Intermediate Electrical interview questions

150 questions with answers (EEE-151 to EEE-300). Try each one aloud before reading the answer.

Circuit Theory

EEE-151 · How do you improve power factor?

Add shunt capacitor banks, use synchronous condensers or over-excited synchronous motors, or use active power factor correction in power electronic loads. These supply reactive power locally.

EEE-152 · How is the capacitor rating for power factor correction calculated?

Q_c = P (tan φ₁ − tan φ₂), where P is active power and φ₁, φ₂ are the initial and target power factor angles.

EEE-153 · What is the quality factor of a series RLC circuit?

Q = ω₀L/R = 1/(ω₀CR) = (1/R)√(L/C). It equals the voltage magnification across L or C at resonance and f₀/bandwidth.

EEE-154 · What is the time constant of an RC and RL circuit?

τ = RC for an RC circuit and τ = L/R for an RL circuit. In one time constant the response reaches about 63.2% of its final change.

EEE-155 · State the maximum power transfer theorem.

For a DC network, maximum power is transferred when R_L = R_th, giving P_max = V_th²/(4R_th). For AC, the load impedance must be the complex conjugate of Z_th.

EEE-156 · How is three-phase power calculated?

P = √3 V_L I_L cos φ for a balanced load, regardless of star or delta connection.

EEE-157 · How do you convert a delta network to star?

Each star resistance equals the product of the two adjacent delta resistances divided by the sum of all three: R_a = R_ab R_ca/(R_ab + R_bc + R_ca). For equal resistors, R_Y = R_Δ/3.

EEE-158 · What is the difference between nodal and mesh analysis?

Nodal analysis applies KCL to find node voltages and suits circuits with many parallel branches. Mesh analysis applies KVL to find loop currents and suits planar circuits with many series elements.

EEE-159 · What happens at parallel resonance?

In an ideal parallel LC circuit, impedance becomes maximum and the line current minimum, with large circulating current between L and C. It is used as a rejector circuit.

EEE-160 · What is the bandwidth of a resonant circuit?

The frequency range between the half-power points, where power is half the maximum (current is 0.707 of maximum). BW = f₀/Q = R/(2πL) for a series RLC circuit.

EEE-161 · Why does a capacitor block DC but pass AC?

X_C = 1/(2πfC) is infinite at f = 0, so in steady state no DC current flows. At AC frequencies X_C is finite and current flows through charging and discharging.

EEE-162 · How do you find the energy stored in an inductor and a capacitor?

W_L = ½ L I² in the magnetic field and W_C = ½ C V² in the electric field, both in joules.

EEE-163 · What is the two-wattmeter method?

A method to measure total power in a three-phase, three-wire system: P = W₁ + W₂. For a balanced load, tan φ = √3 (W₁ − W₂)/(W₁ + W₂).

Electromagnetics

EEE-164 · How are eddy current losses expressed?

P_e = k_e B_m² f² t² V, where t is lamination thickness and V volume. They rise with the square of frequency and thickness.

EEE-165 · How are hysteresis losses expressed?

By Steinmetz's equation, P_h = η B_m^1.6 f V, where η is the Steinmetz coefficient. They rise linearly with frequency.

EEE-166 · What is magnetic saturation?

At high H, most magnetic domains in a ferromagnetic material are aligned, so B increases only slowly and permeability falls toward μ₀.

EEE-167 · What is leakage flux?

Flux produced by one winding that does not link the other winding. It is represented as leakage reactance in machine and transformer equivalent circuits.

EEE-168 · What is fringing in an air gap?

The spreading of flux lines outward at an air gap, which makes the effective gap area larger than the core cross-section.

EEE-169 · How does a magnetic circuit compare with an electric circuit?

MMF corresponds to EMF, flux to current and reluctance to resistance. Unlike current, flux needs no energy to maintain, and leakage is much larger than in electric circuits.

EEE-170 · What is the energy stored in a magnetic field per unit volume?

w = B²/(2μ) = ½ B H J/m³. Most energy in a magnetic circuit with an air gap is stored in the gap.

