One of the first steps of solving an engineering challenge is to define the problem. When discussing energy, the potential challenges include reducing usage and environmental impact, improving grid uptime and efficiency, and protecting equipment. All of these call for the same approach: Measure it. Fix it.
Power is calculated from the measurements of voltage and current. So, if you want to know how much power it takes to run a cell phone charger, a residential air conditioning system, a manufacturing plant, or a neighborhood, you need to measure voltage and current. This article covers sensors and hardware for voltage and current measurements in AC systems as well as the calculations for power using NI LabVIEW software.
Waveform primer for AC voltages
Talking about voltage and current measurements can get a little confusing so here are some basics using voltage waveforms as an example. Waveforms can be expressed using a variety of descriptors:
- VPK—Voltage “peak” is a notation of the highest instantaneous potential measured over a waveform.
- VRMS—Voltage “RMS” is the root mean square of the waveform. Residential outlets around the world refer to the RMS measurement of the waveform. (120 VRMS or 240 VRMS)
- V (L-L), (L-N), or (L-E)—This notation is voltage specific as voltage is the potential between two points on a circuit. These letters denote the two points between which the measurement is taken—line to line, line to neutral, or line to earth. Going back to the residential example, because there is only one phase, the implied notation is line to neutral. For multiphase systems, typically three-phase, it is important to note when the measurement is indicating L-L or L-N in addition to which line (that is, A, B, C or 1, 2, 3).
- Cycle—A single portion of a repetitive waveform as measured from similar points (peak to peak, zero cross to zero cross, and so on)
- RMS x 1.414 = Peak (for a pure sign wave)
- VL-N x sqrt(3) = VL-L
Read the rest of the report here.
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