Measuring AC Power with Advanced Math

Question: How do I measure the AC power using the advanced math capabilities – I wanted to use the equation p = |V| x |I| x cos(Phase( V-I)) but don’t seem to get the correct answer.

Answer: Just to recap, it looks like you are making a power quality measurement on the input to your power supply.  To do that, you are measuring voltage and current on the input, then performing math to determine the Real Power from the measured Apparent Power. 

Whenever you are making power measurements, it’s critical to deskew your probes.  Your voltage and current probes have different propagation delays, and even the pathways behind each channel of your oscilloscope have different delays and gain.  The differences in pathway for the voltage measurement and current measurement introduce timing errors and amplitude errors in to your power measurement, since power is the product of voltage and current. This, of course, will distort your phase measurement and affect your results.

To deskew your probes, you’ll need to adjust the delay and offset of each oscilloscope channel to compensate for the differences in pathways.  To do this, you can use a deskew kit.  This deskew kit provides a fixture and pulse generator.  The deskew pulse generator provides a stimulus signal to the deskew fixture which is then routed to the voltage and current probes.  The propagation delay and gain of each path can then be adjusted using the channel adjustments (deskew and offset) in the scope to align the two waveforms.

Or, if you are using automated power analysis software like DPOPWR or DPO4PWR/DPO3PWR, you can use automated deskew in the software.  The static de-skew function automatically adjusts the delay between selected voltage and current channels based on an embedded table of propagation times for the probes.  Or, each probe may have its propagation delay embedded in its internal memory which the oscilloscope reads.  This technique offers a quick and easy method to minimize de-skew.  DPOPWR even provides an automatic deskew function in which the scope adjusts the waveforms for you.

Automated power analysis software will also automatically measure power quality parameters like apparent power, reactive power and real power (also known as true power) for you.

For more resources on power measurements, I’d suggest looking at www.tektronix.com/power.

Best of luck!

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