EEE-171 · What is the difference between soft and hard magnetic materials?

Soft materials (silicon steel, ferrite) have narrow hysteresis loops and low coercivity, suiting cores. Hard materials (NdFeB, ferrite magnets, alnico) have high coercivity and retentivity, suiting permanent magnets.

EEE-172 · State Ampere's circuital law.

The line integral of H around a closed path equals the total current enclosed: ∮H·dl = I_enclosed.

EEE-173 · What is the force between two parallel current-carrying conductors?

F/l = μ₀ I₁ I₂/(2πd) N/m. Currents in the same direction attract and in opposite directions repel.

EEE-174 · Why does a rotating magnetic field form in a three-phase winding?

Three windings displaced 120° in space carry currents displaced 120° in time. Their MMFs add to a constant-magnitude field (1.5 times the peak of one phase) rotating at synchronous speed.

EEE-175 · What is the lifting force of an electromagnet?

F = B² A/(2μ₀) newtons per pole face, where A is pole-face area and B air-gap flux density.

Transformers

EEE-176 · What are the losses in a transformer?

Iron (core) losses, made of hysteresis and eddy current losses, which are roughly constant at fixed voltage and frequency. Copper losses, which vary with the square of load current (I²R).

EEE-177 · Why are transformer cores laminated?

Thin insulated laminations raise the resistance of eddy current paths, reducing eddy current losses, which vary with the square of lamination thickness.

EEE-178 · What does the open-circuit test of a transformer determine?

Performed at rated voltage on the LV side with HV open, it gives core loss and the magnetizing branch parameters R₀ and X₀.

EEE-179 · What does the short-circuit test of a transformer determine?

Performed at reduced voltage on the HV side with LV shorted until rated current flows, it gives full-load copper loss and equivalent impedance R_eq and X_eq.

EEE-180 · At what load does a transformer have maximum efficiency?

When copper loss equals iron loss. The load fraction is x = √(P_iron/P_cu,FL).

EEE-181 · What is all-day efficiency?

The ratio of energy output to energy input over 24 hours. It matters for distribution transformers, which are energized continuously but lightly loaded much of the time.

EEE-182 · What conditions are needed to parallel two transformers?

Same voltage ratio, same polarity, same phase sequence and vector group (for three-phase), and similar per-unit impedance with a similar X/R ratio for proper load sharing.

EEE-183 · What is inrush current in a transformer?

A large transient magnetizing current when a transformer is energized, caused by core flux offset driving the core into saturation. It can reach several times rated current and is rich in second harmonic.

EEE-184 · What is the vector group of a transformer?

A code showing winding connections and phase displacement, such as Dyn11: HV delta, LV star with neutral, LV leading HV by 30° (11 o'clock position).

EEE-185 · Why is the no-load current of a transformer so small?

The core has high permeability, so very little magnetizing current is needed. It is typically a few percent of rated current and lags voltage by nearly 90°.

EEE-186 · How does secondary load current affect the primary?

Secondary current produces an MMF that would reduce core flux. The primary draws extra current so that I₁′N₁ = I₂N₂, keeping the flux nearly constant.

EEE-187 · How are transformers cooled?

Using codes such as ONAN (oil natural, air natural), ONAF (forced air), OFAF (forced oil and air) and OFWF (forced oil and water), chosen by rating.

EEE-188 · What is the percentage impedance of a transformer?

The percentage of rated voltage needed on one winding to circulate rated current with the other shorted. It sets fault level and voltage regulation.

DC Machines

EEE-189 · What are the conditions for voltage build-up in a self-excited DC generator?

Residual magnetism in the poles, field MMF aiding residual flux, field resistance below critical resistance and speed above critical speed.

EEE-190 · What is critical field resistance?

The field circuit resistance at which the field resistance line is tangent to the linear part of the open-circuit characteristic. Above it, the generator will not build up voltage.

EEE-191 · How is the speed of a DC shunt motor controlled?

By flux (field) control for speeds above base, armature voltage or resistance control for speeds below base, or by a controlled converter supply.

