Electrical and Electronic Measurements
Electrical and Electronic Measurements tests indicating instruments, bridges, potentiometers, instrument transformers, wattmeters and electronic measurement. ESE asks definition-based numericals on loading, errors and rectifier-type meters.
Core rules
- PMMC responds to average value and has a uniform scale; moving iron responds to RMS and is usable on AC and DC.
- A rectifier-type meter is calibrated for the RMS of a sine wave; its actual movement is average. Form factor for sine is 1.11.
- Wheatstone bridge balances when R1/R2=R3/R4; Maxwell bridge measures inductance, Schering bridge capacitance and dissipation factor, Wien bridge frequency.
- CT secondary must never be open-circuited while primary current flows; a PT secondary must not be short-circuited.
- Wattmeter reading is P=VI cos(phi); the pressure coil and current coil connections determine the sign of deflection.
Worked example
A full-wave rectified sine with peak 10 V is read by a PMMC meter calibrated in RMS for sine. Average value=2Vm/pi=6.37 V. The meter multiplies by 1.11, so it indicates 7.07 V. The reasoning: the coil responds to average current, while the scale is drawn for sine RMS.
Traps
Assuming a PMMC meter reads true RMS on a distorted wave; forgetting that MI meters have a non-uniform scale; ignoring voltmeter loading; and opening a CT secondary. In bridge problems, check whether the detector is at null and whether the bridge is balanced for the stated component.