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mqhelen
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Join Date: Jun 2011
Posts: 1

Two Wattmeter Method

05/19/2013 11:14 AM

When the input power to a 3phase load is mesuared by a two meter method, one meter is giving me a reverse reading, while i connected the two meters correctly. What maybe the reason/cause for the reverse reading. Am now confused on how to determine the power consumed.
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lyn
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#1

Re: Two wattmeter method

05/19/2013 11:19 AM

What happens if you reverse the leads on the meter that is reading backwards?
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GW
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Join Date: Oct 2006
Location: Surrey BC Canada
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#2

Re: Two Wattmeter Method

05/19/2013 10:55 PM

what size and type of load are you measuring?

rated Volts, amps, phases, frequency, expected load, etc.

We need a sanity check.
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dkwarner
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Location: Placerville, CA (38° 45N, 120° 47'W)
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#3

Re: Two Wattmeter Method

05/19/2013 11:55 PM

Your title reads "Two Wattmeter Method". Is it correct that both meters are Watt meters? What powers are these meters measuring? A labelled connection diagram would be most helpful in answering your question.

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olehwi
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Join Date: Jun 2010
Location: Canada
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#4

Re: Two Wattmeter Method

05/20/2013 1:34 AM

My, oh my, . . . the assistance you are getting is from people who know less about "the two wattmeter connection" than you.

YES, it is quite normal for one of the wattmeters to read NEGATIVE when the power factor of the load is less than 0.50. Is your load PF this low?

You should draw a vector diagram of your voltages and currents, . . . and then everything will be more easily understood.

It is interesting to note that if you have a 0.0 PF load, the power in one wattmeter will be + while the other wattmeter will read the same value - but (-), . . . with the sum being = 0.0
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lyn
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#7
In reply to #4

Re: Two Wattmeter Method

05/26/2013 11:37 AM

There is no forum rule that says you must know more about a subject than the person that you are advising.

It is up to the OP to separate the wheat from the chaff, assuming the are capable.
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67model
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Join Date: Dec 2010
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#5

Re: Two Wattmeter Method

05/20/2013 6:59 AM

OLEHWI, post #4 has the truth of it. The wattmeter with a reverse reading should be regarded as a negative value. Usually, one has to swap polarity of current coil on "reversed" meter to get a measurement. Another point is that if power factor is 0.5, one wattmeter reads zero.

You could look at p773-775 of "Electrical Measurements and Measuring Instruments" by Golding and Widdis (Pitman ISBN 0 273 41058 X) or similar textbook if you can find one in a library. Second hand may be available, it is a comprehensive and useful book, although old - try www.abebooks.com which brings up alternatives on entering title. The following diagrams come from that.

67model
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67model
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Join Date: Dec 2010
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#6
In reply to #5

Re: Two Wattmeter Method

05/20/2013 7:25 AM

To complete on post #5, if , in Fig 20.11, you make angle phi = 60 degrees (cos phi = 0.5 for angle I1 to E10), then current vector I1 is perpendicular to V12, so W1 reads zero. Note the diagram is drawn for a balanced load (but it does not have to be balanced for correct watts).

Sorry about lack of symbol for phi like diagram but CR4 brings up blank tables for "symbols" (omega) and "faces" buttons since change of owner.

67model
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67model (2); dkwarner (1); GW (1); lyn (2); olehwi (1)

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