Electrical Engg (EEE) - MCQ Practice Questions

Flux reduces drastically when field is short-circuited. As back EMF reduces, armature current increases causing motor speed to increase dangerously.

Transformer efficiency is maximum when core loss (Pi) equals copper loss (Pcu), i.e., Pi = Pcu.

Induction motors draw 4-7 times full-load current during starting because rotor speed is zero and rotor resistance appears as reactance.

Standard starting methods are DOL, star-delta, autotransformer, and rotor resistance. Frequency variation is used for speed control, not starting.

DC series motors have high starting torque (proportional to load current) and are used in electric vehicles, cranes, and hoists.

Induction motors operate at lagging power factor (0.85-0.95) due to reactive magnetizing current required for flux production.

Rotor frequency fr = slip × stator frequency = s × fs, where fs = 50 Hz for 50 Hz supply.

Damper windings provide asynchronous starting torque to accelerate the rotor near synchronous speed before synchronous operation begins.

Turns ratio k = N2/N1 = $\frac{100}{1000}$ = 0.1. Voltage ratio V1/V2 = N1/N2 = $\frac{10}{1}$ or 10:1 (step-down).

Speed = (V - IaRa)/(kΦ). Decreasing field current reduces flux Φ, increasing motor speed.

Rotor frequency = s × f. Therefore, 2 = 0.04 × f, giving f = 50 Hz

The damper winding (squirrel cage) in a synchronous motor provides the initial asynchronous torque for starting until synchronism is achieved.

R'2 = R1 × (N2/N1)² = 2 × ($\frac{230}{2300}$)² = 2 × 0.01 = 0.02 Ω

P = √3 × V × I × cosφ = 1.732 × 400 × 50 × 0.8 = 27,712 W ≈ 27.71 kW

Back EMF in DC motor Eb = PΦZn/(60A), where P = poles, Φ = flux, Z = conductors, n = rpm, A = parallel paths

Ns = 120×$\frac{50}{4}$ = 1500 rpm; Nr = Ns(1-s) = 1500×0.95 = 1425 rpm

Both star-delta and autotransformer starting reduce starting current. Star-delta reduces current by $\frac{1}{3}$ of DOL current.

At starting, back EMF is zero. Starting resistance limits initial armature current to safe values: Ia = V/(Ra + Rstart)

Star-delta connection reduces neutral point problems on primary (generator side) and utilizes delta on secondary for better voltage utilization.

Compound DC motors combine advantages of series (high starting torque) and shunt (stable speed) motors, making them ideal for cranes requiring both high starting torque and controlled speed.