Two Watt Meter Method Power Measurement

Are CT's used? Quick and easy solution if they are.

ESO is always a possibility.

[I've suffered with this condition, too. ]

Read these.

Measurement of Three Phase Power | Electrical4u

Measurement of Three-Phase Power by Two-Wattmeter ...

Power Measurement In a Three-Phase System | EEP

Follow these instructions.

If TonyS is too subtle, here are a few more hints:

1. Power = volts x amps

2. What happens to current waveform if you reverse the CT leads (or run the wire through in the other direction)?

Assuming that you paid strict attention to both the voltage and current polarities (and got them correct), then the cause is improper thinking that leads to the conclusion that something is wrong when it isn't; i.e., not fully understanding the techniques that you are using.

You rectify the situation by paying attention to detail and learning the basics before attempting to analyze the results. 67Model explains the theory, reread his post.

Taken from one of Lyn's links....and assuming 3-phase power...

"...When using the two-wattmeter method, it is important to note that the reading of one wattmeter should be reversed if the power factor of the system is less than 0.5. In such a case, the leads of one wattmeter may have to be reversed in order to get a positive reading. In the case of a power factor less than 0.5, the readings must be subtracted instead of being added...."

....not that I'm much wiser... I don't know the power factor...and I don't know which reading to reverse.... or which wires to reverse (amps or volts or both)...and having done so... which readings are then added or subtracted.....

I suppose I could work it out by myself with a bit more study....but not at the moment.

Food for thought for the OP. I mention it for what it is worth.

Horace40,

I suggest you look at my post #4. I looked at a reference textbook before I wrote it and was careful what I did and did not say.

I do not think what you quote from Lyn's link is accurate or helpful to the "beginner". If the 3 phase load is perfectly balanced (power factor and current/voltage) then one wattmeter reads zero at 0.5 power factor.

I suspect that (again balanced) for power factors >0.5, both wattmeters read +, while for <0.5, one reads negative.

Also my reference said "reverse the voltage coil" ( to accurately read the negative value, if the needle goes backwards onto a zero stop) - that is far easier and less error-prone to do than breaking or switching a current circuit - also far safer than switching a CT current circuit!!

But a real situation is that the load is rarely perfectly balanced and one is measuring to get the facts (and likely to need a convenient way to switch voltage polarity while on load!!).

One phase could be near zero power factor and another resistive (you would have to move your two meters to get phase by phase readings to get a better "grip" on the loads). I would look closely at the idea that a negative reading always means power factor <0.5.

If I did not have the maker's connection diagram, I would proceed by connecting a suitable resistive load to a wall socket via the wattmeter (and step-down transfo if needed) (also voltmeter & ammeter as watts check). This would find standard polarities for a positive reading It would not matter if current and voltage polarities were reversed compared to the maker's diagram so long as you keep with the source - load side and terminal connections you have discovered.

67model

Hello 67model

I started my post before i saw yours. By the time I finished reading Lyn's links and writing my reply, your post (that answers some of my questions thank you) came in before mine was posted 7th on the list. Just unlucky timing.

What I quoted was 100% accurate - inasmuch that it was a direct 'copy-and-paste' from Lyn's link. See for yourself about half way down 1st page :-

http://electrical-engineering-portal.com/power-measurement-in-a-three-phase-system

A power factor of less than 0.5 might be a direct hit and a complete answer to the OP in terms of the limited info we have been given.

But if not, then the reasons in the link about reverse wiring and reverse readings are OK - but not explained clearly - leaving just as many queries - which is why I made the point - that hopefully can be answered by more knowledge gleaned from your post and others.

The OP should be able to make some sense out of it by now.

horace40

I did not mean to suggest you misquoted. But what was in the reference about 0.5 power factor was probably based on the load being balanced - without including that limitation.

regards,

67model

I wonder what Kaydee the OP thought about it all?