Why are analog watt meters and demand meters wired?
Why are analog watt meters and demand meters wired?
(OP)
I have an incoming three phase 3w system at 12.5kV. It goes into a switchgear housing that has a pf, a wattmeter, a voltmeter and an ammeter. I want to know how these things work, so I did some digging in my text book, but I am really surprised how most of my engineering books lack the detail on metering circuits. :(
Thus, I did go to the library and I found some diagrams from the Handbook of Electric Power Calculations however, it still lacks the details I am looking for. I did photocopied them and I shall posted them below.
Ammeter and voltmeter is straightforward. Power factor you can get by measuring the current and phase, pretty easy as well. But the wattmeter bugs me, the diagram is shown here:
http://i48 .photobuck et.com/alb ums/f223/m ajesus/wat tmeter.jpg
It requires CTs on two legs and PTs on two phases as well.
Since W= VLL*Iph*cos(O)*3^.5
Why can't we just simply measure the Line current? Why do we need the input from two current and two PTs?
Similar, the demand meter requires three CT and two PTs.
I wonder why?
http:// i48.photob ucket.com/ albums/f22 3/majesus/ demand.jpg
Thus, I did go to the library and I found some diagrams from the Handbook of Electric Power Calculations however, it still lacks the details I am looking for. I did photocopied them and I shall posted them below.
Ammeter and voltmeter is straightforward. Power factor you can get by measuring the current and phase, pretty easy as well. But the wattmeter bugs me, the diagram is shown here:
http://i48
It requires CTs on two legs and PTs on two phases as well.
Since W= VLL*Iph*cos(O)*3^.5
Why can't we just simply measure the Line current? Why do we need the input from two current and two PTs?
Similar, the demand meter requires three CT and two PTs.
I wonder why?
http://






RE: Why are analog watt meters and demand meters wired?
However;
I was in a spot trying to get a plant online and the delivery of the metering equipment was behind schedule. We used single phase metering on one leg of a 277/480 volt service. We multiplied by three for the three phases. Our total multiplier was 360. When the proper equipment arrived, we left the single phase meter connected for 2 or 3 months as a check. It was tracking within about 1 1/2%. (I knew going in that the load was well balanced.)
I was working in a very permissive jurisdiction.
respectfully
RE: Why are analog watt meters and demand meters wired?
Futhermore, why would a demand meter require three current and only two voltage?
I may have to buy a specific book on this, because I must of opened a dozen or so distribution engineering books from the CRC, IEEE, my university books and books we have at my company and the library, but I couldn't find much that would give a good description as to how they work. I did find some info on single phase, but this is three phase. Much more interesting :)
RE: Why are analog watt meters and demand meters wired?
Blondel's Theorem: "In a system of N conductors, N-1 meter elements, properly connected, will measure the power or energy taken. The connection must be such that all potential coils have a common tie to the conductor in which there is no current coil."
RE: Why are analog watt meters and demand meters wired?
RE: Why are analog watt meters and demand meters wired?
Yeah I found a bunch of books online that looked great. "Electrical Measurements" by Golding, I'll do a library search and see is I can locate it.
Thank you guys for the great help.
RE: Why are analog watt meters and demand meters wired?
RE: Why are analog watt meters and demand meters wired?
The reason you need both the voltage and the current is so you can measure the angle between them to calculate Watts.
RE: Why are analog watt meters and demand meters wired?
RE: Why are analog watt meters and demand meters wired?
Anyways a great read on Blondel's theorem is here:
https
Blondel's theorm is a very neat and simple idea, Blondel was a smart guy.
RE: Why are analog watt meters and demand meters wired?
https:
"USE OF A SINGLE ELEMENT WATTMETER OR WATT TRANSDUCER
ON A BALANCED THREE-PHASE THREE-WIRE LOAD
WILL NOT WORK. HERE'S WHY."
https://www.ohiosemitronics.com/techLibrary.asp
provides a lot of good reads.
RE: Why are analog watt meters and demand meters wired?
A single element wattmeter may be used to measure a phase to neutral load on a four wire system. The phase current will be in phase with the phase to neutral voltage and the 30 deg. shift is avoided.
If the voltages and currents are equal, the the single phase reading can be extrapolated to obtain the total three phase consumption.
respectfully
RE: Why are analog watt meters and demand meters wired?
On page 5 of: "USE OF A SINGLE ELEMENT WATTMETER OR WATT TRANSDUCER ON A BALANCED THREE-PHASE THREE-WIRE LOAD
WILL NOT WORK. HERE'S WHY CONCLUSION."
CONCLUSION
Use of a single element watt transducer measuring the power between two phases of a three phase connected system does not work except at unity power factor (phase angle shift of 0°).
BTW, how did you do the quote insert in your post?
RE: Why are analog watt meters and demand meters wired?
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RE: Why are analog watt meters and demand meters wired?