3 Phase wattmeter

[tte]

We have a wattmeter hooked up on the mains 3 phase 480V AC input to our compressor.
For each phase is a separate coil to measure the AC current.The direction of each coil was determined by the Right Hand Rule and all 3 coils for that meter were placed in the same direction.

I hooked up another exact type and model wattmeter as a test meter to check if the first meter was working ok.
It was but what I found was that when I positioned 2 coils in one direction and the third one in the opposite direction in the 2nd meter the readings from both meters were the same.

Why would the meter readings be the same if one meter has 2 of its coils in one direction and the 3rd coil in the oppostite direction to the current flow and the other meter has all its 3 coils correctly orientated but different from the other meter?

Cheers,
Roy

 

[DanDel]

How are the voltage connections polarized?

 

[tte]

How do u mean dan?

Roy

 

[buzzp]

I believe he was referring to the voltage connections to the meter. Make sure they are not hooked up opposite on the other meter. On the surface (not knowing your meter) it would appear that this would not affect a wattmeters readings.

Not sure of the size or type of your CT's, but I have had toroids placed in upside by the manufacturer compared with the polarity markings, etc.

 

[busbar]

What happens to the second-meter readings if the third coil is "corrected"?

Specifically, how are both meters' potential leads connected? Blondel’s Theorem always gets dragged into wattmeter connections. Is that discussed in the meter application material?

Are different readings obtained connecting each mater in an identical manner?

 

[tte]

Yes. I did that. I experimented for about a week and hooked up the both meters the same way and the readings were the same...only a variation on 1-2 on the meters. I also corrected the 3rd coil orientation o the second meter and the readings were the same as the first meter...The readings were all the same no matter what i did...

Any thoughts?

Roy

 

[busbar]

Is it possible the meter(s) are operating in a 3-wire, 2-element mode and ignoring the third current?

 

[jbartos]

Comment: If there happen to be CTs then the possibilities of various connections increase.