Application 6:
Operating the Agilent 6812A and 6813A at Low Frequencies
Overview of application
For some applications, such as the simulation of European railway power systems, a low frequency (16.6 Hz) ac waveform is required. Generating low frequency waveforms with ac sources can present testing challenges due to output power derating and programming inaccuracies, depending on the regulation technique used. If the ac source has measurement capability, control of the measure- ment sample period is necessary to capture suffi- cient cycles of the output waveform to ensure measurement accuracy.
The 6812A and 6813A can meet the above chal- lenges at frequencies below 45 Hz. These two models provide precise control of the waveform generation and measurement system for optimal operation at low frequencies. For this example, the equipment under test will require an ac input at 230 Vrms and 16.6 Hz. Real power, apparent power, and power factor will be accurately meas- ured using the high resolution current measure- ment range.
Agilent 6812A and 6813A features used
•Programmable Vrms and frequency
•Realtime regulation mode
•Sinewave generation
•Programmable voltage and current sample rate
•Power measurement (power factor, VA, and Watts)
•x10 current measurement range
Advantages/benefits of the Agilent 6812A and 6813A solutions
•Programmable regulation mode allows for accu- rate output voltage and current limit control.
•Programmable measurement sample rate pro- vides high accuracy measurements for low frequency signals.
•Use of the FETCh command provides fast measurements from the same data buffers.
•x10 current measurement range increases the accuracy for low current and low power measurements.
Implementation details
How the 6812A and 6813A generate low frequency ac waveforms and perform low frequency ac measurements A 230 Vrms sinewave is programmed at a frequency of 16.6 Hz. To perform measurements at this out- put frequency, the programmable sample period is increased from 25 microseconds to 75 microsec- onds. A triggered acquisition of voltage and current provide the data from which the power measure- ments are calculated. This method of acquisition is necessary since the MEASure command will reset the sample period back to the default value of 25 microseconds. A FETCh command is sent to return the real power (Watts), power factor, and apparent power measurements from the same voltage and current data buffer.
Agilent 6800 series setup
•Connect the ac source output to the ac input of the equipment under test.
•Set the regulation mode to Realtime.
•Set the rms voltage to Fixed mode.
•Set the frequency to Fixed mode.
•Set the shape to sine.
•Set the rms voltage to 230 V.
•Set the frequency to 16.6 Hz.
•Enable the output.
•Set the measurement sample period to 75 microseconds.
•Trigger the acquisition of voltage and current data.
•Fetch the real power.
•Fetch the power factor.
•Fetch the apparent power.
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