Junior Engineer (Electrical/Electronics) MCQs – Series 2 (Q31-40)
Q31. The back EMF in a DC motor is directly proportional to:
- A) Supply voltage only
- B) Armature resistance
- C) Armature current only
- D) Speed and flux
Answer: D
Back EMF, Eb = kΦN, is proportional to both flux (Φ) and speed (N).
Q32. Which DC generator characteristic curve is used to determine the critical resistance for self-excitation?
- A) Internal characteristic
- B) External characteristic
- C) Load characteristic
- D) Open circuit characteristic (O.C.C.)
Answer: D
The critical field resistance is found from the tangent to the O.C.C. of a self-excited generator.
Q33. In a DC series motor, if the load is suddenly removed, the motor:
- A) Reverses direction
- B) Tends to run at dangerously high speed
- C) Runs at rated speed
- D) Stops immediately
Answer: B
Since flux is proportional to armature current in a series motor, removing load causes flux to drop sharply, causing dangerously high speed.
Q34. The efficiency of a DC machine is maximum when:
- A) Copper losses are zero
- B) Variable (copper) losses equal constant (iron) losses
- C) Copper losses are twice the iron losses
- D) Iron losses are zero
Answer: B
Maximum efficiency condition for a DC machine occurs when I²Ra (copper loss) equals the constant losses.
Q35. The commutator in a DC machine is primarily used to:
- A) Improve power factor
- B) Increase machine speed
- C) Convert the AC induced in the armature to DC output
- D) Reduce armature resistance
Answer: C
The commutator, along with brushes, performs mechanical rectification converting the AC generated in the armature into DC.
Q36. Armature reaction in a DC machine causes:
- A) Increase in speed only
- B) No effect on flux
- C) Distortion and weakening of the main field flux
- D) Increase in field flux
Answer: C
The armature’s own magnetic field distorts and, under normal saturation, weakens the main field flux (cross-magnetizing and demagnetizing effects).
Q37. Compared with a DC series motor of similar rating, the starting torque of a DC shunt motor is:
- A) Higher
- B) Zero
- C) Equal
- D) Lower
Answer: D
Since torque in a series motor is proportional to the square of armature current at low speeds, its starting torque exceeds that of a shunt motor.
Q38. Which method is commonly used to control the speed of a DC shunt motor above its rated (base) speed?
- A) Series resistance in the armature
- B) Field flux control (field weakening)
- C) Armature voltage control
- D) Ward-Leonard method only
Answer: B
Weakening the field flux increases speed above the rated value since N is inversely proportional to Φ for constant Eb.
Q39. The condition for maximum efficiency of a DC generator, in terms of load current I, is:
- A) I²Ra = constant (iron + friction) losses
- B) Ra = 0
- C) I²Ra = 0
- D) I = Ra
Answer: A
Maximum efficiency occurs when the variable copper loss (I²Ra) equals the constant losses of the machine.
Q40. In DC machines, interpoles (commutating poles) are mainly used to:
- A) Increase machine speed
- B) Reduce sparking at the commutator caused by armature reaction
- C) Increase armature resistance
- D) Reduce field current
Answer: B
Interpoles neutralize the reactance voltage and cross-magnetizing effect of armature reaction, reducing sparking at the brushes.
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