Measurements and Instrumentation
Measurements and Instrumentation
This domain covers indicating instruments, bridges, instrument transformers, wattmeters and energy meters. Questions test both connections and error calculations.
Core rules
- PMMC instruments work on DC only and read average value; the scale is uniform. Moving-iron instruments read RMS and work on AC or DC. Electrodynamometer instruments are transfer instruments and are used in wattmeters.
- An ammeter has low resistance and is connected in series; a voltmeter has high resistance and is connected in parallel. Meter loading changes the circuit being measured.
- A multiplier extends voltmeter range: Rm = V/Ig − Rcoil. A shunt extends ammeter range.
- A CT secondary must never be open-circuited while primary current flows; a PT secondary must never be short-circuited.
- Wheatstone bridge measures resistance; Maxwell bridge measures inductance; Schering bridge measures capacitance and loss angle.
- Energy meter constant is expressed in revolutions per kWh.
Worked example
A PMMC movement has full-scale current 100 μA and coil resistance 1 kΩ. For a 50 V range, the multiplier is Rm = 50/(100×10^-6) − 1000 = 500,000 − 1000 = 499 kΩ. The multiplier dissipates almost all the voltage, so its power rating must be adequate.
Traps
Do not ignore the meter coil resistance. Do not connect a wattmeter current coil in parallel with the load. Do not open a live CT secondary. Do not treat a PMMC reading as RMS on AC.