EEE-192 · Why must a DC series motor never be started without load?

At no load, armature current and flux are very small. Since N ∝ 1/Φ, speed rises to dangerous levels and can destroy the armature.

EEE-193 · What is Swinburne's test?

A no-load test on a DC shunt machine run as a motor to find constant losses, from which efficiency at any load is calculated. It cannot be used on series motors.

EEE-194 · What is the Hopkinson test?

A regenerative back-to-back test on two identical DC machines, one as motor and one as generator, which supplies only the losses and allows full-load testing.

EEE-195 · What is the condition for maximum efficiency of a DC machine?

Variable copper loss equals constant loss (core, friction and shunt field losses).

EEE-196 · What are compensating windings?

Windings in slots in the main pole faces, connected in series with the armature. They neutralize armature reaction under the poles, preventing flashover in large machines with heavy or fluctuating loads.

EEE-197 · What causes sparking at the brushes?

Poor commutation from reactance voltage, armature reaction shifting the neutral axis, brush misalignment, worn or rough commutator, or wrong brush grade.

EEE-198 · How does a DC shunt motor's speed vary with load?

Speed falls slightly from no load to full load (about 5–10%) because of the I_a R_a drop, making it nearly constant-speed.

EEE-199 · Why is a differential compound motor rarely used?

Series flux opposes shunt flux, so heavy load weakens flux and can cause instability, runaway speed or reversal on starting.

EEE-200 · How is power developed in a DC motor's armature?

Mechanical power developed P_m = E_b I_a = T ω. Output power is P_m minus rotational losses.

Induction Machines

EEE-201 · Why can't an induction motor run at synchronous speed?

At synchronous speed there is no relative motion between rotor and field, so no EMF is induced in the rotor, no current flows and no torque is produced. Slip is always greater than zero in motoring.

EEE-202 · How is rotor copper loss related to air-gap power?

P_cu,rotor = s P_ag and mechanical power P_m = (1 − s) P_ag, so P_ag : P_cu : P_m = 1 : s : (1 − s).

EEE-203 · At what slip does maximum torque occur?

At s_max = R₂/X₂ (rotor quantities referred to stator, neglecting stator impedance). Maximum torque is independent of rotor resistance.

EEE-204 · How does rotor resistance affect the torque-slip curve?

Higher rotor resistance increases the slip at which maximum torque occurs and raises starting torque, without changing the maximum torque value.

EEE-205 · How does induction motor torque vary with supply voltage?

Torque at a given slip is proportional to V². A 10% voltage drop reduces torque by about 19%.

EEE-206 · What are the no-load and blocked-rotor tests?

The no-load test at rated voltage gives core, friction and windage losses and magnetizing reactance. The blocked-rotor test at reduced voltage and rated current gives equivalent resistance and leakage reactance.

EEE-207 · How is induction motor speed controlled?

Variable frequency (V/f) control, pole changing, stator voltage control, rotor resistance control and slip power recovery for slip-ring motors.

EEE-208 · What is a double-cage rotor?

A rotor with an outer high-resistance, low-reactance cage for high starting torque and an inner low-resistance, high-reactance cage for efficient running.

EEE-209 · What is cogging in an induction motor?

The failure of a motor to start because rotor and stator slot numbers lock magnetically. It is avoided by choosing suitable slot combinations.

EEE-210 · What is crawling in an induction motor?

Stable running at a low speed (around one-seventh of synchronous) caused by harmonic torques, especially from the seventh space harmonic. Skewed rotor slots reduce it.

EEE-211 · Why are rotor slots skewed?

To reduce magnetic locking (cogging), noise and harmonic torques, giving smoother starting and running.

EEE-212 · Why does an induction motor have a low power factor at light load?

The magnetizing current needed for the air gap remains nearly constant while active current is small, so current lags voltage heavily at light load.

EEE-213 · Why is the air gap in an induction motor kept small?

A smaller gap reduces magnetizing current, improving power factor and efficiency. It is limited by mechanical clearance and noise.

Synchronous Machines

EEE-214 · What are the V-curves of a synchronous motor?

