Interview Questions
Electrical Engineering
Interview Questions
11 categories · 55 questions · concise expert answers
Basic Electrical Engineering
Ohm's Law states that the current through a conductor is directly proportional to the voltage across it and inversely proportional to its resistance: V = IR. It applies only to linear, bilateral, and time-invariant elements at constant temperature. Limitations include inapplicability to non-linear devices (diodes, transistors), unilateral elements, and electrolytes where resistance varies with temperature and concentration.
EMF (Electromotive Force) is the total voltage generated by a source under no-load conditions — it represents the work done per unit charge by the source. Terminal voltage is the actual voltage available at the source's terminals under load. The difference is the voltage drop across the internal resistance of the source: V_terminal = EMF − I × r_internal.
KCL (Kirchhoff's Current Law) states that the algebraic sum of all currents entering and leaving a node is zero — conservation of charge. KVL (Kirchhoff's Voltage Law) states that the algebraic sum of all voltages around any closed loop is zero — conservation of energy. These two laws form the foundation for circuit analysis and are valid for all lumped-parameter circuits at any frequency.
DC (Direct Current) flows in one direction with constant magnitude, as produced by batteries or rectifiers. AC (Alternating Current) periodically reverses direction, typically as a sinusoidal waveform with defined frequency (50 Hz or 60 Hz). AC is preferred for power transmission due to easy voltage transformation via transformers; DC is used in electronics and high-voltage long-distance links (HVDC).
Power factor (PF) is the ratio of active (real) power to apparent power: PF = P/S = cos φ. A low power factor means more current is drawn for the same real power delivery, increasing I²R losses, requiring larger conductors, and reducing system efficiency. Utilities penalize industrial consumers for low PF; capacitor banks or synchronous condensers are used for power factor correction to bring it close to unity.
Electrical Machines
An electrical machine is an electromechanical device that converts energy between electrical and mechanical forms. Motors convert electrical energy to mechanical energy using the force on current-carrying conductors in a magnetic field (F = BIL). Generators do the reverse, using Faraday's law of electromagnetic induction — a conductor moving in a magnetic field generates an EMF proportional to the rate of flux change.
Faraday's Law states that an EMF is induced in a conductor whenever the magnetic flux linking it changes: e = −dΦ/dt. The negative sign (Lenz's Law) indicates the induced EMF opposes the change causing it. This principle is the basis for transformers, generators, and induction motors. The magnitude of induced EMF depends on the rate of flux change and the number of turns in the coil.
Core losses consist of hysteresis losses (due to repeated magnetization/demagnetization cycles, proportional to frequency and Bmax^1.6) and eddy current losses (due to induced currents in the iron core, proportional to f² and B²). They are minimized by using silicon steel laminations (thin, insulated sheets) which restrict eddy current paths, and by selecting core materials with low hysteresis coefficients and operating at appropriate flux densities.
Insulation class ratings define the maximum operating temperature the insulating materials can withstand continuously without significant deterioration. Classes range from A (105°C) to H (180°C) and beyond. Proper class selection ensures long machine life — operating beyond the rated temperature halves insulation life for approximately every 10°C rise (Montsinger's rule). Higher-class insulation enables more compact machine designs or extended service life.
Synchronous machines rotate at exactly synchronous speed (Ns = 120f/P) and require external DC excitation for the rotor field. They are used for precise speed control and power factor correction. Asynchronous (induction) machines run slightly below synchronous speed with a slip, and the rotor current is induced electromagnetically — no external excitation is needed. Induction motors are the most widely used industrial motors due to rugged construction and low cost.
Switchgear & Protection
Switchgear refers to the combination of electrical disconnect switches, fuses, and circuit breakers used to control, protect, and isolate electrical equipment. Its primary functions are: switching (connecting/disconnecting circuits), protection (isolating faulty sections to prevent damage), and measurement (through associated metering). Switchgear is essential for safe operation of power systems from generation to distribution.
A fuse is a one-time protective device that melts and breaks the circuit when current exceeds its rating — it must be replaced after operation. A circuit breaker is a reusable switching device that automatically trips on overcurrent or fault conditions and can be reset manually or automatically. Circuit breakers offer adjustable trip settings, can interrupt both overloads and short circuits, and provide remote operation capabilities not available in fuses.
A differential relay protects electrical equipment (transformers, generators, bus bars) by continuously comparing the current entering and leaving the protected zone. Under normal conditions, the algebraic sum of currents is zero (Kirchhoff's law). During an internal fault, the balance is disturbed; when the differential current exceeds a set threshold, the relay triggers the circuit breakers to isolate the faulty equipment quickly and selectively.
