Electrical Engineering
PSPCL JE Electrical questions typically test circuit laws, single- and three-phase AC quantities, transformer and induction-motor basics, and power-system units. Most items are short: either a one-line conceptual distinction or a two-step numerical. Marks usually hinge on phase conventions, RMS values, and correct units rather than long derivations.
Core rules and formulas
- Ohm's law: V = IR. Series resistances add; for parallel branches use 1/R_p = Σ(1/R_i). KCL: ΣI_in = ΣI_out at a node. KVL: ΣV = 0 around a closed loop.
- DC power: P = VI = I²R = V²/R. Single-phase AC: P = VI cosφ, Q = VI sinφ, S = VI. Three-phase: P = √3 V_L I_L cosφ, where V_L is line voltage.
- Star connection: V_L = √3 V_ph, I_L = I_ph. Delta connection: V_L = V_ph, I_L = √3 I_ph.
- Transformer EMF: E = 4.44 f N Φ_m; turns ratio V1/V2 ≈ N1/N2; frequency is unchanged.
- Induction motor: N_s = 120f/P; slip s = (N_s − N_r)/N_s; rotor frequency f_r = s f.
Worked example
A 4-pole, 50 Hz, three-phase induction motor runs at 1440 r/min. N_s = 120 × 50 / 4 = 1500 r/min. s = (1500 − 1440)/1500 = 60/1500 = 0.04 = 4%. f_r = 0.04 × 50 = 2 Hz. If the motor is star-connected to a 400 V line, phase voltage is 400/√3 ≈ 231 V.
Traps
- Substituting peak voltage into P = VI cosφ instead of RMS.
- Omitting √3 in three-phase power or using phase voltage as line voltage.
- In delta, treating line current as equal to phase current.
- Using synchronous speed as actual rotor speed.
- Applying transformer turns ratio to frequency; frequency does not change.
- Mixing kW and kVA without the power factor.