Electrical and electronics engineering interviews test circuit fundamentals, machines, power systems, control and embedded systems. These questions suit campus placements, PSU interviews and roles in power, automation and electronics design.
Fundamentals, definitions and simple scenarios. Good for freshers and warm-ups.
Ohm's law: the current through a conductor is proportional to the voltage across it, V = IR, at constant temperature. Kirchhoff's current law: the sum of currents entering a node equals the sum leaving it, based on conservation of charge. Kirchhoff's voltage law: the sum of voltage rises and drops around any closed loop is zero, based on conservation of energy. Together they let you analyse any circuit.
With AC, transformers can easily step voltage up for transmission and down for use. Transmitting at high voltage means lower current for the same power, which greatly reduces I²R losses in the lines. AC generators and induction motors are also simple and robust. High-voltage DC is still used for very long distances, undersea cables and links between grids that are not synchronised.
A transformer transfers electrical energy between two windings by mutual induction. AC in the primary winding creates a changing magnetic flux in the core, which induces a voltage in the secondary. The voltage ratio equals the turns ratio, V1/V2 = N1/N2. It only works with AC, because a steady DC current produces no changing flux. Its losses are copper losses in the windings and core losses (hysteresis and eddy currents).
A fuse contains a wire that melts and breaks the circuit when the current is too high, and must be replaced after operating. A miniature circuit breaker trips (a thermal element for overloads, a magnetic element for short circuits) and can be reset. An RCCB is different again: it detects earth leakage current and protects people from electric shock.
Applied problems, trade-offs and questions about your own projects.
Power factor is the ratio of real power (kW) to apparent power (kVA), equal to cos φ for sinusoidal supply. A low power factor, usually caused by inductive loads like motors, means more current is drawn for the same useful power, increasing losses and requiring bigger cables and transformers, and utilities may charge penalties. It is improved with capacitor banks, synchronous condensers or active power factor correction.
The three-phase supply in the stator windings creates a rotating magnetic field at synchronous speed, Ns = 120f/P. This field cuts the rotor conductors and induces currents in them, and the interaction produces torque that turns the rotor in the same direction. The rotor always runs slightly slower than synchronous speed, because if it matched, no current would be induced; this difference is called slip.
A PID controller computes its output from the error between the setpoint and the measured value. The proportional term reacts to the current error, but on its own can leave a steady-state error. The integral term adds up past error and removes that steady-state error, but can cause overshoot. The derivative term reacts to how fast the error is changing and damps the response. The three gains are tuned, for example with the Ziegler-Nichols method.
A microprocessor is mainly a CPU and needs external memory and peripherals, which suits general-purpose computing like PCs. A microcontroller puts the CPU, RAM, flash memory, timers, ADCs and input/output ports on a single chip, so it is cheaper, smaller and lower-power, which suits embedded systems such as appliances, sensors and controllers. Examples are the 8051, AVR and ARM Cortex-M families.
System design, deep internals, leadership and tough follow-ups.
Overcurrent and earth-fault relays for feeders; distance protection for transmission lines; differential protection for transformers, generators and busbars, which compares current entering and leaving the zone; and the Buchholz relay for internal faults in oil-filled transformers. Relays trip circuit breakers, and their settings are coordinated so the device nearest the fault operates first, keeping the rest of the system running.
A programmable logic controller runs a continuous scan cycle: it reads all the inputs, executes the user program (commonly written in ladder logic), then updates all the outputs, typically in milliseconds. PLCs are rugged and reliable for industrial automation, such as conveyors, pumps and packaging lines. SCADA systems sit above them to monitor and supervise many PLCs, with HMIs for operators.
Earthing provides a low-resistance path to the ground for fault current. If a live wire touches a metal enclosure, a large fault current flows through the earth conductor so the fuse, MCB or RCCB operates quickly, and the voltage on the enclosure stays low enough to be safe to touch. System earthing also stabilises the voltage relative to ground and protects equipment from surges and lightning.
An interrupt is a signal that pauses the main program so the processor can immediately run an interrupt service routine (ISR), then return to where it left off. Interrupts can come from hardware, such as a timer, a button or data arriving on a UART, or from software. Good practice is to keep ISRs very short, declare shared variables as volatile, protect shared data, set priorities, and debounce mechanical inputs.