🔌DC Machines Quiz🔌

📘 Summary – DC Machines MCQs (Q1–100)

This complete set of 100 MCQs covers every major concept of DC machines, ensuring thorough preparation for PSU and competitive exams.

• Electromechanical Energy Conversion – torque, co‑energy, stored energy, and electromagnetic force principles.
• Losses – copper, iron, stray, mechanical, windage, and their dependence on speed, load, and frequency.
• Constructional Features – yoke, armature, commutator, brushes, laminations, slot wedges, pole shoes, and ventilation ducts.
• Armature Windings – lap, wave, simplex, duplex, winding pitches, equalizer rings, commutator segments, and coil spans.
• Generator Characteristics – shunt, series, compound types, OCC curves, residual magnetism, voltage build‑up, and regulation.
• Parallel Operation – equalizer bars, load sharing, compounding methods, and stability conditions.
• DC Motors – back emf, torque production, current relations, speed‑torque characteristics, and energy conversion principles.
• Applications – welding generators, exciters, boosters, traction loads, and industrial uses.

✅ By practicing all 100 questions with explanations, you’ll master the fundamentals of DC machines and be well prepared for SSC JE, GENCO, TRANSCO, NTPC, BHEL, and other PSU exams.

🔌⚡ DC Machines Quiz – Part 1 (Q1–10) ⚡🔌

👋 This is Part 1 of the DC Machines quiz. It covers electromechanical energy conversion, torque, losses, and basic construction concepts. Each question includes a detailed explanation.

1In an electromechanical energy conversion system, the electromagnetic force or torque that develops will naturally act to…

a) increase stored energy when flux is constant.
b) reduce stored energy when flux is constant.
c) lower co‑energy at constant mmf.
d) decrease stored energy at constant mmf.

Check Answer

Answer: b) reduce stored energy when flux is constant
Explanation: The torque produced in electromechanical systems always acts to oppose changes in stored magnetic energy. At constant flux, this means the torque tends to decrease stored energy, ensuring the system remains stable and balanced.

2For a linear electromagnetic circuit, which statement holds true?

a) Field energy equals co‑energy.
b) Field energy is greater than co‑energy.
c) Field energy is less than co‑energy.
d) Co‑energy is zero.

Check Answer

Answer: a) Field energy equals co‑energy
Explanation: In linear systems, the magnetization curve is straight. Hence, stored energy and co‑energy are equal, simplifying analysis.

3The torque developed in an electromechanical energy conversion device depends on…

a) Stator field strength and torque angle.
b) Stator field and rotor field strength.
c) Stator field, rotor field strengths, and torque angle.
d) Stator field strength only.

Check Answer

Answer: c) Stator field, rotor field strengths, and torque angle
Explanation: Torque is proportional to the product of stator and rotor field magnitudes and the sine of the angle between them.

4Why is a small air gap left between the stator and rotor in DC machines?

a) To reduce reluctance of the magnetic path.
b) To increase flux density in the air gap.
c) To permit mechanical clearance.
d) To avoid saturation of the field.

Check Answer

Answer: c) To permit mechanical clearance
Explanation: The air gap ensures safe clearance between rotating and stationary parts, preventing contact and mechanical wear.

5Reluctance torque in rotating machines occurs when…

a) The air gap is not uniform.
b) Reluctance seen by stator mmf varies.
c) Reluctance seen by rotor mmf varies.
d) Reluctance seen by the working mmf varies.

Check Answer

Answer: d) Reluctance seen by the working mmf varies
Explanation: Reluctance torque arises when reluctance changes with rotor position, aligning the rotor to minimize reluctance.

6If the self‑inductance of both stator and rotor windings is independent of rotor position, the machine will not develop…

a) Starting torque.
b) Synchronizing torque.
c) Hysteresis torque.
d) Reluctance torque.

Check Answer

Answer: d) Reluctance torque
Explanation: Reluctance torque requires inductance variation with rotor position. If inductance is constant, no reluctance torque develops.

7A metallic disc placed in a vertical magnetic field is rotated horizontally. The induced emf will be…

a) Zero.
b) Constant, independent of disc size.
c) Increasing radially outward.
d) Decreasing radially outward.

Check Answer

Answer: a) Zero
Explanation: Since flux linkage does not change with rotation in this orientation, no emf is induced.

8The emf induced in a conductor rotating in a bipolar field is…

a) DC.
b) AC.
c) Both DC and AC.
d) None of these.

Check Answer

Answer: b) AC
Explanation: Rotation in a bipolar field produces alternating emf due to sinusoidal variation of flux linkage.

9In a DC motor, windage loss is proportional to…

a) Supply voltage.
b) Square of supply voltage.
c) Square of flux density.
d) Square of armature speed.

Check Answer

Answer: d) Square of armature speed
Explanation: Windage loss is due to air friction, which increases with the square of speed.

10Which of the following are variable losses in a rotating machine?

a) Core loss and mechanical loss.
b) Core loss and stray load loss.
c) Copper and mechanical losses.
d) Copper loss and stray load loss.

Check Answer

Answer: d) Copper loss and stray load loss
Explanation: Copper losses vary with load current, and stray load losses vary with load conditions. Core and mechanical losses are constant.

➡️ Continue to Part 2 (Q11–22)