Measurements and Instrumentation
Measurements and Instrumentation tests indicating, recording and integrating instruments, bridges, transducers and instrument transformers. The exam favors principle-based questions and simple bridge-balance numericals.
Instruments
- PMMC instruments respond to the average value and are used for DC; moving-iron instruments respond to RMS and suit AC/DC.
- Electrodynamometer instruments measure AC/DC power; induction energy meters integrate power over time and are calibrated by the meter constant.
- Ammeters are connected in series and have low resistance; voltmeters are connected in parallel and have high resistance.
- CTs and PTs isolate and scale high currents and voltages; burden must not exceed the rated value.
Bridges and transducers
- Wheatstone bridge measures resistance; Maxwell bridge measures inductance; Schering bridge measures capacitance and loss angle.
- At balance, the detector current is zero and the ratio of opposite arms is equal.
- LVDT converts displacement to AC voltage; strain gauges use resistance change; thermocouples use the Seebeck effect.
Worked example
A Wheatstone bridge has R1 = 100 Ω, R2 = 200 Ω and R3 = 50 Ω, with R4 unknown. At balance R1/R2 = R3/R4, so R4 = R2R3/R1 = 200 × 50/100 = 100 Ω. If the bridge is unbalanced, the galvanometer deflects.
Traps that cost marks
Assuming a PMMC meter reads RMS on AC-it reads zero average-and confusing bridge balance ratios are common. Also check whether the unknown is resistance, inductance or capacitance before choosing a bridge. In energy meters, the meter constant is revolutions per kWh, not kWh per revolution.