Plots of armature current against field current at constant load. The minimum current occurs at unity power factor; under-excitation gives lagging PF and over-excitation leading PF.

EEE-215 · How is the power output of a synchronous machine expressed?

P = (E V/X_s) sin δ per phase, neglecting resistance, where δ is the load angle. Maximum (steady-state) power occurs at δ = 90°.

EEE-216 · What is the load angle?

The angle δ between the excitation EMF E and terminal voltage V. It increases with load and determines the power transferred.

EEE-217 · What is short-circuit ratio (SCR)?

The ratio of field current for rated open-circuit voltage to field current for rated armature short-circuit current. It approximately equals 1/X_d in per unit.

EEE-218 · How is voltage regulation of an alternator found?

By the EMF (synchronous impedance) method, MMF method, zero power factor (Potier) method or ASA method. The EMF method tends to be pessimistic and the MMF method optimistic.

EEE-219 · What is the effect of changing excitation on an alternator connected to an infinite bus?

Active power stays the same (set by the prime mover) but reactive power changes. Over-excitation supplies lagging VARs; under-excitation absorbs them.

EEE-220 · What is the effect of increasing prime mover input on a grid-connected alternator?

Load angle δ increases and the machine delivers more active power, while frequency remains fixed by the infinite bus.

EEE-221 · How does armature reaction depend on power factor in an alternator?

At unity PF it is mainly cross-magnetizing, at lagging PF it is demagnetizing and at leading PF it is magnetizing.

EEE-222 · What are the methods of synchronizing alternators?

Dark-lamp method, bright-lamp (two bright, one dark) method and the synchroscope, with modern auto-synchronizers checking voltage, frequency and phase.

EEE-223 · What are pitch factor and distribution factor?

Pitch factor K_p = cos(α/2) for a short-pitched coil with chording angle α. Distribution factor K_d = sin(mβ/2)/(m sin(β/2)) for m slots per pole per phase with slot angle β.

EEE-224 · Why are short-pitched coils used in alternators?

They reduce specific harmonics in the EMF waveform, save copper in end connections and slightly reduce the fundamental EMF.

EEE-225 · What are the direct-axis and quadrature-axis reactances?

In salient-pole machines, X_d acts along the pole axis where the air gap is small and X_q acts between poles where the gap is large, so X_d > X_q.

Power Generation

EEE-226 · How does a governor control generator frequency?

A governor senses speed and adjusts steam, gas or water input. With droop, a rise in load causes a proportional drop in frequency, letting units share load in proportion to their ratings.

EEE-227 · What is speed droop?

Droop = (no-load speed − full-load speed)/rated speed × 100%. A typical droop of around 4–5% lets parallel units share load changes stably.

EEE-228 · What is the function of a surge tank in a hydro plant?

It absorbs pressure rises (water hammer) when turbine gates close quickly and supplies water when gates open suddenly.

EEE-229 · How are hydro turbines selected by head?

Pelton (impulse) for high heads, Francis (reaction) for medium heads, and Kaplan or propeller (reaction) for low heads with large flow.

EEE-230 · What is the function of an economizer and air preheater in a boiler?

The economizer heats feed water and the air preheater heats combustion air, both using flue gas heat, improving boiler efficiency.

EEE-231 · What is a moderator in a nuclear reactor?

A material such as light water, heavy water or graphite that slows fast neutrons to thermal energies so they are more likely to cause fission in U-235.

EEE-232 · What is the function of control rods?

Control rods, made of neutron absorbers such as boron or cadmium, regulate the chain reaction and shut down the reactor by absorbing neutrons.

EEE-233 · What is maximum power point tracking (MPPT)?

A control technique that continuously adjusts the PV array's operating voltage to extract maximum power as irradiance and temperature change. Common algorithms are perturb-and-observe and incremental conductance.

EEE-234 · How do temperature and irradiance affect a PV module?

Short-circuit current rises roughly in proportion to irradiance. Rising temperature lowers open-circuit voltage noticeably, so power output falls in hot conditions.

EEE-235 · What is the heat rate of a power plant?

