SSC JE General Engineering (Electrical) Syllabus

Write the relations for active, reactive, and apparent power in an AC circuit.

Active power P = VI cos φ (watts); Reactive power Q = VI sin φ (VAR); Apparent power S = VI (VA). They relate as S² = P² + Q².

What is resonance in a series RLC circuit and the resonant frequency formula?

Series resonance occurs when inductive reactance equals capacitive reactance (XL = XC); impedance is minimum (= R) and current is maximum. Resonant frequency fr = 1/(2π√(LC)).

Define inductive reactance and capacitive reactance with formulas.

Inductive reactance XL = 2πfL (increases with frequency). Capacitive reactance XC = 1/(2πfC) (decreases with frequency). Both measured in ohms.

In a purely inductive and purely capacitive AC circuit, what is the phase relationship?

In a pure inductor, current lags voltage by 90°. In a pure capacitor, current leads voltage by 90°.

Classify electrical measuring instruments into the three basic types by function.

Indicating instruments (show present value, e.g., ammeter/voltmeter), Integrating instruments (totalize over time, e.g., energy meter), and Recording instruments (produce a continuous record, e.g., chart recorder).

What is the difference between absolute and secondary instruments?

Absolute instruments give the quantity in terms of instrument constants (no calibration needed, e.g., tangent galvanometer). Secondary instruments are pre-calibrated and read directly from a scale (e.g., ammeters, voltmeters).

Name the three controlling/operating torques required in an indicating instrument.

Deflecting torque (moves the pointer), Controlling torque (provides position/restoring force, by spring or gravity), and Damping torque (brings the pointer to rest quickly without oscillation).

What is the operating principle of a PMMC (Permanent Magnet Moving Coil) instrument and what does it measure?

It works on the motor principle—a current-carrying coil in a permanent magnetic field experiences a torque. It measures only DC, has a uniform (linear) scale, and is used with rectifiers/shunts for other ranges.

How is a galvanometer converted into an ammeter and into a voltmeter?

Into an ammeter: connect a low-resistance shunt in parallel. Into a voltmeter: connect a high-resistance multiplier in series.

What does a wattmeter measure and what is the basic operating type used?

A wattmeter measures electrical power (watts). The common type is the electrodynamometer (dynamometer) wattmeter, having a fixed current coil and a moving voltage (pressure) coil; deflection ∝ VI cos φ.

What is the working principle of an energy meter (induction type)?

It works on the induction principle: interacting fluxes from current and voltage coils induce eddy currents in an aluminium disc, producing torque that rotates the disc at a speed proportional to power; the number of revolutions measures energy (kWh).

What is a Wheatstone bridge used for and the balance condition?

It measures unknown resistance. At balance (no galvanometer current), P/Q = R/S, i.e., the products of opposite arms are equal: P·S = Q·R.

State the EMF equation of a DC generator.

Eg = (PΦZN)/(60A) volts, where P = poles, Φ = flux per pole, Z = total armature conductors, N = speed in rpm, A = number of parallel paths.

What is back EMF in a DC motor and why is it important?

Back EMF (Eb = PΦZN/60A) is the EMF induced in the armature that opposes the supply voltage. It regulates armature current automatically: Ia = (V - Eb)/Ra, and makes the motor self-regulating with load.

How are DC machines classified based on field excitation?

Separately excited (field from external source) and self-excited; self-excited types are series-wound, shunt-wound, and compound-wound (further long/short shunt, cumulative/differential).

State the torque equation of a DC motor.

Armature torque Ta = (PΦZIa)/(2πA) N·m, i.e., torque is proportional to flux and armature current (Ta ∝ Φ·Ia).

What is the EMF equation of a transformer?

E = 4.44 f N Φmax volts, where f = frequency, N = number of turns, and Φmax = maximum flux. The turns ratio E1/E2 = N1/N2 = I2/I1.

What are the two main losses in a transformer and how do they vary?

Core (iron) losses—hysteresis and eddy current—are constant and independent of load. Copper losses (I²R) are variable and proportional to the square of load current. Maximum efficiency occurs when copper loss = iron loss.

What is the working principle of a three-phase induction motor and slip?

A rotating magnetic field produced by 3-phase stator currents induces currents in the rotor (transformer action), producing torque. Slip s = (Ns - N)/Ns, the fractional difference between synchronous speed and rotor speed.

Give the formula for synchronous speed of an AC machine.

Ns = 120f/P rpm, where f is supply frequency in Hz and P is the number of poles.

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

At standstill the rotor cannot follow the rapidly rotating stator field, so net average torque is zero. It is started using damper (amortisseur) windings as an induction motor, or by an auxiliary/pony motor, then synchronized.

What is the effect of varying excitation in a synchronous motor (V-curves)?

At constant load, under-excitation makes the motor draw lagging current, over-excitation makes it draw leading current, and normal excitation gives unity power factor (minimum armature current). Plotting Ia vs field current gives V-curves.

What is a synchronous condenser?

An over-excited synchronous motor running on no load, used to supply leading reactive power (VAR) for power factor improvement and voltage control in power systems.

What is voltage regulation of an alternator (synchronous generator)?

It is the rise in terminal voltage when full load is removed (field excitation and speed constant), expressed as a percentage: %Reg = [(E - V)/V] × 100, where E is no-load EMF and V is full-load terminal voltage.

Differentiate base load and peak load power plants.

Base load plants (e.g., thermal, nuclear, large hydro) supply the constant minimum demand continuously at high load factor. Peak load plants (e.g., gas turbines, pumped storage, diesel) supply short-duration peak demand and start/stop quickly.

Why is power transmitted at high voltage over long distances?

For a given power, higher voltage means lower current, which reduces I²R line losses and voltage drop, allows thinner conductors, and improves transmission efficiency.

Why is electrical power generated and transmitted as three-phase AC rather than DC or single-phase?

Three-phase AC enables easy voltage transformation, gives constant power, requires less conductor material than single-phase for the same power, and produces a rotating magnetic field for motors. AC is easily stepped up/down by transformers.

Define load factor and diversity factor.

Load factor = average load / maximum demand (over a period); higher is more economical. Diversity factor = sum of individual maximum demands / maximum demand of the whole system (always ≥1).

What methods of electric heating use induced/eddy currents and dielectric loss?

Induction heating heats conducting materials via eddy currents induced by an alternating magnetic field. Dielectric heating heats insulating materials by molecular friction from a high-frequency alternating electric field.

What is the ideal speed-time curve / important requirement for traction motors, and which motor is preferred for DC traction?

Traction needs high starting torque and good speed control. The DC series motor is preferred because it provides high starting torque, automatically reduces speed on heavy load, and is self-regulating (Ta ∝ Ia²).

Differentiate intrinsic and extrinsic semiconductors, and N-type vs P-type doping.

Intrinsic = pure semiconductor (e.g., Si, Ge). Extrinsic = doped. N-type is doped with pentavalent atoms (electrons are majority carriers); P-type is doped with trivalent atoms (holes are majority carriers).