Measurements, Instrumentation and Control Systems
Measurements, Instrumentation and Control Systems tests analog and digital instruments, bridges, transducers, error analysis, and feedback control.
Core rules
- PMMC responds to average DC and has a uniform scale; moving iron responds to RMS and works on AC/DC; electrodynamometer is used in wattmeters and transfer instruments.
- A wattmeter has a current coil (low resistance, in series) and pressure coil (high resistance, in parallel). Energy meter measures kWh; creeping is unwanted rotation at no load.
- CT and PT scale current and voltage for measurement/protection; burden must be within rating.
- Wheatstone bridge measures medium resistance; Kelvin bridge low resistance; Maxwell bridge inductance; Schering bridge capacitance and loss.
- Feedback reduces sensitivity to disturbances and widens bandwidth but can cause instability. PID: proportional to error, integral removes offset, derivative damps.
Worked example
An energy meter marked 10 rev/kWh makes 5 revolutions in 2 minutes. Energy=5/10=0.5 kWh. Time=2/60=1/30 h, so average power=0.5/(1/30)=15 kW. If true power were 12 kW, the meter would be fast.
Traps
PMMC cannot directly measure AC without a rectifier. Instrument errors are not the same as gross mistakes. Do not connect a CT secondary to an open circuit; do not confuse resolution with accuracy.