When a circuit breaker opens under load, an arc forms between contacts due to the ionized path and inductive energy. Arc quenching is the process of extinguishing this arc. Methods include: air blast (high-pressure air cools and displaces arc), oil (arc is quenched in oil which vaporizes and deionizes), SF₆ gas (superior arc-quenching properties), and vacuum (contacts open in vacuum where arc cannot sustain). SF₆ and vacuum breakers dominate modern HV applications.
A power system is divided into overlapping protection zones, each covering a specific piece of equipment (generator, transformer, bus bar, transmission line). Each zone has its own protective relays and circuit breakers. Overlapping zones ensure no unprotected areas exist; a fault at the boundary is detected by both adjacent zone protections. This arrangement provides selective fault isolation — only the faulted zone is disconnected, minimizing disruption to the rest of the system.
Power System
The per-unit (pu) system expresses electrical quantities as fractions of a chosen base value, eliminating the need to refer values through transformer turns ratios. It simplifies calculations in multi-voltage systems, allows direct comparison of machines of different ratings, and makes transformer impedances appear identical on both sides. Equipment nameplate impedances expressed in pu are typically similar regardless of size, simplifying system analysis.
Load flow analysis determines the steady-state voltages, currents, and power flows throughout a power network under specified loading conditions. It solves a set of nonlinear algebraic equations (power balance at each bus) using iterative methods such as Gauss-Seidel, Newton-Raphson, or fast-decoupled methods. Results are used for planning, operation, contingency analysis, and optimizing real and reactive power dispatch across the network.
Voltage stability is the ability of a power system to maintain acceptable voltage levels at all buses under normal and disturbed conditions. Voltage collapse occurs when reactive power demand exceeds supply capability — typically during heavy loading, reactive power shortage, or loss of reactive compensation. Beyond the critical loading point on the P-V curve (nose point), voltage cannot be maintained and cascading voltage collapse may occur, leading to widespread blackouts.
SCADA (Supervisory Control and Data Acquisition) is a control system architecture that collects real-time data from remote terminal units (RTUs) across the power network and transmits it to a central control center. Operators can monitor voltages, currents, breaker states, and equipment status, and can remotely operate switching devices. SCADA enables centralized monitoring, fault detection, energy management, and rapid system restoration after outages.
Economic dispatch is the process of allocating the required total power generation among available generating units to minimize the total fuel cost while satisfying system constraints (power balance, unit operating limits, network constraints). The optimal solution occurs when the incremental cost (λ) of all on-line generators is equal — this is the "equal incremental cost" criterion. Modern systems use EMS software with optimization algorithms to perform economic dispatch in real-time.
Power Generation
A thermal power plant burns fossil fuel (coal, oil, gas) to heat water in a boiler, producing high-pressure steam. The steam drives a turbine, which mechanically couples to an alternator (synchronous generator) to produce electricity. The Rankine cycle governs the thermodynamic process: heat addition (boiler), expansion (turbine), condensation, and pumping. Efficiency is typically 35–42%, limited by Carnot's theorem; supercritical plants achieve higher efficiency.
Hydroelectric plants convert potential energy of stored water into electrical energy. Water stored at height is directed through penstocks to hydraulic turbines (Pelton, Francis, or Kaplan, depending on head and flow). The turbine drives a synchronous generator. Output power = ρgQHη, where Q is flow rate, H is effective head, and η is efficiency. Hydro plants have fast response times, long life, and zero fuel costs, making them ideal for peak-load management.
An Automatic Voltage Regulator (AVR) maintains the generator terminal voltage at a set reference value by continuously adjusting the field excitation current. When load increases and terminal voltage tends to drop, the AVR increases excitation to boost voltage, and vice versa. It also controls reactive power output and helps the machine remain stable during system disturbances. Modern AVRs use closed-loop feedback with PID controllers and thyristor-based excitation systems.
Nuclear power plants produce large amounts of electricity from small amounts of fuel (uranium/plutonium) through fission, with zero direct CO₂ emissions during operation. They have high capacity factors (90%+), providing reliable baseload power independent of weather. Fuel costs are relatively low and stable. The main challenges include high initial capital cost, radioactive waste management, long construction timelines, and public perception regarding safety (though modern reactors have excellent safety records).
