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Three phase question help

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danrogers

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Hi all, I'm having some trouble with an assignment on three phase systems. I'm stuck on this question (picture attached).

Some help would be greatly appreciated :)
**broken link removed**
 
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Hello there,

Have you worked in the past with AC voltage sources that have a phase angle associated with them such as 120v@120deg, etc.?
I ask because for a three phase system you have to work with voltages that are at angles of 0 degrees, 120 degrees, and 240 degrees.
 
At first reading there is a catch here, we dont know if there is a neutral connection.

To calculate the power for the delta connection:

1 For each load, calculate the impedance. (Z = √(R² + X²))

2 For each load, find the current. (I = V/Z)

3 For each load, find the power. (W = I²R)

The total power is now the sum of the power in each load.

Now, for a star connection where the star point is connected to neutral, just do the same as for the delta connection. (Remember that the phase voltage will be line voltage divided by √3).

However, if the star point is floating the voltage across each load will be different because this is an unbalanced circuit.
At this point my three phase mathematical abilities run out and I dont know how to handle this case.
Running through the problem and assuming a neural connection, the numbers seem to work out quite easily.
(Hint: for the delta connection I make the total power 458watts).

JimB
 
Hello Jim,

Yes that's a good point. They dont specify the source connections. I would have assumed because they said "line voltage" and not "line to line voltage" that the source is a wye, but they may have simply worded it incorrectly.
Another good point that the wye neutral is probably connected to the source neutral, unless this is a slightly more advanced course.

If he hasnt yet learned to work with three phase voltages with phase angles then i would bet the wye neutral is connected to the source neutral so the problem is simplified.
 
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I have covered AC breifly in the past but it remains somehwat a mystery for a light current electronics person like myself!

Thanks for taking the time to explain that JimB, one question I have before I try and run through is the value of X in (Z = √(R² + X²))?

Thanks very much
 
X is the reactance of the inductor. (X = 2.pi.f.L)

JimB
 
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