AFCAT (Air Force Common Admission Test) EKT - Electrical and Mechanical Engineering Stream Syllabus

What is the EMF equation of a transformer?

E = 4.44·f·N·φm = 4.44·f·N·Bm·A, where f = frequency, N = number of turns, φm = maximum flux, Bm = max flux density, A = core area. This applies to both primary and secondary with their respective turns.

State the transformation ratio of a transformer relating voltage, current and turns.

K = V2/V1 = N2/N1 = I1/I2 (ideal). Voltage transforms in the ratio of turns; current transforms in the inverse ratio. Impedance referred across transforms by K^2.

What do the open-circuit and short-circuit tests of a transformer determine?

Open-circuit (no-load, rated voltage on LV side): gives core/iron losses and the shunt magnetizing branch parameters (R0, X0). Short-circuit (rated current, reduced voltage on HV side): gives copper losses and the equivalent series impedance (Req, Xeq).

State the condition for maximum efficiency of a transformer.

Maximum efficiency occurs when copper (variable) losses equal iron (constant) losses: I^2·Req = Pi. The load fraction for max efficiency = √(Pi/Pcu,full-load).

Why are transformer cores laminated and made of silicon steel?

Laminations (thin insulated sheets) reduce eddy-current losses by breaking up induced current paths; silicon steel has low hysteresis loss and high permeability. Together they minimize the core (iron) losses.

Write the slip equation of an induction motor and the relation between rotor frequency and slip.

Slip s = (Ns - N)/Ns, where Ns = synchronous speed = 120f/P and N = rotor speed. Rotor frequency fr = s·f. At standstill s=1; at synchronous speed s=0.

Why can a three-phase induction motor never run at synchronous speed?

If the rotor reached synchronous speed, there would be no relative motion between the rotating field and rotor conductors, so no flux cutting, no induced EMF, no rotor current, and hence no torque. It must run slightly below Ns (positive slip) to produce torque — it is asynchronous.

Compare squirrel-cage and slip-ring (wound-rotor) induction motors.

Squirrel-cage: rugged, cheap, low maintenance, fixed rotor (bars short-circuited), lower starting torque, no external rotor control. Slip-ring: wound rotor with slip rings allowing external rotor resistance for high starting torque and speed control, but costlier and more maintenance.

Why is a synchronous motor not self-starting, and how is it started?

At standstill the rotor (DC field) cannot follow the rapidly rotating stator field — average torque is zero. It is started as an induction motor using damper (amortisseur) windings, then the DC field is applied to pull the rotor into synchronism.

What is a synchronous condenser?

An over-excited synchronous motor running on no load that draws leading reactive current, used to improve power factor and provide voltage support/reactive-power compensation in power systems.

How does varying excitation of a synchronous machine affect power factor (V-curves)?

Under-excitation → motor draws lagging current; normal excitation → unity PF (minimum armature current); over-excitation → draws leading current. Plotting armature current vs field current gives the characteristic V-curves.

List the typical voltage levels in the sequence of power generation, transmission and distribution in India.

Generation ~11 kV (also 6.6/15.75 kV); stepped up for transmission to 132/220/400/765 kV; sub-transmission ~33/66 kV; primary distribution 11 kV; secondary/utilization distribution 400 V (3-phase) / 230 V (1-phase).

Why is electrical power transmitted at high voltage?

For a given power, raising voltage lowers current (P = √3·VL·IL·cosφ), which reduces I^2R line losses and voltage drop, and allows smaller conductor cross-section — improving efficiency and cutting cost over long distances.

Compare AC and HVDC transmission.

AC: easy voltage transformation, simpler switchgear, but suffers skin effect, charging current, stability limits, and corona; needs reactive compensation. HVDC: no reactive power/charging current, lower losses over very long distances and undersea cables, allows asynchronous interconnection, but needs costly converter stations and difficult circuit breaking.

What is the 'string efficiency' of a suspension insulator and how is it improved?

String efficiency = (voltage across whole string)/(n × voltage across the disc nearest the conductor). Voltage distribution is unequal due to shunt capacitance to the tower. It is improved by using a grading/guard ring, longer cross-arms, or capacitance grading.

What is corona in transmission lines and what factors affect it?

Corona is the partial ionization of air around a conductor when the surface electric field exceeds the breakdown strength, causing a hissing sound, violet glow, ozone, radio interference and power loss. It increases with higher voltage, smaller conductor diameter, closer spacing, and humid/dirty air; reduced by bundled/larger conductors.

What is a thyristor (SCR) and what are its three terminals and operating states?

An SCR is a four-layer (PNPN) three-terminal power semiconductor device with Anode, Cathode and Gate. It conducts (turns ON) when a gate pulse is applied with the anode positive; it is a latching device that stays ON until anode current falls below the holding current. States: forward-blocking, forward-conducting, reverse-blocking.

How is a conducting SCR turned off (commutation)?

