Chapter 14: Electrical and Electronic Technology

Electrical power systems

Electricity is generated (hydropower, thermal, wind, solar) and transmitted at high voltage through transmission lines to substations, then distributed at lower voltage to consumers. In Sri Lanka, electricity is supplied at a nominal 230 V (single phase) and 400 V (three phase) at 50 Hz. A three-phase system has three AC voltages displaced by . In a star connection, line voltage , so a 400 V line supply gives 230 V across each phase. The power in a balanced three-phase load is

where is the power factor. Example: V, A, power factor 0.8 gives kW.

Electrical machines

Transformers: , efficient (often above 95%), with losses in copper and iron.

DC motors and generators: easy speed control; used in vehicles and tools.

AC induction motors: the most common industrial motor. The synchronous speed is (rpm, with the number of poles); at 50 Hz with 4 poles, rpm. The slip ; at 1440 rpm, .

Single-phase motors (fans, pumps, appliances) and synchronous machines (alternators).

Wiring and protection

Domestic wiring includes the consumer unit with MCBs and an RCD, circuits for lighting (small current rating) and sockets (higher rating), earth conductors, switches, and sockets. The cable size must carry the current without overheating and with limited voltage drop. Protection: fuses and MCBs protect against overload and short circuit; RCDs protect against earth leakage; earthing and bonding limit touch voltage. Wiring must follow the national code of practice (Sri Lanka Standards Institution, n.d.) and be done by competent, certified people. Always isolate, lock off, test dead, and tag before working on any circuit.

Electronics

Semiconductor devices include diodes, transistors, thyristors, and integrated circuits. A bridge rectifier (four diodes) with a smoothing capacitor gives DC from AC; the average value of a full-wave rectified sine wave is (Boylestad & Nashelsky, 2013). Regulators hold the voltage steady. Transistors switch loads on and off and amplify signals. Operational amplifiers use negative feedback: an inverting amplifier has gain . The 555 timer makes oscillators and delays.

Digital electronics uses logic gates (AND, OR, NOT, NAND, NOR, XOR), flip-flops, counters, and registers; microcontrollers and programmable logic controllers (PLCs) run industrial automation. Communication technologies include radio, optical fibre, and wireless networks.

Instrumentation and control

Sensors convert physical quantities to electrical signals: thermocouples and RTDs (temperature), strain gauges (force), LVDTs (position), pressure transducers, photodiodes, proximity sensors. An analogue-to-digital converter (ADC) turns a signal into numbers for a computer. Control systems may be open-loop (no feedback) or closed-loop (feedback corrects errors). A PID controller computes

combining proportional, integral, and derivative action on the error . Simple on–off control (a thermostat) is common.

Renewable energy

Solar photovoltaic (PV): Example sizing: a daily load of 6 kWh, with 5 peak-sun-hours per day and 25% system losses, needs an array of about kWp.

Wind: power in the wind .

Mini-hydro: ; for , m, and , kW.

Biogas and biomass (Chapter 20).

Energy storage: batteries, pumped storage.

Energy efficiency saves money and the environment: LED lighting uses far less energy than incandescent lamps, motors can be fitted with variable-speed drives, and power-factor correction reduces losses.

Common mistakes

Ignoring the power factor in AC power calculations.

Working on equipment that is not isolated and locked off.

Using a wire with too small a cross-section for the load.

Practice questions

Find the line current of a three-phase 400 V motor taking 11 kW at power factor 0.85. [≈18.7 A]

A 6-pole induction motor runs at 50 Hz and 960 rpm. Find the slip. [4%]

A mini-hydro scheme has , m, and . Find the output power. [≈20.6 kW]