Govt. Exams
Entrance Exams
For two-wattmeter method in three-phase systems: tan(φ) = √3(W₁ - W₂)/(W₁ + W₂), used to calculate power factor.
Gauge factor = (ΔR/R₀)/strain; 2.1 = 5/(R₀ × 0.001); R₀ = 5/(2.1 × 0.001) ≈ 2381 Ω.
Kelvin's double bridge uses cross-connection to eliminate lead resistance errors when measuring very low resistances in industrial applications.
Using AC bridge balance: Z₁/Z₂ = Z₃/Z₄; therefore 100/50 = 200/Z₄, which gives Z₄ = 100 Ω
For unbalanced three-phase loads, three wattmeters are required—one in each phase—to measure total power accurately.
The voltmeter and resistor form a parallel circuit. Using voltage divider: V_true = 50 × (1000 + 100) / 1000 ≈ 54.55 V
Full-bridge phase-shift topology offers high efficiency (>97%), ZVS operation, and high isolation ratio suitable for high-power fast-charging applications.
In resonant DC-link inverters, the resonant frequency is designed to equal or track the switching frequency for zero-voltage switching.
For AC controller: Vo/Vi = √[(π-α+sin(2α)/2)/π] with α=45°≈0.785rad gives ≈0.642.
In a five-level inverter, maximum voltage across each switch is Vdc/4, compared to Vdc/2 in a two-level inverter, reducing stress by half.