By reducing the anode current below the holding current. Natural (line) commutation: the AC supply reverses and naturally brings current to zero (used in AC circuits). Forced commutation: an external LC/auxiliary circuit forces current to zero in DC circuits.

Compare a power BJT, a power MOSFET and an IGBT.

BJT: current-controlled, slow, low cost. MOSFET: voltage-controlled, very fast switching, high frequency but higher on-state loss at high voltage. IGBT: voltage-controlled like MOSFET with low conduction loss like BJT — best for medium/high power, moderate-high frequency.

What does a rectifier do versus an inverter?

A rectifier (converter) converts AC to DC. An inverter converts DC to AC. A device that can do AC↔DC is a converter; a DC-DC converter is a chopper; an AC-AC converter is a cycloconverter.

Give the average output voltage of single-phase half-wave and full-wave (bridge) controlled rectifiers (firing angle α).

Half-wave controlled: Vdc = (Vm/2π)(1 + cosα). Full-wave/bridge controlled (full-converter): Vdc = (2Vm/π)·cosα. (Uncontrolled full-wave: Vdc = 2Vm/π.)

What is a chopper, and what are buck, boost and buck-boost converters?

A chopper is a DC-DC converter using switching. Buck (step-down): Vo = D·Vs. Boost (step-up): Vo = Vs/(1-D). Buck-boost: Vo = Vs·D/(1-D) (output inverted). D = duty cycle (Ton/T).

What is PWM in inverters and why is it used?

Pulse Width Modulation varies the width of switching pulses to control the inverter's output voltage/frequency and to shift harmonics to higher frequencies that are easier to filter, producing a near-sinusoidal output with improved waveform quality.

What are the four quadrants of operation of an electric drive?

Defined by torque vs speed sign: Q1 forward motoring (+speed,+torque), Q2 forward braking/regenerating (+speed,−torque), Q3 reverse motoring (−speed,−torque), Q4 reverse braking (−speed,+torque). Quadrants 1 and 3 are motoring; 2 and 4 are braking.

Name and distinguish the three electric braking methods for motors.

Regenerative braking: machine acts as generator, energy returned to supply (only above synchronous/no-load speed). Dynamic (rheostatic) braking: armature/stator energy dissipated in a resistor. Plugging (reverse-voltage) braking: supply reversed to give rapid braking torque (energy wasted).

State the fundamental torque (dynamics) equation of an electric drive.

T - TL = J·(dω/dt), where T = motor torque, TL = load torque, J = total moment of inertia, ω = angular speed. During acceleration T > TL; at steady state T = TL (dω/dt = 0).

What is a moving-iron versus a moving-coil (PMMC) instrument, and which reads RMS?

PMMC (permanent-magnet moving-coil): responds to average current, used for DC, has a linear/uniform scale. Moving-iron: works on attraction/repulsion, responds to RMS, usable for both AC and DC, has a non-uniform (cramped at low end, square-law) scale.

What is the principle of a Wheatstone bridge and its balance condition?

A four-arm bridge for measuring an unknown resistance by balancing; at balance the galvanometer reads zero and the condition is P/Q = R/S, i.e. the products of opposite arms are equal (PS = QR). Independent of supply voltage.

How are voltmeter and ammeter ranges extended, and where are they connected?

An ammeter is connected in series and its range is extended by a low-value parallel shunt resistor. A voltmeter is connected in parallel and its range is extended by a high-value series multiplier resistor. Ideal ammeter has zero resistance; ideal voltmeter has infinite resistance.

What is the principle of a single-phase induction-type (disc) energy meter?

It measures kWh (energy) using an aluminium disc rotated by eddy currents from two fluxes (from voltage and current coils). Disc speed ∝ power; the number of revolutions ∝ energy consumed. A braking magnet provides damping proportional to speed.

What are the main casting defects and a common process classification?

Casting pours molten metal into a mould cavity. Common defects: blowholes/porosity (trapped gas), shrinkage cavity, cold shut/misrun (incomplete fill), hot tears (cracks), inclusions, and shifts (core misalignment). Processes are classed as expendable-mould (sand, investment, shell) or permanent-mould (die, gravity, centrifugal) casting.

Differentiate brazing, soldering and welding.

Welding: base metals are melted (with/without filler) and fused, giving the strongest joint. Brazing: filler metal melts above ~450°C but base metal does NOT melt; joined by capillary action. Soldering: same as brazing but filler melts below ~450°C; weakest, used for electrical joints.

Define hot working versus cold working in metal forming.

Hot working: deforming above the recrystallization temperature — low forces, large deformation, no strain hardening, refined grains, but poor surface finish/tolerance. Cold working: below recrystallization temperature — better finish and tolerance, increased strength via strain hardening, but needs higher force and may require annealing.

Define cutting speed, feed and depth of cut in machining.

Cutting speed: relative surface velocity between tool and workpiece (m/min). Feed: distance the tool advances per revolution/stroke (mm/rev). Depth of cut: thickness of material removed in one pass (mm), measured perpendicular to the machined surface.