tomschaefer
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- buffalo, new york
How accurate is the current measurement in the third leg when you amp two of the 3 legs together?
3 phase current measurement
- Thread starter tomschaefer
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ron
Senior Member
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- New York, 40.7514,-73.9925
If it is a balanced 3 phase load then I1 + I2 + I3 = 0
If you know two of the three phases, then you know the third I1 + I2 = -I3
If you know two of the three phases, then you know the third I1 + I2 = -I3
kwired
Electron manager
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- NE Nebraska
Are you clamping two legs at same time or are you separately measuring two lines and trying to calculate the third?
Tony S
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- Resting under the Major Oak UK
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- Placerville, CA, USA
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- Retired PV System Designer
You have to be a little careful how you apply that. The equation you give is a vector equation, and all that your meter is going to give you is the magnitude of whichever vector you are measuring.
So if you know |I1| and |I2| you do not know the magnitude of |I3|.
But by clamping two wires together and taking one reading you do get |(I1+I2)| rather than |I1| + |I2|. And it is correct that |(I2+I2)| is equal to |I3| (No minus sign in this equation.)
ron
Senior Member
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- New York, 40.7514,-73.9925
Is it possible for there to be a delta system (no neutral) that is unbalanced?
Without a neutral, what would carry the unbalanced current?
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- Placerville, CA, USA
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- Retired PV System Designer
It can certainly be unbalanced in that the three line currents are not equal.
Consider, for example, a 208Y/120 that has a 10A load from A to C. The current in B is zero. That is unbalanced but has no neutral current.
If you have a wye, then you could have a load from A to neutral, with equal current in A and neutral, and the vector equation I1 + I2 + I3 = 0 would not hold. But the vector equation I1 + I2 + I3 + IN = 0 would.
This was my thought as well. Picture a motor that for whatever reason is drawing unbalanced phase currents say 10A, 11A and 12A (pretty common). These phase currents are unbalanced however there is no neutral and any current on a given phase returns (or is balanced) by the other two phases. At any point in this example I1+I2+I3=0.
kwired
Electron manager
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- NE Nebraska
Put two ten amp single phase loads on any three phase system connected A to B and B to C. A draws 10 amps, C draws 10 amps, B draws 17.32 amps.
Another good example. In this case I1+I2+I3=0 as well.
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