complete cancellation of the main field
Rationale: The armature MMF interacts with the main field
MMF. The resulting field is no longer distributed symmetrically
in the same manner as the no-load field. In practical machines,
armature reaction can also produce a demagnetizing
component that reduces the useful average flux per pole.
,4. Under no-load conditions, the magnetic neutral axis (MNA)
of a DC machine is approximately:
A. 90° electrical from the GNA
B. Parallel to the armature shaft
C. Coincident with the geometric neutral axis
D. Located at the interpole axis in every machine
Rationale: With no armature current, there is essentially no
armature MMF to distort the main field. Consequently, the
magnetic neutral axis is approximately aligned with the
geometric neutral axis. When load current flows, armature
reaction shifts the MNA from this original position.
5. What happens to the magnetic neutral axis of a generator
when it is loaded?
A. It remains fixed under all load conditions
B. It shifts in the direction of rotation
C. It shifts opposite to the direction of rotation
D. It disappears completely
Rationale: In a conventional DC generator, armature reaction
distorts the flux distribution and shifts the magnetic neutral
plane. The MNA is displaced in the direction of rotation of the
armature. This shift is important because the brushes must
, ideally lie along the magnetic neutral plane to minimize the emf
in the coil being commutated.
6. What is the principal effect of armature reaction on the air-
gap flux distribution?
A. It makes the flux perfectly uniform
B. It completely eliminates the pole flux
C. It distorts the flux distribution under the pole faces
D. It converts magnetic flux into mechanical torque
Rationale: Armature MMF combines with the main field MMF,
strengthening the flux in some portions of the pole arc and
weakening it in others. Therefore, the flux distribution becomes
distorted. This distortion is one of the most important
consequences of armature reaction.
7. Armature reaction can be divided into which two principal
components?
A. Resistive and inductive components
B. Mechanical and electrical components
C. Cross-magnetizing and demagnetizing components
D. Capacitive and inductive components
Rationale: The armature MMF can be resolved into components
relative to the main field. The cross-magnetizing component
Correct Answers (VerifiedAnswers) Plus
Rationales 2026 Q&A Instant Download PDF
1. What is armature reaction in a DC machine?
A. The heating effect produced by the armature winding
B. The mechanical reaction between the armature and field
poles
C. The effect of armature-produced magnetomotive force on
the main field flux
D. The voltage drop caused by armature resistance
Rationale: Armature reaction refers to the influence of the
magnetic field produced by the armature current on the main
magnetic field established by the field poles. This interaction
changes the distribution and, depending on operating
conditions and machine design, the magnitude of the resultant
air-gap flux. It can cause flux distortion, changes in generated
emf, and difficulties in commutation.
2. The magnetic field produced by armature current is
commonly called:
,A. Main field
B. Residual field
C. Leakage field
D. Armature field
Rationale: Current flowing through the armature conductors
produces its own magnetomotive force (MMF) and magnetic
field. This field is called the armature field. Its interaction with
the main pole field is the fundamental cause of armature
reaction.
3. In a DC generator, the armature reaction primarily causes:
A. Increase in mechanical losses only
B. Elimination of brush voltage
C. Distortion of the main magnetic field and possible reduction
of useful flux
D. Complete cancellation of the main field
Rationale: The armature MMF interacts with the main field
MMF. The resulting field is no longer distributed symmetrically
in the same manner as the no-load field. In practical machines,
armature reaction can also produce a demagnetizing
component that reduces the useful average flux per pole.
,4. Under no-load conditions, the magnetic neutral axis (MNA)
of a DC machine is approximately:
A. 90° electrical from the GNA
B. Parallel to the armature shaft
C. Coincident with the geometric neutral axis
D. Located at the interpole axis in every machine
Rationale: With no armature current, there is essentially no
armature MMF to distort the main field. Consequently, the
magnetic neutral axis is approximately aligned with the
geometric neutral axis. When load current flows, armature
reaction shifts the MNA from this original position.
5. What happens to the magnetic neutral axis of a generator
when it is loaded?
A. It remains fixed under all load conditions
B. It shifts in the direction of rotation
C. It shifts opposite to the direction of rotation
D. It disappears completely
Rationale: In a conventional DC generator, armature reaction
distorts the flux distribution and shifts the magnetic neutral
plane. The MNA is displaced in the direction of rotation of the
armature. This shift is important because the brushes must
, ideally lie along the magnetic neutral plane to minimize the emf
in the coil being commutated.
6. What is the principal effect of armature reaction on the air-
gap flux distribution?
A. It makes the flux perfectly uniform
B. It completely eliminates the pole flux
C. It distorts the flux distribution under the pole faces
D. It converts magnetic flux into mechanical torque
Rationale: Armature MMF combines with the main field MMF,
strengthening the flux in some portions of the pole arc and
weakening it in others. Therefore, the flux distribution becomes
distorted. This distortion is one of the most important
consequences of armature reaction.
7. Armature reaction can be divided into which two principal
components?
A. Resistive and inductive components
B. Mechanical and electrical components
C. Cross-magnetizing and demagnetizing components
D. Capacitive and inductive components
Rationale: The armature MMF can be resolved into components
relative to the main field. The cross-magnetizing component