Load factor is the ratio of average load to maximum demand over a period; it indicates efficient use of installed capacity. A high load factor means more uniform loading and better economic return. Plant capacity factor is the ratio of actual energy generated to the maximum possible energy if the plant ran at full capacity continuously. It reflects overall plant utilization including outages. High capacity factors indicate efficient, well-maintained plants with good fuel and resource availability.
Transmission System
For a given power (P = VI), increasing voltage reduces current proportionally. Since line losses are P_loss = I²R, reduced current dramatically decreases resistive losses. High voltage transmission (110 kV to 765 kV) allows the same power to be carried with significantly smaller conductors, reducing material cost and line losses — typically to 2–5% of transmitted power. Step-up transformers at the sending end and step-down transformers at the receiving end make this economically feasible.
The Ferranti effect is the phenomenon where the receiving end voltage of a long transmission line is higher than the sending end voltage under no-load or light-load conditions. It occurs due to the charging current of the line's shunt capacitance flowing through the series inductance, creating a voltage rise. The effect is more pronounced for longer lines and higher voltages. Shunt reactors are connected to absorb the excessive reactive power and counteract this overvoltage condition.
FACTS (Flexible AC Transmission Systems) are power electronic devices that enhance the controllability and power transfer capacity of AC transmission systems. Key devices include SVC (Static VAR Compensator), STATCOM, TCSC (Thyristor-Controlled Series Capacitor), and UPFC (Unified Power Flow Controller). They provide fast, continuous control of voltage, impedance, and power flow, improving system stability, reducing losses, and allowing transmission corridors to be utilized closer to their thermal limits.
The skin effect is the tendency of AC current to concentrate near the outer surface of a conductor at higher frequencies, effectively reducing the conducting cross-sectional area and increasing resistance. It is caused by induced eddy currents opposing current flow at the conductor's center. At power frequency (50/60 Hz), the effect is moderate but significant for large conductors. ACSR (Aluminium Conductor Steel Reinforced) conductors exploit this by placing cheaper steel at the center where current density is lowest.
Surge Impedance Loading (SIL) is the power delivered by a transmission line when it is terminated by its characteristic (surge) impedance Zc = √(L/C). At SIL, the reactive power generated by line capacitance exactly equals that absorbed by line inductance, resulting in unity power factor at both ends and flat voltage profile. Lines loaded below SIL generate excess reactive power (capacitive); above SIL they absorb it. SIL is a useful reference for assessing a line's natural loading capability.
Distribution System
A radial distribution system has power flowing from one source to consumers along a single path — it is the simplest and cheapest design but has poor reliability since a fault isolates all downstream consumers. A ring (loop) system connects consumers in a closed loop from two ends of the feeder, providing an alternate supply path; a fault can be isolated without interrupting supply to healthy sections. Ring systems offer better reliability at higher cost and are used in urban and critical areas.
A distribution transformer steps down medium-voltage distribution (11 kV, 33 kV) to utilization voltage (415V/240V LV) for end consumers. They are typically oil-immersed or dry-type, mounted on poles or in substations. Standard ratings range from 25 kVA to 2500 kVA. They are designed for high efficiency at partial load (typically 70–80% loading in daily operation) and must comply with no-load and load loss standards to minimize energy waste in the distribution network.
Voltage regulation is the change in voltage at the consumer terminals as load varies, expressed as a percentage of full-load voltage: VR% = (V_no-load − V_full-load)/V_full-load × 100. In distribution systems, acceptable limits are typically ±6% to ±10% of nominal voltage. Poor voltage regulation causes equipment malfunction, reduced motor efficiency, and lighting flicker. Tap-changing transformers, shunt capacitors, and voltage regulators are used to maintain acceptable voltage profiles throughout the feeder.
A smart grid integrates digital communication technology, advanced sensors, and automation into the conventional power distribution infrastructure. It enables two-way communication between utilities and consumers, real-time monitoring of grid conditions, automatic fault detection and self-healing, integration of distributed generation (solar, wind), demand response programs, and accurate metering through AMI (Advanced Metering Infrastructure). Smart grids improve reliability, reduce losses, enable dynamic pricing, and facilitate the transition to renewable energy.
A Distribution Management System (DMS) is a software platform used by utilities to monitor, control, and optimize the performance of the distribution network. It integrates SCADA data with network models and advanced applications such as fault location, isolation and restoration (FLISR), volt/VAR optimization, load balancing, and outage management. DMS helps operators make real-time decisions, reduces outage duration, optimizes energy delivery, and coordinates the growing number of distributed energy resources on the network.