Heat input per unit of electrical output, in kJ/kWh. Efficiency = 3600/heat rate, so a lower heat rate means higher efficiency.

EEE-236 · What is a DFIG wind generator?

A doubly-fed induction generator with its stator on the grid and rotor fed through a back-to-back converter rated at a fraction of machine power, allowing variable-speed operation.

EEE-237 · What is spinning reserve?

Generating capacity that is online and synchronized but unloaded, able to pick up load within seconds to minutes if a unit trips.

Transmission & Distribution

EEE-238 · What are the ABCD constants of a transmission line?

They relate sending and receiving quantities: V_S = A V_R + B I_R and I_S = C V_R + D I_R. For a reciprocal network, AD − BC = 1; for a symmetrical one, A = D.

EEE-239 · What is surge impedance?

Z_c = √(L/C) for a lossless line, typically about 400 Ω for overhead lines and 40–60 Ω for cables.

EEE-240 · What is surge impedance loading (SIL)?

SIL = V_L²/Z_c. At SIL, reactive power generated by line capacitance equals that absorbed by line inductance, giving a flat voltage profile.

EEE-241 · What is transposition of transmission lines?

Exchanging conductor positions at regular intervals so each phase occupies each position for one-third of the length, balancing inductance and capacitance between phases.

EEE-242 · What are bundled conductors?

Two or more sub-conductors per phase held apart by spacers. They raise effective radius, lowering inductance, corona loss and radio interference.

EEE-243 · What is string efficiency of a suspension insulator?

String efficiency = voltage across the string ÷ (number of discs × voltage across the disc nearest the conductor). It is less than 100% due to shunt capacitance to the tower.

EEE-244 · How is string efficiency improved?

Using longer cross-arms, capacitance grading of discs, or a guard (grading) ring that equalizes voltage distribution.

EEE-245 · How is voltage controlled in a power system?

With generator excitation (AVR), on-load tap changers, shunt capacitors and reactors, synchronous condensers, SVCs and STATCOMs.

EEE-246 · Why is a shunt reactor used on long lines?

It absorbs reactive power generated by line capacitance at light load, limiting receiving-end overvoltage.

EEE-247 · What is series compensation?

Capacitors inserted in series with a line to cancel part of its inductive reactance, increasing power transfer capability and stability.

EEE-248 · What is the inductance per phase of a single-circuit three-phase line?

L = 2 × 10⁻⁷ ln(D_m/r′) H/m, where D_m is geometric mean distance and r′ = 0.7788 r is the GMR of a solid conductor.

EEE-249 · What is the purpose of a distribution transformer's delta-star connection?

The delta primary traps triplen harmonics and isolates zero-sequence currents. The earthed star secondary gives a neutral for single-phase loads.

EEE-250 · What is a ground (shield) wire on a transmission tower?

An earthed conductor strung above the phase conductors that intercepts lightning strokes and shields the phase wires.

Protection & Switchgear

EEE-251 · What is the inverse definite minimum time (IDMT) characteristic?

Operating time falls as current rises but never drops below a definite minimum at high currents. The IEC standard inverse curve is t = TMS × 0.14/((I/I_s)^0.02 − 1).

EEE-252 · What are plug setting multiplier and time multiplier setting?

PSM = fault current ÷ (relay pickup current referred to the primary). TMS scales the operating time curve vertically to achieve grading.

EEE-253 · How is an arc extinguished in an AC circuit breaker?

The breaker exploits the natural current zero. Arc-quenching media cool and deionize the gap so dielectric strength recovers faster than the restriking voltage rises.

EEE-254 · What is restriking voltage and recovery voltage?

Restriking voltage is the transient voltage across contacts just after current zero. Recovery voltage is the power-frequency voltage that appears after the transient dies away.

EEE-255 · Why is SF₆ used in circuit breakers?

SF₆ is electronegative, captures free electrons and has dielectric strength about 2–3 times that of air, giving fast arc quenching and compact switchgear. It is a potent greenhouse gas, so leakage must be controlled.

EEE-256 · Why are vacuum circuit breakers popular at medium voltage?

