Electrical, Electronics & Telecommunication Engineering - IESL Part II
IESL Part II Electrical, Electronics & Telecommunication Engineering examines the whole discipline breadth in a single paper: three-phase circuits, electrical machines, power systems, power electronics, control, analogue and digital electronics, measurement, and communication. Expect short numericals that jump between per-phase, per-unit and load-scaled reasoning. The formulae are elementary; the marks are lost in assumptions and units.
Core rules
- Balanced three-phase: S = √3·V_L·I_L and P = √3·V_L·I_L·cos φ; solve one phase and multiply by three.
- Star–delta: Z_Y = Z_Δ/3; V_L = √3·V_ph applies only in star, I_L = √3·I_ph only in delta.
- Per-unit: convert an impedance by (V_base,old/V_base,new)² × (S_base,new/S_base,old).
- Machines: N_s = 120f/P, slip s = (N_s − N)/N_s, DC machine E = kΦω.
- Losses: core loss is fixed with load; copper loss scales as (load fraction)².
- Sri Lankan practice: 50 Hz, 400/230 V LV, 11 kV and 33 kV distribution, 132 kV and 220 kV transmission.
Worked example
A 100 kVA transformer has 1.2 kW core loss and 2.0 kW full-load copper loss. At full load, 0.8 pf lagging: output = 100 × 0.8 = 80 kW; losses = 1.2 + 2.0 = 3.2 kW; η = 80/83.2 = 96.2%. Maximum efficiency occurs where copper loss equals core loss, at load fraction k = √(1.2/2.0) = 0.775, i.e. 77.5 kVA; output there is 77.5 × 0.8 = 62 kW and η = 62/(62 + 2.4) = 96.3%.
Traps that cost marks
- Entering kVA as kW in the efficiency fraction.
- Scaling copper loss linearly with load instead of with its square.
- Applying √3 twice to a quantity already expressed per phase.
- Mixing rms and peak values in AC power; P uses rms.
- In communication items, taking modulation index for depth, or forgetting Carson's rule B = 2(Δf + f_m) for FM.