Industrial Electrical
A Variable Frequency Drive (VFD) is a power electronics device that controls the speed of an AC induction motor by varying the frequency and voltage of the power supply. Since motor speed is proportional to supply frequency (N = 120f/P), VFDs provide precise speed control without mechanical losses. Benefits include 20–50% energy savings on variable-torque loads (fans, pumps), reduced mechanical stress, soft starting, and protection against overloads — making them indispensable in modern industrial automation.
Earthing (grounding) provides a low-impedance path for fault currents to flow safely to ground, protecting personnel from electric shock and equipment from damage. It stabilizes system voltages relative to earth, enables protective relays to detect ground faults reliably, and reduces electromagnetic interference. Industrial earthing systems include equipment earthing (all metal enclosures connected to earth) and system earthing (neutral of supply transformer solidly or impedance-grounded). Earth resistance should typically be below 1Ω for effective protection.
A Programmable Logic Controller (PLC) is a ruggedized industrial digital computer designed to control manufacturing processes and machinery. It reads inputs from sensors (switches, proximity sensors, encoders), executes a user-written control program (ladder logic, structured text, function block diagram), and drives outputs (motors, solenoids, indicators). PLCs replaced relay-based control panels, offering easy reprogramming, self-diagnostics, communication capability, and high reliability in harsh environments.
IP (Ingress Protection) ratings, per IEC 60529, classify the degree of protection provided by an enclosure against solid particles and liquids. The first digit (0–6) indicates dust protection; the second (0–9) indicates water protection. For example, IP65 is fully dust-tight and jet-water resistant. Selecting appropriate IP ratings ensures equipment survives its operating environment — dusty foundries, outdoor substations, washdown areas, or explosive atmospheres — preventing premature failures and ensuring safety compliance.
Load shedding is the deliberate disconnection of pre-selected, non-critical loads when the available power supply is insufficient to meet total demand — preventing a complete system collapse. In industrial plants, an automatic load shedding scheme uses under-frequency or bus voltage as a trigger, disconnecting loads in priority order until balance is restored. Critical processes (safety systems, critical motors) are protected while auxiliary loads (HVAC, non-essential pumps) are shed first. It is essential for plants with captive generation or unreliable utility supply.
Electrical Circuits
Thevenin's theorem states that any linear two-terminal network can be replaced by an equivalent circuit consisting of a single voltage source (V_th) in series with a single impedance (Z_th). V_th is the open-circuit voltage at the terminals; Z_th is the impedance seen from the terminals with all independent sources deactivated. This greatly simplifies analysis of complex networks — especially useful when analyzing the effect of changing a single load element, as only the equivalent circuit needs recalculation.
Resonance in an RLC circuit occurs when the inductive reactance (XL = ωL) equals the capacitive reactance (XC = 1/ωC), giving a resonant frequency f₀ = 1/(2π√LC). At resonance in a series RLC circuit, impedance is minimum (purely resistive = R), and current is maximum. In a parallel RLC circuit, impedance is maximum and current is minimum. Resonance has applications in tuned filters, oscillators, and RF circuits, but can cause dangerously high voltages/currents if the Q factor is high.
The Superposition theorem states that in a linear network with multiple independent sources, the response (voltage or current) at any element equals the algebraic sum of responses caused by each source acting alone while all others are deactivated (voltage sources short-circuited, current sources open-circuited). It applies only to linear circuits and cannot be used to calculate power directly (since power is proportional to I², making it non-linear). It simplifies multi-source circuit analysis but requires n separate analyses for n sources.
The time constant (τ) of an RC circuit is τ = RC, where R is resistance (ohms) and C is capacitance (farads), giving τ in seconds. It represents the time required for the voltage or current to reach approximately 63.2% of its final value (or fall to 36.8% during discharge). After 5τ, the circuit is considered to have reached steady state (99.3%). The time constant governs the speed of charge/discharge and is critical in designing timing circuits, filters, and transient analyses.
Norton's theorem states that any linear two-terminal network can be replaced by an equivalent current source (I_N) in parallel with an impedance (Z_N). I_N is the short-circuit current at the terminals; Z_N equals Thevenin's impedance (Z_th = Z_N). The two theorems are duals — Thevenin uses a series voltage source, Norton uses a parallel current source — and are interconvertible: V_th = I_N × Z_N. Norton's equivalent is preferred for analyzing parallel circuits and current-driven systems.
