Harmonics & Power Measurement
Power-line quality is of increasing concern these days,
especially in industrial settings. The TekScope THS-720P
provides harmonics analysis and power quality measurements to
address these problems.
This waveform shows the AC voltage being supplied to a switching power supply. It shows some distortion, but how much?
You can find out using the 720P's harmonics mode. It is accessed through the Display menu, using the left bezel button.
When harmonics mode is turned on, the scope automatically selects Channel 1, and presents a live display of the harmonic content. As this display shows, the signal has some distortion from the odd harmonics, especially the 3rd and 5th.
The total harmonic distortion (THD) and RMS of the input signal are shown near the top of the display. In this case, the THD is 3.7%, and the input signal is 122 V RMS.
The 720P supports two types of total harmonic distortion measurements. The default is THD-F, which measures the total harmonic distortion relative to the fundamental. Through the Display menu, "THD-R" can be selected, which measures the THD relative to the RMS of the composite signal.
This display also provides information about the selected harmonic, including frequency, amplitude, amplitude as a percentage of the fundamental, and phase relative to the fundamental.
In this case, it is at 60 Hz, and 122 VRMS. Of course, its amplitude is 100% (of itself), and phase is zero.
The ± rocker can be used to select a specific harmonic. In this signal, the 3rd harmonic is at 180 Hz, and about 2-1/2 VRMS. It is about 2% the amplitude of the fundamental, and lags it by 45°.
This is the current being drawn by the power supply. Clearly, this signal has much more harmonic distortion!
Selecting channel 2 in harmonics mode displays the harmonic information for the current waveform. The supply is drawing about 1/4 ARMS, and has almost 150% harmonic distortion!
Using the ± rocker to select the 3rd harmonic, we can quickly see that it is larger than the fundamental (103%).
Since the only significant harmonic content in this signal is in the odd harmonics, use the Display menu and the second bezel button to select "Show Odd from 1 to 21" to show only the odd harmonics.
The 720P can measure out to the 31st harmonic, but only displays 11 harmonics at a time. You can choose to show all harmonics in a given range (Fundamental-11, 11-21, or 21-31), only the odd harmonics (F-21, or 11-31), or only the even harmonics (2-22, or 12-30).
To look at the 31st harmonic, use the Display and second bezel buttons to bring up the "Show" menu, then use the ± rocker to select the "11-31" subrange.
After clearing the menu, use the ± rocker to select the 31st harmonic.
The 31st harmonic is at 2.1% of the fundamental. This is about as high as the 3rd harmonic of the voltage signal!
The 720P also provides information about the power waveform. It automatically sets up the math waveform to be the product of the voltage and current waveforms. This is what the power used by the switching power supply looks like.
In harmonics mode, selecting MATH fills the screen with measurements. These include true power (W), apparent power (VA), reactive power (VAR), true and apparent power, and phase angle (theta).
The instrument also provides the min, max, and average values for the power, voltage, and current amplitudes.