Power systems operate at frequencies of 50 or 60Hz, but some devices, including personal computers, printers, and some industrial equipment, present a non-linear load and create currents and voltages with frequencies at harmonic frequencies. These harmonics get fed back into the power system and can cause other devices connected to the power source to malfunction. There are several standards that specify the level of harmonics that a particular device can produce.
The Asterion AC Series power source makes it easy to determine the harmonic content produced by a device under test (DUT). It uses advanced digital signal process (DSP) techniques to digitize its output voltage and current waveforms, and then perform a harmonic analysis of the data. To do this, the Asterion Series performs a fast Fourier transform (FFT) on both voltage and current. The resulting frequency spectrum (DC through 49th harmonic) can be displayed on the LCD display in a tabular as well as a graphical format.
Acquiring FFT data
To perform an FFT analysis on the output of the power source, you must first acquire data. Using the front panel display, you perform the following steps:
- Navigate to the HARMONICS menu of the MEASUREMENTS screen.
- Scroll to the FUNCTION field and select VOLT or CURRENT.
- Scroll to the VIEW field and select the TABLE or BAR display mode.
- Scroll to the DATA field and select ABSOLUTE or RELATIVE. The ABSOLUTE display format will show all harmonic components in volts or amps. The RELATIVE display format will use the fundamental as a 100% reference and display all harmonics as a percentage of the fundamental. Phase angles are always shown with respect to the fundamental frequency.
- Tap the MODE field and select SINGLE or CONTINUE. The SINGLE mode will acquire the data once and show the result, while the CONTINUE mode will update the data continuously.
- Tap the SOUR field and select IMMEDIATE; alternate trigger mode is PHASE.
- Tap the START field to start the analysis. The display mode that was selected will be opened and the results displayed. If the trigger mode, CONTINUE, was selected, the data will be continually updated.
- Returning to the HARMONICS menu can be done by tapping the UP arrow button. To display the data in a different format, the selections are changed as desired, and a new acquisition started by tapping the START field.
Analyzing FFT data
Once you have acquired the data, you can analyze the data. The Asterion will perform a FFT analysis on the data and display the entire data set using either tabular or graphical formats. For tabular display, as shown in the figure below, the Asterion shows harmonics in five groups with ten harmonics per group. You use the LEFT and RIGHT arrows to scroll through the data vertically.
You can also display the FFT data in bar chart format as shown in the figure below.
In this display, the Asterion shows only the amplitude information graphically. The phase data is displayed in numeric format at the right-side of the display. The display can show up to 25 harmonic components at a time. The triangle at the bottom of the display shows the currently selected component for which numeric data is shown on the right-side. This data includes the harmonic number (DC through 50), the harmonic frequency, the absolute or relative amplitude (depending on selection in DATA field), and the phase angle with respect to the fundamental. The rotary encoder could be used to scroll through the displayed harmonics horizontally, or the touch-screen could be used to directly select an individual harmonic.
You can, of course, also access these measurements via the remote digital interface using the Asterion Virtual Panels or SCPI commands. This capability allows you to do more in-depth analyses as well as automate harmonic analysis tests. For more information on the Asterion Series, contact AMETEK Programmable Power by sending email to firstname.lastname@example.org or phoning 800-733-5427.