Calculate Watts from RMS Values

[MeCoOp]

A bit of background the project I'm currently working on was specified to read rms voltage and current on a single or three phase system.  To do this I purchased 3 true RMS ac current and 3 true RMS voltage transducers and coupled theme with the NI9203.  Now they would like to calculate wattage.  From what I've found the answer appears to be no but to be sure is there a way to calculate the wattage using the sensors I've already purchased or will I have to use a different approach?

Thanks.

[johnsold]

No.  Only if your load is purely resistive will the transducers you have give you the power.  What you can calculate is volt-amperes.  To get  the power you need the phase angle between voltage and current or knowledge of the power factor of the load.  If your load contains electronic speed controls or other non-linear circuits, you might need to do something much more involved to get the true power.

Lynn

[MeCoOp]

Yeah, unfortunately I won't know the power factor ahead of time either.  Thanks for the reply electrical theory is my strong suit so I needed a second opnion before I look into another solution.

[MeCoOp]

Yeah, unfortunately I won't know the power factor ahead of time either.  Thanks for the reply electrical theory is my strong suit so I needed a second opnion before I look into another solution.

[Henrik_Volkers]

Without more information about the load ... no chance.

Here are some nice appnotes on power calculations:

http://www.zes.com/download/application-notes/zes_applicat_102_standby_power_e.pdf

http://www.zes.com/download/application-notes/zes_applicat_105_leistungsmessung_e.pdf

[rex1030]

How do power companies measure three phase power to a building?

[johnsold]

In the past (1880s to 200x) they used electromechanical watt-hour meters often assisted by potential and current transformers.  These devices are basically a specially calibrated motor which turns at a rate proportional to the power being measured.  The magnetic properties of the meter and the configuration of the voltage and current coils results in a measurement of the true power.  A similar device configured to operate 90 degrees out of phase can measure the "volt-amperes reactive" or the imaginary component.  The readings can be combined to measure power factor.

"Smart" meters may use sampled data techniques.

Lynn

[Henrik_Volkers]

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@rex1030 wrote:

How do power companies measure three phase power to a building?

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For the math and physics read the articles linked in my first reply.

See whats wikipedia is telling

Some nice pictures and stories about electricity meter can be found in the meter cabinet of the PTB (german language only, click the green field lower left corner to see some pictures of old meters)