Electrical Engineering
Electrical Engineering in SSC JE tests basic circuit laws, DC and AC machines, transformers, power systems, measurements, and basic electronics. Numerical questions are usually short but sensitive to RMS and phase conventions.
Core rules
- For a DC shunt motor, back emf E_b = V − I_a R_a; armature current depends on load torque, not on supply voltage alone.
- In a balanced three-phase system, P = √3 V_L I_L cos φ. In star, V_L = √3 V_ph and I_L = I_ph; in delta, I_L = √3 I_ph and V_L = V_ph.
- Transformer emf per turn is constant: E1/N1 = E2/N2; for an ideal transformer, I1/I2 = N2/N1.
- For a sine wave, RMS = peak/√2; average over a half cycle = 2 × peak/π.
- Copper loss = I²R varies with load; iron loss is approximately constant at rated voltage and frequency.
- Power-factor correction reduces line current for the same active power; it does not reduce the active power consumed by the load.
Worked example
A 220 V DC shunt motor has armature resistance 0.5 Ω and draws an armature current of 20 A. Find back emf. E_b = V − I_a R_a = 220 − (20 × 0.5) = 220 − 10 = 210 V. The reasoning: the armature resistance drop subtracts from terminal voltage; the remainder is the back emf that limits armature current. If the question gives line current in a shunt motor, subtract field current before using I_a.
Traps
- Using line current as armature current in a shunt machine.
- Swapping line and phase quantities in star/delta.
- Treating peak value as RMS in power calculations.
- Assuming power-factor correction reduces the load's active power.
- Using transformer current ratio in the same direction as the turns ratio.