Armature Reaction in DC Generator – MCQs for SSC JE Electrical

What is armature reaction in a DC generator?
A) Increase in armature resistance
B) Effect of armature flux on main field flux
C) Effect of speed on EMF
D) Change in commutation
Answer: B
Explanation: Armature reaction refers to the impact of armature magnetic field on the main field flux, causing distortion and weakening of the flux.
Armature reaction causes which of the following in a DC generator?
A) Increase in terminal voltage
B) Sparking at brushes
C) Smooth commutation
D) Constant flux
Answer: B
Explanation: Armature reaction distorts the main field, leading to poor commutation and sparking at brushes.
Under loaded condition, armature reaction in DC generator causes the flux to:
A) Strengthen
B) Remain unchanged
C) Get distorted and weakened
D) Change direction
Answer: C
Which component is affected directly due to armature reaction?
A) Brushes
B) Shaft
C) Commutator
D) Magnetic field
Answer: D
In a DC generator, armature reaction at full load causes magnetic neutral axis (MNA) to:
A) Remain unchanged
B) Shift in the direction of rotation
C) Shift opposite to the direction of rotation
D) Disappear
Answer: B
Explanation: The MNA shifts in the direction of rotation in generators due to the armature reaction.
Which type of armature reaction leads to a decrease in generated EMF?
A) Cross-magnetizing
B) Demagnetizing
C) Magnetizing
D) Both A and B
Answer: B
Interpoles are used in DC machines to:
A) Reduce hysteresis
B) Improve commutation
C) Increase speed
D) Reduce core loss
Answer: B
What is the primary effect of armature reaction on the main field?
A) Enhances flux
B) Distorts and weakens flux
C) Keeps it stable
D) Reverses flux direction
Answer: B
Which of the following is NOT a result of armature reaction?
A) Shifting of MNA
B) Sparking at brushes
C) Increase in main field strength
D) Flux weakening
Answer: C
In a DC generator, armature reaction can be reduced by:
A) Decreasing field current
B) Using compensating windings
C) Decreasing armature resistance
D) Increasing load current
Answer: B
Cross magnetization effect due to armature reaction is more dominant in:
A) No-load condition
B) Light load
C) Full load
D) Overload
Answer: C
The main cause of poor commutation in a DC generator is:
A) High field current
B) Armature reaction
C) Large number of turns
D) Low armature resistance
Answer: B
Demagnetizing effect of armature reaction occurs when brushes are:
A) At geometrical neutral axis
B) Advanced from GNA
C) Retarded from GNA
D) Perpendicular to MNA
Answer: B
How is armature reaction neutralized in large DC machines?
A) By reducing the load
B) By placing brushes at MNA
C) By using interpoles and compensating windings
D) By increasing the number of poles
Answer: C
The axis at which there is no induced EMF in coil sides is known as:
A) Magnetic neutral axis
B) Geometrical neutral axis
C) Electrical axis
D) Rotor axis
Answer: A
Which effect of armature reaction is responsible for shifting of magnetic neutral axis?
A) Demagnetizing
B) Cross-magnetizing
C) Magnetizing
D) Capacitive
Answer: B
What does a compensating winding do in a DC machine?
A) Increases armature resistance
B) Reduces eddy current losses
C) Neutralizes armature reaction
D) Decreases load current
Answer: C
In lap winding, demagnetizing ampere-turns per pole is given by:
A) (Ia × Z × θ) / (360 × A)
B) (Ia × Z × P) / (A)
C) (V × I) / t
D) (Z × E) / R
Answer: A
When brushes are shifted in the direction of rotation in a generator, it causes:
A) Cross-magnetizing effect
B) Demagnetizing effect
C) Magnetizing effect
D) No effect
Answer: B
The value of armature reaction increases with:
A) Speed of the machine
B) Load current
C) Field current
D) Number of poles
Answer: B

🎯 SSC JE PYQs on Armature Reaction (2017–2025)

Below are previous year questions (PYQs) asked in the SSC JE Electrical Engineering exam from 2017 to 2023, specifically on the topic of Armature Reaction in DC machines and synchronous machines. These are directly picked from official shifts and reliable sources like Testbook, Reddit, and SSC Portal.

✅ 1. SSC JE 25‑Jan‑2018 – Afternoon Shift

Q: Armature reaction effect causes flux to reduce with:

Decreasing torque
Increasing torque
Increase current
Both increasing torque & increase current

✔️ Correct Answer: Increase current

Explanation: As the load increases, armature current increases, leading to a stronger armature flux that distorts the main field flux, reducing net flux.

✅ 2. SSC JE 25‑Jan‑2018 – Morning Shift (General Engineering Paper)

Q: In which of the following methods, the effect of armature reaction is more?

Field weakening method
Armature resistance control
Same in both methods
Cannot be determined

✔️ Correct Answer: Field weakening method

Explanation: In field weakening, the field flux is intentionally reduced. Any distortion by the armature current becomes more prominent, thus increasing armature reaction effects.

✅ 3. SSC JE 10‑Oct‑2023 – Shift‑2 (Official Paper)

Q: The armature reaction effect is high in ________.

Both the armature and field control methods
Field control method
Armature control method
Series parallel control method

✔️ Correct Answer: Field control method

Explanation: Reducing the field current increases armature reaction's impact on flux weakening.

✅ 4. SSC JE 16‑Nov‑2022 – Shift‑3

Q: Which of the following statements about armature reaction of an alternator is NOT correct?

It increases the generated voltage and sparking at brushes reduces
It reduces the generated voltage by distorting the main flux and it causes sparking at the brushes
It depends on both load current and power factor
When the generator is loaded, the armature current sets up its own magnetic field

✔️ Correct Answer: Option 1 – It increases the generated voltage and sparking at brushes reduces

Explanation: This is false. Armature reaction causes distortion and weakening of flux, leading to voltage drop and increased sparking.

📝 Summary Table

Year / Shift Question Focus Correct Option
Jan 2018 (PM) Flux reduction vs current increase Increasing armature current
Jan 2018 (Morning) Method with higher armature reaction effect Field weakening method
Oct 2023 Shift‑2 Effect high in which method Field control method
Nov 2022 Shift‑3 Incorrect statement about armature reaction “Increases voltage…”
Mar 2017 paper (sync) Dependence on load current & power factor (synchronous) Both load current & PF
Mar 2017 paper (sync) Armature reaction nature at zero‑PF lagging load Demagnetizing