DC/AC Circuits and Measuring Instruments
DC/AC Circuits and Measuring Instruments
This domain checks DC circuit analysis and single-phase AC quantities, plus correct use of ammeters, voltmeters, wattmeters, energy meters, multimeters and clamp meters.
Core rules
- For a sine wave: V_rms = 0.707 × V_peak; V_avg = 0.637 × V_peak; form factor = 1.11; peak factor = 1.414.
- India uses 50 Hz; period T = 1/f = 20 ms.
- AC power: P = V_rms I_rms cos φ for single phase; power factor = cos φ.
- Impedance Z = √(R² + (X_L − X_C)²); X_L = 2πfL; X_C = 1/(2πfC).
- Ammeter in series, voltmeter in parallel, wattmeter current coil in series and voltage coil in parallel.
- PMMC meters read average and suit DC; moving-iron meters suit AC RMS.
Worked example
A 230 V RMS, 50 Hz supply feeds a 2 kW resistive heater. I = P/V = 2000/230 = 8.70 A. The peak voltage is 230 × 1.414 = 325 V, but power calculations use RMS values. For a 10 A load with power factor 0.8, P = 230 × 10 × 0.8 = 1840 W, while apparent power S = 2300 VA.
Traps that cost marks
Using peak voltage in P = VI; treating VA as W at unity power factor only; connecting a clamp meter around both live and neutral so fields cancel; reading a DC meter on AC; ignoring power factor in motor and inverter loads.