Thursday, 29 December 2011

Adventures in AC waveform measurements (part 2)

Let's have a look at some voltage/current/power waveforms that can be measured with simple household items...

Let's look at a CFL. (compact fluorescent or "energy-saving" lamp)

The lamp in question is an Ikea 7W energy saving lamp. The details stamped on it are:

  • Voltage: 220-240 V AC
  • Current: 0.061   A
  • Power:   7       W

On the basis of these details, and assuming it draws the stated current at the bottom end of the voltage range, we can calculate the expected power factor.

PF = Real Power / Apparent Power

PF =     7      / (220 * 0.061) = 0.522

Here is the plot from the Picoscope software. Note the non-sinusoidal current waveform that is typical of this style of lamp.

Looking at the averages from the measurements we have on the Picoscope screenshot:

V = 247.6
I = 0.0475
P = 6.725

PF = 6.725 / (247.6 * 0.0475) = 0.572

The next plot is from a dehumidifier unit with only the fan running.

From the measurements on the screenshot:

V = 245.7
I = 0.136
P = 33.01

From the plot we can see the power factor should be pretty good

PF = 33.01 / (245.7 * 0.136) = 0.988

A quick look inside the device with a torch reveals the presence of a 1.0uF capacitor that is providing power factor correction for the fan motor.

Now with the compressor running:


V = 244.7
I = 1.535
P = 329.8

A tell-tale sign that the power factor is no longer 1 is the presence of negative values in the power waveform (green trace).

PF = 329.8 / (244.7 * 1.535) = 0.878

That'll do for now I think. Time to go and ponder how to interface the voltage and current waveforms into a microprocessor.
Seeing as the majority of 'modern' electronic devices are non-linear I think the use of something like the dsPIC will be required to properly interpret the waveforms...

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