RRB JE Electrical Engineering — Solved Practice Question Bank

Q: Why is transmission voltage kept high (e.g. 400 kV) instead of low? Sol: For the same power transmitted, higher voltage means lower current (P = VI), and since transmission line loss is I²R, a lower current drastically reduces power loss over long distances. Q: A 4-pole, 50 Hz induction motor runs at 1440 rpm. Find the slip. Sol: Ns = 120f/P = 120×50/4 = 1500 rpm; slip s = (Ns−N)/Ns = (1500−1440)/1500 = 60/1500 = 0.04 = 4%. Q: Why must a DC series motor never be started without load? Sol: With no load, the motor accelerates to a dangerously high, potentially destructive speed because series motor speed varies inversely with armature current, and with no load the current (and hence counter-torque) is very low.

Q: In a purely capacitive AC circuit, what is the phase relationship between voltage and current? Sol: Current leads voltage by 90°. Q: Two resistors of 10Ω and 20Ω are in parallel. Find the equivalent resistance. Sol: 1/R = 1/10 + 1/20 = 2/20+1/20 = 3/20; R = 20/3 ≈ 6.67Ω. Q: State the condition for resonance in a series RLC circuit. Sol: XL = XC (inductive reactance equals capacitive reactance) — at this frequency, impedance is purely resistive and minimum, and current is maximum.

Q: Why must an ammeter have very low resistance? Sol: Because it is connected in series with the circuit — a high ammeter resistance would add significant extra resistance to the circuit, altering the very current it is meant to measure. Q: What happens if a Current Transformer's secondary is left open while the primary carries current? Sol: A dangerously high voltage spike develops across the open secondary (since there is no counter-MMF from secondary current to limit core flux), which can damage insulation and pose a safety hazard. Q: What null-deflection instrument is used to measure unknown resistance with high precision? Sol: Wheatstone bridge — balanced when P×R = Q×S for its four arms.