1. Armature Current Calculation
Question: A 150 V DC motor has an armature resistance of 0.4 Ω and a back EMF of 142 V. The armature current is:
Options:(1) 100 A
(2) 10 A
(3) 20 A
(4) 150 A
Correct Answer: (3) 20 A
Explanation: Iₐ = (Vₜ - E₆)/Rₐ = (150 - 142)/0.4 = 20 A
2. DC Shunt Generator EMF vs. Speed
Question: A shunt generator running at 1000 rpm generates 100 V. If speed increases to 1200 rpm, the new EMF is:
Options:(1) 140 V
(2) 120 V
(3) 240 V
(4) 360 V
Correct Answer: (2) 120 V
Explanation: E ∝ N ⇒ E₂/100 = 1200/1000 ⇒ E₂ = 120 V
3. DC Generator Classification
Question: A DC generator can be termed as:
Options:(1) Rotating amplifier
(2) Prime mover
(3) Power pump
(4) None of these
Correct Answer: (1) Rotating amplifier
Explanation: It converts mechanical input into amplified electrical output.
4. Slip and Rotor Speed
Question: A 6-pole, 50 Hz single-phase induction motor has 3% slip. Its rotor speed is:
Options:(1) 1455 rpm
(2) 728 rpm
(3) 970 rpm
(4) 960 rpm
Correct Answer: (3) 970 rpm
Explanation: Nₛ = (120×50)/6 = 1000 rpm, Nᵣ = 1000(1 - 0.03) = 970 rpm
5. 3-Phase Induction Motor Slip
Question: A 4-pole, 50 Hz induction motor has 4% slip. Its actual speed is:
Options:(1) 1440 rpm
(2) 720 rpm
(3) 1550 rpm
(4) 1460 rpm
Correct Answer: (1) 1440 rpm
Explanation: Nₛ = (120×50)/4 = 1500 rpm, Nᵣ = 1500(1 - 0.04) = 1440 rpm
6. Synchronous Motor Power Factor
Question: A synchronous motor can operate at:
Options:(1) Leading power factor only
(2) Lagging power factor only
(3) Unity power factor only
(4) Lagging, leading, and unity power factors
Correct Answer: (4) Lagging, leading, and unity power factors
Explanation: Excitation control allows operation at any power factor.
7. Over-Excited Synchronous Motor
Question: An over-excited synchronous motor is used for:
Options:(1) Variable speed loads
(2) Low torque loads
(3) Power factor correction
(4) High torque loads
Correct Answer: (3) Power factor correction
Explanation: It supplies reactive power to improve system power factor.
8. Starting Torque
Question: A single-phase induction motor with one stator winding has:
Options:(1) Zero starting torque
(2) Finite but moderate torque
(3) High starting torque
(4) Infinite torque
Correct Answer: (1) Zero starting torque
Explanation: Requires auxiliary winding (capacitor/inductor) to start.
9. Capacitor-Run Motor
Question: A two-value capacitor-run motor uses:
Options:(1) High capacitor for both start/run
(2) Low capacitor for start, high for run
(3) Low capacitor for both
(4) High capacitor for start, low for run
Correct Answer: (4) High for start, low for run
Explanation: High capacitance for starting torque, low for running efficiency.
10. Power Losses in Machines
Question: Power loss consideration is essential for:
Options:(1) Operating cost
(2) Temperature rise
(3) Voltage drops
(4) All of these
Correct Answer: (4) All of these
Explanation: Losses affect efficiency, heating, and voltage regulation.
11. Constant Speed Motor
Question: A constant speed motor is:
Options:(1) DC shunt motor
(2) Synchronous motor
(3) Induction motor
(4) DC compound motor
Correct Answer: (2) Synchronous motor
Explanation: Runs at synchronous speed (Nₛ = 120f/P).