Vacuum has very high dielectric recovery, the arc is short and of metal vapour, contacts wear little and maintenance is minimal.

EEE-257 · What is distance (impedance) protection?

A relay measures V/I at its location. If the apparent impedance falls inside its set reach, a fault is inside the protected zone and it trips. Zones 1, 2 and 3 provide primary and backup protection.

EEE-258 · What is the Merz-Price circulating current scheme?

A differential scheme where matched CTs on both sides of the protected equipment circulate current through pilot wires. Only an internal fault produces spill current through the relay.

EEE-259 · What is restricted earth fault (REF) protection?

A differential scheme covering earth faults in a star winding, comparing the residual of phase CTs with a neutral CT. It is sensitive to faults near the neutral.

EEE-260 · What is auto-reclosing?

Automatic reclosing of a breaker after a trip on overhead lines, since most faults there are transient. If the fault persists, the breaker trips and locks out.

EEE-261 · What is the breaking capacity of a circuit breaker?

The maximum fault current, expressed in kA (symmetrical) or MVA, that the breaker can interrupt at rated voltage.

EEE-262 · What is a directional overcurrent relay?

An overcurrent relay that trips only when current flows in a set direction, using a voltage reference. It is used on ring mains and parallel feeders.

EEE-263 · What protections are used on a large generator?

Differential, stator earth fault, rotor earth fault, loss of excitation, negative phase sequence, reverse power, over/under frequency, overvoltage and backup impedance or overcurrent.

Power Electronics

EEE-264 · How is an SCR turned on?

Gate triggering is the normal method. It can also turn on, often unwantedly, from forward breakover voltage, high dv/dt, temperature or light (in light-activated SCRs).

EEE-265 · What are the methods of SCR commutation?

Natural (line) commutation, where AC supply reverses current, and forced commutation using auxiliary circuits, classed as Class A to F.

EEE-266 · What is a snubber circuit?

A series RC network across a device that limits dv/dt and suppresses voltage spikes during switching. A series inductor limits di/dt.

EEE-267 · What is the average output voltage of a single-phase full converter?

V_dc = (2V_m/π) cos α for continuous conduction, where α is firing angle. For α > 90° it operates as an inverter.

EEE-268 · What is the average output of a three-phase fully controlled bridge?

V_dc = (3√2 V_L/π) cos α ≈ 1.35 V_L cos α, where V_L is RMS line voltage.

EEE-269 · What is the output of an ideal boost converter?

V_o = V_in/(1 − D) in continuous conduction, so output is always greater than input.

EEE-270 · What is the output of a buck-boost converter?

V_o = −D V_in/(1 − D). It can step up or down, with output polarity inverted.

EEE-271 · What is the difference between voltage source and current source inverters?

A VSI has a stiff DC voltage link (large capacitor) and outputs defined voltage. A CSI has a stiff DC current link (large inductor) and outputs defined current.

EEE-272 · What is sinusoidal PWM?

A sine reference is compared with a triangular carrier. The switching pulses give an output whose fundamental is proportional to the modulation index, with harmonics near the carrier frequency.

EEE-273 · What is dead time in an inverter leg?

A short delay between turning off one switch and turning on the complementary switch, preventing shoot-through of the DC link.

EEE-274 · How does a MOSFET compare with an IGBT?

MOSFETs switch faster and suit lower voltages and higher frequencies. IGBTs have lower conduction loss at high voltage and current but slower turn-off due to tail current.

EEE-275 · Why do diode rectifiers cause harmonics?

They draw current only near voltage peaks to charge the DC capacitor, producing a pulsed current rich in odd harmonics and a poor true power factor.

EEE-276 · What is the ripple factor of a rectifier?

Ripple factor = RMS of AC component ÷ DC value. For a single-phase full-wave rectifier with resistive load it is about 0.48; half-wave is about 1.21.

Electrical Measurements

EEE-277 · How is the range of an ammeter extended?

By connecting a low-value shunt in parallel: R_sh = R_m/(m − 1), where m is the multiplying factor.

