# Electronics Protection - Summer 2017 - 19

```Feature
then the number of channels increases to nine. The flexibility
of being able to specify up to 16 channels in the digitizer configuration is a major advantage in this type of measurement.
The phase voltages are shown in the top row of Figure 7.
Phase currents appear in the center row. The phase power
appears in the bottom row. The sum of all three-phase power
waveforms appears in the leftmost grid labeled 'Total Power.'
Note that the total power is relatively constant. Parameters,
shown in the Info pane on the left read the average values of
the individual phase power waveforms along with the total
power. The sum of the average values of the three-phase
power measurements equals the average total power. The
measured result for the total power is 850.9 Watts

Figure 7. SBench 6 graphical display of the analysis results
of a three-phase measurement

An alternative technique is the two-wattmeter method, which requires measurement of only two line voltages and
two phase currents. In mathematical form: PT (t) = Vac(t) ia(t) + Vbc(t) ib(t)
This can be derived as follows. It can be verified using the following mathematical derivation:
PT = Va(t) ia(t) + Vb(t) ib(t) + Vc(t) ic(t)
However, using Kirchhoff's current law: ia + ib + ic = 0 or +ic = - ia - ib
PT(t) = Va(t) ia(t) - Vc(t) ia(t) - Vc(t) ib(t)+ Vb(t) ib(t)
PT (t) = Vac(t) ia(t) + Vbc(t) ib(t)
Where: Va - Vc ≡ ac and Vb - Vc ≡ bc
Figure 8 shows an example of applying the twowattmeter method which can be accomplished using a
four-channel Spectrum digitizer along with two differential voltage and two current probes.
As in the example computing total power based on
the individual phase voltages and currents this method
uses two line voltages (Vac and Vbc) and two phase
currents (Ia and Ib).
Line voltages appear in the top row, phase currents
appear in the middle row, and the individual power
waveforms appear in the bottom row. As before, the
total power appears in the leftmost grid marked 'Total
Power.' The average or mean values of each of the
power waveforms is shown in the Info grid on the
extreme left. Again, the nominal power is 851 Watts.

Figure 8. Display of three-phase analysis using the two-wattmeter
method.

Modular digitizers can be used to make the power measurements that are commonly required to evaluate the
performance of devices or circuits. Digitizers are voltage responding measurement instruments that can also measure current using suitable current probes or current shunts. Acquiring current and voltage next leads to the ability
to calculate power based on the product of the acquired current and voltage waveforms. A modular digitizer along
with a versatile software analysis tool is therefore a versatile instrument for making basic power measurements in AC
circuits and devices.
www.ElectronicsProtectionMagazine.com * Summer 2017

19

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http://www.spectrum-instrumentation.com http://www.ElectronicsProtectionMagazine.com

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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