Electronics Protection - Summer 2017 - 17

Feature
Measuring Single Phase Power

Let's look at an example measuring the power used by a small, line-powered cooling fan. The measurements were
made using a digitizerNETBOX model DN2.496.04, which is the model with 4 analog channels, 16 bit resolution, 60
MS/s sample rate, and a 30 MHz bandwidth (see sidebar, "Selecting a Digitizer").
A Tektronix model P6042 current probe and a pair of passive
oscilloscope probes were used to acquire the current and voltage waveforms. Line voltage was measured differentially so that
neither the hot nor neutral lead of the power line was grounded.
The resulting measurement is displayed using Spectrum's SBench
6 software to control and process the acquired data as shown in
Figure 2.
The two passive probes differentially measure the input voltage
and are connected to channels Ch2 and Ch3. The channels are
combined and shown as channel Ch2 in the top center grid. The
Figure 2. SBench 6 display showing the measurements of
readout is scaled to reflect the attenuation of the probes. The
the voltage and current probes
output of the current probe appears on channel Ch0 in the lower
center grid. This data is also scaled by the sensitivity of the current probe so it reads in vertical units of Amperes. The
peak to peak and effective (rms) values of both current and voltage appear in the Info pan on the left of the display.
The SBench 6 software's analog calculation computes the instantaneous power by multiplying the current and voltage waveforms. The power is displayed in the leftmost grid. The peak to peak and average values of the power also
are listed in the Info pane. The average of the instantaneous power represents the real power and is recorded as
6.6 Watts.
Apparent power is computed by taking the product
of the effective values of the line current and voltage.
Based on the measured values (121.5 V and 63.2 mA)
the apparent power is 7.68 VA.
This lets us compute the power factor as 0.86. As seen in
the horizontally expanded view of the current and voltage waveform in the rightmost two grids in Figure 2, the
voltage waveform (top-right) leads the current waveform. This indicates an inductive characteristic. Cursors
marking the positive slope zero crossing show that the
voltage waveform is 1.44 ms in advance of the current
waveform. This represents a phase advance of about 31
degrees. A more accurate calculation is the cosine of the
power factor (0.86) or 30.68 degrees, avoiding cursor
placement uncertainties.

Figure 3. Display of voltage and current harmonics

Once we have the current and voltage waveforms, we can expand the analysis to the frequency domain. Figure 3
shows the average spectra of both the line current (lower left) and line voltage (upper left) waveforms. The line
voltage spectrum has more, higher order harmonics. The odd order harmonics are the most prominent. The current
spectrum has a lower overall harmonic content, but it too is predominantly odd harmonics.

Three-Phase Power

Three-phase electric power used to power large motors and other heavy electrical loads. It is a type of poly-phase
AC electrical distribution system of electric power generation, transmission, and distribution. A three-phase system is
usually more economical than an equivalent single-phase or two-phase system at the similar voltage levels because it
uses less conductor material to transmit electrical power.
Spectrum continued on page 18
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Table of Contents for the Digital Edition of Electronics Protection - Summer 2017