EEE-278 · How is the range of a voltmeter extended?

By connecting a high-value multiplier resistance in series: R_s = R_m (m − 1).

EEE-279 · What is loading effect?

Error caused when an instrument draws power from the circuit, such as a low-resistance voltmeter across a high-resistance branch. Voltmeter sensitivity is given in Ω/V.

EEE-280 · What is a Kelvin double bridge used for?

Measuring low resistances (below about 1 Ω), eliminating errors from lead and contact resistance.

EEE-281 · What is the Maxwell bridge used for?

Measuring medium-Q inductance by comparing it with a standard capacitance and resistances.

EEE-282 · What is the Schering bridge used for?

Measuring capacitance and dielectric loss (tan δ) of insulation, especially at high voltage.

EEE-283 · What is the ratio error of a CT?

Ratio error = (K_n I_s − I_p)/I_p × 100%, where K_n is nominal ratio. It is caused by magnetizing and core loss current.

EEE-284 · Why must a CT secondary never be left open?

With no secondary MMF to oppose it, primary current drives the core into deep saturation, inducing dangerously high secondary voltage and overheating the core.

EEE-285 · How is earth resistance measured?

With a fall-of-potential tester using a current spike and a potential spike. The potential spike is typically placed at about 62% of the distance to the current spike.

EEE-286 · What is the polarization index test?

The ratio of 10-minute to 1-minute insulation resistance. A higher value indicates dry, clean insulation; a value near 1 suggests moisture or contamination.

EEE-287 · What is a digital storage oscilloscope used for in power work?

Capturing and analyzing voltage and current waveforms, transients, harmonics and switching behaviour, with isolated or differential probes for safety.

EEE-288 · What is the difference between average-responding and true-RMS meters?

Average-responding meters are calibrated for sine waves using form factor 1.11 and read wrongly on distorted waveforms. True-RMS meters compute RMS directly and are accurate for non-sinusoidal signals.

Electrical Safety, Wiring & Drives

EEE-289 · What is the purpose of the earth wire?

It gives fault current a low-resistance path back to the source, so protective devices operate quickly, and it keeps exposed metal near earth potential to protect people from shock.

EEE-290 · What are TN-S, TN-C-S and TT earthing systems?

TN-S has separate neutral and protective conductors from the source. TN-C-S combines them in the supply (PEN) and splits them at the installation. TT uses a local earth electrode independent of the supply earth.

EEE-291 · What is touch voltage and step voltage?

Touch voltage is between a hand on earthed metal and the feet during a fault. Step voltage is between two feet a step apart on the ground near a fault.

EEE-292 · How is cable size selected?

By current-carrying capacity after derating for ambient temperature, grouping and installation method, then checking voltage drop and short-circuit withstand.

EEE-293 · What is V/f control in a VFD?

Voltage is varied in proportion to frequency to keep air-gap flux constant, so the motor delivers rated torque over the speed range below base speed.

EEE-294 · What are the four quadrants of drive operation?

Forward motoring, forward braking, reverse motoring and reverse braking, defined by the signs of speed and torque.

EEE-295 · What is regenerative braking?

Braking where the motor acts as a generator and returns kinetic energy to the supply, possible when back EMF exceeds supply voltage or the converter can reverse power flow.

EEE-296 · What is dynamic (rheostatic) braking?

The motor is disconnected from supply and connected to a resistor. It acts as a generator and dissipates kinetic energy as heat.

EEE-297 · What is plugging?

Braking by reversing the supply to the motor (swapping two phases or reversing armature polarity), producing reverse torque. It is fast but causes high currents and heating.

EEE-298 · What is a soft starter?

A device using anti-parallel thyristors to ramp up motor voltage gradually, reducing starting current and mechanical shock. It does not vary frequency.

EEE-299 · What is IP rating?

Ingress Protection rating, such as IP65, where the first digit gives protection against solids and the second against water.

EEE-300 · What is the minimum approach distance principle?

Workers must stay beyond a defined distance from exposed live parts based on voltage, unless the equipment is isolated, earthed and proven dead.

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