Electrical Engineering: Machines, Power Systems and Measurements
What the electrical paper tests
KPTCL AE/JE Electrical questions concentrate on network laws, three-phase power, transformers, rotating machines, transmission and distribution, protection, and measurements. Answers often hinge on choosing line or phase quantities and on rated versus actual operating conditions.
Core rules
- KCL and KVL with phasors; impedance in series/parallel; power triangle P=VI cosφ, Q=VI sinφ, S=VI.
- Balanced three-phase: V_L=√3 V_ph, I_L=I_ph in star; I_L=√3 I_ph in delta; P=√3 V_L I_L cosφ.
- Transformer: E=4.44 f N Φ_m; OC test gives core loss, SC test gives copper loss; regulation depends on load power factor.
- Induction motor: N_s=120f/P; slip s=(N_s−N)/N_s; rotor frequency sf.
- Transmission: surge impedance √(L/C); Ferranti effect on light-load long lines; corona and skin effect increase effective resistance.
Worked example
A balanced 400 V, three-phase load draws 50 A at 0.8 lagging. P=√3×400×50×0.8=27,713 W≈27.7 kW. Q=√3×400×50×0.6=20.8 kvar. The 400 V is line voltage; do not multiply by 3 unless phase voltage and phase current are both used.
Traps
Mixing line and phase values in delta; treating transformer OC test as copper loss; using synchronous speed instead of rotor speed in slip; forgetting lagging power factor makes regulation larger; and ignoring CT ratio when calculating primary current.