The Philosophy and Consequence of Measurement – Power

The Philosophy and Consequence of Measurement – Power (continued)

Published: 25th March 2014

‘When you cannot measure… your knowledge is of a meagre and unsatisfactory kind’. Lord Kelvin

We continue looking into the need and specifications for test equipment by looking at 6Ps (Precision, Power, Performance) - (Productivity, Portability, Package) and from Precision we continue our look at Power measurements which is basically how much oomph a system needs to carry the quantity as far and as for long as needed.

We’ll split these in two (without a divider)

- Electrical Power Measurements which can be measured with some form of DMM or Oscilloscope; clamp; Wattmeter
- Spectral Power Measurements using a Power meter or Spectrum Analyser

In this article we’ll specifically glance at Electrical Power Measurements

Resistive:- For DC circuits and purely-resistive AC circuits, power is the product of voltage and . You first measure the volts then the current and multiply the two that will be your power, say 220 Volt multiplied with 5.9090909 amps = 1300 watts or 1.3 kW. kW = kilowatts.

Reactive:- For reactive AC circuits, the product of voltage and current is termed Apparent Power, which is measured in volt amperes (symbol: V.A). To measure the true power of reactive AC circuits, in Watts, one needs to take into account the circuit's power factor, which is the cosine of the angle by which the current leads or lags the supply voltage.

A typical power measurement requires one measurement device to capture the voltage across the terminals of a load, and the second device to capture the current going through the load. However, the actual power calculation depends on the resistive and reactive components (capacitors and/or inductors) in the circuit. The power dissipation in a purely resistive circuit is always a function of the voltage drop and current draw through the circuit.

Reactive circuits appear to function like resistive circuits because they produce voltage drops and draw current. However, reactive circuits actually store or return power. The reactive components cause a phase shift (up to 90 degrees) between the voltage and current waveforms which reduces the overlap between the two curves and effectively delivers less power to the loads. This phenomenon is represented by three different power measurements: reactive power, apparent power, and real power. These three power measurements have a phase relationship that can be visualised as a power triangle.

Apparent Power:- is the measure of a circuit’s impedance (Z) and is represented by an S, which has a unit measure of Volt-Amps (VA). Apparent power is the combination of reactive power and real power, without reference to a phase angle. You calculate apparent power by using the formula: Apparent Power (S) = Vrms * Irms

The pursuit of measurement of absolute power will come next where in our world absolute power does not corrupt absolutely but absolute power has inherent corruption.

Ref John Dalberg-Acton, 1st Baron Acton

We’ll discuss that next. Do leave your comments and inputs. These articles are solely an opinion on the measurements we are interested in.

Rejoice in your measurements. Demand more and enjoy this series.

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