02-03-2013, 11:27 AM
Power and Energy Measurements
Power and Energy.ppt (Size: 1.61 MB / Downloads: 96)
Power in DC circuits
Power
Can be carried out using a voltmeter and an ammeter (generally)
Two measurement arrangements
Wattmeters:
Dynamometer
Digital wattmeter
Thermal wattmeter
Hall-power meter
DC circuits
Ammeter measures current which flow into the voltmeter and load
Voltmeter measures voltage drop across the ammeter in addition to that dropping across the load
Dynamometer
Power (direct) measurement device for DC and AC systems
Accuracy better than 0,25 %
Two coils: static and movable
Torque is proportional product of current in current coil and current in voltage coil
Hall-power meter
Coil generates magnetic field which is proportional to load current
The sensor excitation current passes through R1 and is proportional to the load voltage
Hall voltage is proportional to load power
Problems: offset and linearity
Power in AC circuits
Instantaneous power (time dependence)
Mean power (usually the most interesting)
Real power (active work), reactive power, apparent power
Measures can be done same way as DC circuit (single-phase)
Line-Frequency Power Measurements
Polyphase Power Measurements
Three-phase systems are most commonly used in industrial applications
Energy and power generation and distribution
“Real power for consumer”
Reactive power also important (loading)
Power can measured several ways
Power factor
Symmetrical Power Systems Supplying Unbalanced Loads
Current amplitudes are different, and their relative phase is not equal 120°
Usually it is caused by some fault (short circuit)
Three- or two wattmeter arrangements (depends on neutral point)
Energy measurements
Automatic remote reading in future
Pricing
Controlling generation/loads
Several system under development (GSM, radio link, phone line…)
Energy meters also in var (reactive power) hours and volt-ampere (apparent power) hours