Electrical Engineering: Power, Machines and Installation Practice
Electrical Engineering covers circuit theorems, AC single- and three-phase power, transformers, induction and synchronous machines, transmission and distribution, protection and installation standards. The Sri Lankan context matters: a 50 Hz system, 230 V phase and 400 V line at utilisation, 220 kV and 132 kV transmission, the Ceylon Electricity Board as transmission licensee, LECO and other distribution licensees, all regulated by the Public Utilities Commission of Sri Lanka.
Power and machines
- Single phase: P = VI cos φ. Balanced three phase: P = √3 V_L I_L cos φ; in star V_L = √3 V_ph, in delta I_L = √3 I_ph.
- Transformer EMF: E = 4.44 f N Φm. Efficiency peaks where copper loss equals iron loss.
- Synchronous speed Ns = 120f/P; slip s = (Ns − N)/Ns.
Worked example
A 100 kW load at 0.8 pf lagging is corrected to 0.95. tan φ1 = 0.750 and tan φ2 = 0.329, so kVAr = P(tan φ1 − tan φ2) = 100 × 0.421 = 42.1 kVAr. The kW stays the same during correction; only kVA and current fall, which is why the cable and switchgear rating improves.
Protection and installation
- Discrimination: the downstream device must clear a fault before the upstream device operates.
- 30 mA RCDs protect socket circuits; earthing resistance and equipotential bonding matter as much as cable size.
- Cable sizing must satisfy current-carrying capacity, voltage drop (commonly ≤ 4% at the terminals) and fault withstand (k²S² ≥ I²t).
Traps
- Applying √3 formulas to single-phase loads.
- Confusing kW with kVA when finding current.
- Ignoring motor efficiency: input current follows output power divided by efficiency and power factor.
- Mixing line and phase quantities for a delta-connected load.