Electronics Protection - Summer 2017 - 16

Feature
Digital Analysis Brings Convenience and Accuracy to
Power Measurement
Oliver Rovini, Technical Manager
Spectrum GmbH
The use of digital measurement tools along with PC-based analysis software brings accuracy, convenience and detailed insight into AC power measurement.
The basic concepts of AC power measurement include the definition of instantaneous, real, apparent, and reactive
power. There are a number of tools and approaches available to accomplish this. For example, digitizers, with a suitable number of channels, can be employed to measure single and multi-phase power systems employing suitable
voltage and current probes.
The digitizer's versatility, ease of communication, and rapid information transfer make it ideal for AC power measurements. PXI cards are available for applications where a mix of modular instruments will work together as part of
a complete test system. PCI and PCIe cards can also be installed directly into most modern PC's turning them into
powerful stand-alone test stations.

How Digitizers Help Measure Power

In terms of basic power measurements, instantaneous power is an abstract value calculated as the product of applied
voltage and current. To measure instantaneous power, and hence the other basic power factors, one must measure
voltage and current. From there one can determine real, reactive and total apparent power.
Voltage measurements require the use of probes. Conventional oscilloscope, high
impedance probes can be used with a digitizer. The ability to scale vertical voltage
data to account for the probes is very useful. Since most power measurements
require line (mains) voltage measurements, it is best to make these measurements
differentially in order to avoid grounding issues associated with single ended
probes. The digitizer should be able to accept two probe inputs and compute the
difference. Alternatively, the voltage on the hot and neutral lines can be acquired
separately and subtracted using waveform calculations. If a differential probe is
available that can also be used.
The most convenient method of making current measurements is to use an appropriate current probe. Make sure that any current probe you use has controls
separate from the measuring instrument. The output of the current probe can be
applied to a digitizer channel with appropriate scaling to display signals from the
probe in units of current.

Figure 1. Relationships of real, reactive and total apparent power

Real power (P), then, is the average or mean value of the instantaneous power measured in Watts. Circuits containing reactive elements, such as inductors or capacitors, can store energy and reverse power flow so that power flows
from the load back to source. This is reactive power (R), measured in units of Volt-Amps Reactive or VAR. The vector
sum of the real and reactive power is called total or apparent power this is illustrated in Figure 1.
Apparent power (S) can be computed as the product of the effective or rms voltage and effective or rms current.
The units of apparent power are Volt-Amps (VA). The angle () enclosed by the real and apparent power vectors Θ
represents the phase difference between the current and voltage waveforms. The cosine of that angle, the ratio of
real power and the apparent power is called the Power Factor (pf). The formula is: pf=cos(Θ)=PS
If the device is purely resistive the current and voltage waveforms are in phase, the apparent and real power are
equal and the power factor equals one. As the reactive component increases the power factor decreases.
16

Summer 2017 * www.ElectronicsProtectionMagazine.com


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Table of Contents for the Digital Edition of Electronics Protection - Summer 2017

Editor's Choice
Smart Electronic Adhesives for Microelectronics – Enabled by Low Viscosity Nanocomposite Materials
Sheilding Tips and Tricks
Digital Analysis Brings Convenience and Accuracy to Power Measurement
Enclosures
Thermal
EMI/EMC/ESD
Power
Hardware
Industry News
Electronics Protection - Summer 2017 - Cover1
Electronics Protection - Summer 2017 - 2
Electronics Protection - Summer 2017 - Editor's Choice
Electronics Protection - Summer 2017 - Smart Electronic Adhesives for Microelectronics – Enabled by Low Viscosity Nanocomposite Materials
Electronics Protection - Summer 2017 - 5
Electronics Protection - Summer 2017 - 6
Electronics Protection - Summer 2017 - 7
Electronics Protection - Summer 2017 - Sheilding Tips and Tricks
Electronics Protection - Summer 2017 - 9
Electronics Protection - Summer 2017 - 10
Electronics Protection - Summer 2017 - 11
Electronics Protection - Summer 2017 - 12
Electronics Protection - Summer 2017 - 13
Electronics Protection - Summer 2017 - 14
Electronics Protection - Summer 2017 - 15
Electronics Protection - Summer 2017 - Digital Analysis Brings Convenience and Accuracy to Power Measurement
Electronics Protection - Summer 2017 - 17
Electronics Protection - Summer 2017 - 18
Electronics Protection - Summer 2017 - 19
Electronics Protection - Summer 2017 - Enclosures
Electronics Protection - Summer 2017 - 21
Electronics Protection - Summer 2017 - Thermal
Electronics Protection - Summer 2017 - 23
Electronics Protection - Summer 2017 - EMI/EMC/ESD
Electronics Protection - Summer 2017 - 25
Electronics Protection - Summer 2017 - Power
Electronics Protection - Summer 2017 - Hardware
Electronics Protection - Summer 2017 - Industry News
Electronics Protection - Summer 2017 - 29
Electronics Protection - Summer 2017 - Cover4
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