three phase power measurement with single phase meter

[electrofelon]

Can someone take a stab at explaining this? I get that you only need two CT's for a delta, but what I dont get is the W=W1+W2.....

 

[jumper]

Watts is power. You can sum watts, you cannot with amps.

 

[electrofelon]

I feel like I must be missing something real obvious. For simplicity, let's say the load is balanced. W=VxIx1.732 doesn't match what you get with the method in the photo??

 

[Jraef]

I feel like I must be missing something real obvious. For simplicity, let's say the load is balanced. W=VxIx1.732 doesn't match what you get with the method in the photo??

Look closer at that diagram. Those meters are showing a connection of current AND voltage simultaneously. So the meter is reading watts (which is V x A x PF) on each phase separately already. So since the watts are already calculated per phase, to get the total you just add them.

Using the 1.732 factor is for when you read phase amps SEPARATELY, because you must integrate them into a total.

 

[jumper]

Diagram is a delta, but labeled Wye or Star.

 

[Bugman1400]

This is simply known as the Two Wattmeter Method and is illustrated perfectly on many websites. So, rather than copy and paste someone's work, you can Google for yourself.

 

[Ingenieur]

Can someone take a stab at explaining this? I get that you only need two CT's for a delta, but what I dont get is the W=W1+W2.....

the power across each coil is V x I/sqrt3
3 coils = 3 x V x I/sqrt3 = sqrt3 x V x I
at peak the power in one coil = 0, so adding any 2 legs gives the total

 

[GoldDigger]

the power across each coil is V x I/sqrt3
3 coils = 3 x V x I/sqrt3 = sqrt3 x V x I
at peak the power in one coil = 0, so adding any 2 legs gives the total

Another way of looking at it is that as long as you know the relative voltages on each conductor, you only need to know any two of the three currents.

And there is a general theorem that can be proved that for N conductors (including the grounded conductor, if potentially current carrying) you need N-1 single phase wattmeters to capture total power.
The standard POCO case of a single wattmeter for single phase 120/240 three wire service actually cheats and makes use of the assumption that the L-N voltages are balanced. With one wattmeter you get only an approximate answer, but one that slightly favors POCO and so is good enough for them.

 

[electrofelon]

Thanks for the explanations

This is simply known as the Two Wattmeter Method and is illustrated perfectly on many websites. So, rather than copy and paste someone's work, you can Google for yourself.

Yeah its great to know what something is called, lots of hits on that.

 

[Sahib]

Load power factor has an effect on the measurement: depending on its value, one of the meters may show a negative value.