DC Machines
A DC machine consists of a stationary stator (field system) with salient poles wound with field windings, and a rotating armature (rotor) with a cylindrical core carrying armature windings embedded in slots. The commutator (a segmented copper cylinder) with carbon brushes converts the alternating EMF induced in the armature into DC at the external terminals (generator) or converts DC supply to AC for armature operation (motor). The yoke provides a magnetic return path and mechanical support.
Back EMF (E_b) is the voltage induced in the rotating armature of a DC motor that opposes the applied supply voltage, per Lenz's Law: E_b = V − I_a × R_a. It is proportional to flux and speed: E_b = KΦN. Back EMF acts as the automatic speed-regulating mechanism — if load increases and speed drops, back EMF decreases, allowing more current to flow, which increases torque to restore speed. It also limits armature current during running and determines the motor's efficiency.
DC generators are classified by their field excitation method. Separately excited generators have field winding powered by an external source — giving independent voltage control. Self-excited generators use their own output: shunt generators have field winding in parallel with armature (constant voltage); series generators have field in series with armature (current-dependent voltage); compound generators combine shunt and series windings to achieve flat or rising voltage characteristics. Compound types are most common in practice for stable regulation.
Armature reaction is the distortion and weakening of the main magnetic field due to the magnetic field produced by armature current. It shifts the magnetic neutral axis (MNA) from the geometric neutral axis (GNA) in the direction of rotation (motor) or against it (generator), causing commutation problems and flux weakening that reduces torque and terminal voltage. Mitigation methods include interpoles (commutating poles) to neutralize armature flux at the commutating zone, and compensating windings embedded in the main pole faces for large machines.
DC motor speed (N ∝ E_b/Φ ∝ (V − I_a R_a)/Φ) can be controlled by three methods: (1) Armature voltage control — varying supply voltage to armature gives speed below base speed, widely used in Ward-Leonard systems and modern choppers; (2) Field flux control — weakening field by adding resistance in series with field winding raises speed above base speed; (3) Armature resistance control — inserting external resistance in armature circuit reduces speed but is inefficient due to I²R losses. Modern drives use thyristor or IGBT-based converters for efficient control.
AC Machines
Slip (s) is the difference between synchronous speed (Ns) and rotor speed (Nr), expressed as a fraction: s = (Ns − Nr)/Ns. It is necessary for induction motor operation — without slip, no relative motion between rotor and rotating field, no induced EMF, no rotor current, and no torque. At no-load, slip is very small (1–3%); at full load, slip is typically 3–8% depending on motor class. Slip determines rotor frequency (f_r = sf), rotor EMF, rotor reactance, and thus torque-speed characteristics.
Induction motors draw 5–8× full-load current at direct-on-line (DOL) starting. Starting methods to limit inrush current include: DOL (smallest motors only), Star-Delta starter (reduces voltage to 1/√3, reducing starting current and torque to 1/3), Auto-transformer starter (adjustable starting voltage, better torque), rotor resistance starter (wound-rotor motors — inserts external resistance to reduce starting current while maintaining torque), and modern VFDs (ramp up frequency/voltage gradually, providing smooth start with adjustable torque control).
A synchronous motor operates by locking its DC-excited rotor field to the rotating stator field, running exactly at synchronous speed. It is not self-starting because at standstill, the stator's rotating field moves at synchronous speed relative to the stationary rotor — the net average torque over one cycle is zero (torque alternates direction). Starting methods include pony motor starting, damper winding (amortisseur) starting (as induction motor until near synchronous speed, then DC excitation applied to pull-in), or VFD starting.
The V-curve of a synchronous motor is a plot of armature current (Ia) versus field excitation current (If) at constant mechanical load and terminal voltage, producing a V-shaped curve. At the bottom of the V (unity power factor), armature current is minimum for the given load. Under-excitation causes lagging PF (motor absorbs reactive power, acting inductively); over-excitation causes leading PF (motor supplies reactive power, acting capacitively). This property makes synchronous motors valuable for power factor correction in industrial systems.
Single-phasing occurs when one of the three supply phases to an induction motor is lost — due to a blown fuse, open contactor contact, or broken supply wire. A three-phase motor under single-phase supply cannot develop starting torque and will not start. If already running, it continues at reduced speed and erratic performance, with remaining two phases carrying √3 times their normal current. This causes rapid overheating of windings, insulation degradation, and motor burnout unless protective devices (phase failure relay, overload relay) detect and disconnect the motor quickly.
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