Line Voltage vs phase voltage

[gar]

130316-1440 EDT

The MTU (measuring and transmitting unit) part of a TED system measures +/- power. TED does not do a good job on low power factor. I believe this is mostly a result of the current transformers being used.


By contrast the Kill-A-Watt handles power factor very well. With a high quality 30 ufd capacitor the measurements are:

123.7 V
1.41 A
174 VA
1 W
Xc = about 90 ohms

So less than 1% power error at a phase shift of 90 deg for the current relative to voltage using Kill-A-Watt.

I have also run an unloaded single phase induction motor at constant voltage and adjusted the shunt capacitance with essentially no variation in the power reading on the Kill-A-Watt. With no capacitance the PF is 0.25 . Simultaneously TED shows a large change in the power reading from no PF correction to about full correction.

TED uses the Cirrus CS5461A chip and this should be as good as the Kill-A-Watt. Thus, my reason to put the blame on the TED current transformers. Kill-A-Watt uses a resistive shunt for current measurement, thus, no phase shift.

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[GoldDigger]

By contrast the Kill-A-Watt handles power factor very well.

I would say instead that that it handles phase power factor very well. I have seen data that indicates that its behavior with distortion power factor on non-linear loads like CFLs and electronics can be spectacularly bad. (In the neighborhood of 25 watts (not VA!) for an 8 watt CFL.)
This is particularly concerning for people who are trying to reduce or at least calculate their true wattage consumption for off-grid limited energy homes.
It is very good for appliances with poor phase power factor like refrigerators and A/Cs.

 

[gar]

130316-1641 EDT

Some Kill-A-Watt EZ data from 3 CFLs:

25 W GE Dimmable, total time about 10 minutes

25.3 W 122.8 V
26.7 W
27.5 W 123.2 V 38,1 VA
24.0 W 121.7 V 34.4 VA
24.1 W 121.7 V 34,2 VA


27 W NuVision, possibly about 10 minutes also

22.3 W 121.9 V 40.1 VA
23.4 W 122.0 V 42.9 VA
27.2 W 121.9 V 47.9 VA
27.5 W 121.7 V 47.5 VA
26.8 W 121.6 V 46.4 VA
26.3 W 121.6 V 46.2 VA


13 W Normal type GE

11.9 W 122.0 V 19.6 VA
12.7 W 122.0 V 20.1 VA
13.3 W 121.9 V 21.0 VA


15 W Incandescent

15.7 W 122.0 V 15.8 VA
15.6 W 122.0 V 15.6 VA
10 minutes later
15.4 W 121.7 V 15.4 VA

The Kill-A-Watt does very well. I could not find my 75 W Simpson, but its accuracy probably is not as good at these lower power levels. I don't remember but full scale accuracy is probably about +/-2%, and therefore possibly +/-6% of reading at 25 W.

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[gar]

130316-2209 EDT

A more severe experiment on the Kill-A-Watt EZ, lower power, and DC.

The load was 10.09 kohms across a 20 ufd high quality capacitor with a half wave rectifier using a 1N5625. Thus, a unidirectional short duration pulse of current at the input voltage peak. Both polarities were tested. One current pulse per cycle vs two for a full wave bridge rectifier.

The results:

+ output voltage relative to neutral. 120 V 60 Hz supply.
122.4 V 2.9 W 8.4 VA 168.5 DC V calculated power 2.82 W

- output voltage relative to neutral. 120 V 60 Hz supply.
122.4 V 2.9 W 8.2 VA 168.3 DC V calculated power 2.81 W

Note: there is some small power dissipation in the diode.

Actually the Kill-A-Watt results are extremely good.

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