X / R RATIO and Equivalent Pf %

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SHORT CIRCUIT PF % = R / Z. and. Z can not be calculated from the X /R ratio. The value of R or X must be known to calculate Z from X / R.
Bottom line here is LV circuit breakers need to be derated if the short circuit X / R is greater than 15. There is a table for this correction factor.
When PF % O.15 is given with X/R 6.6 as lv circuit breaker test values, this turns out to be useless information. And caused my mental block. Now unblocked.
 
One of the things about getting old is that you forget a lot of what you knew.
I am guilty. This is a Correction to my above statements.
Short Circuit PF = 0.15 and X/ R = 6.6. are in fact SYMMETRICAL values from a calculated short circuit study.
And these values are used to find the true ASYMMETRICAL factor for the calculated SYMMETRICAL
short circuit values. And check the lv circuit breaker short circuit rating, ie Interrupting capability.
Sorry for posting my mental block. Now completely cleared. Thought i better post this follow. up.
 
JEFF MILLAR said:
SHORT CIRCUIT PF % = R / Z. and. Z can not be calculated from the X /R ratio. The value of R or X must be known to calculate Z from X / R.
Bottom line here is LV circuit breakers need to be derated if the short circuit X / R is greater than 15. There is a table for this correction factor.
When PF % O.15 is given with X/R 6.6 as lv circuit breaker test values, this turns out to be useless information. And caused my mental block. Now unblocked.
Click to expand...
Please, just read back. The calculation was already given to you and you still believe your idea is correct. The ratio of the reactive power ( or simply put, reactance) to the real power (also represented by resistance) --> X/R is the tangent of the power angle theta. You can also calculate theta as the inverse cosine of the power factor. Once you have the value of theta, you can have the value of X/R, which is the tangent of theta. Did you follow?
 
topgone said:
Please, just read back. The calculation was already given to you and you still believe your idea is correct. The ratio of the reactive power ( or simply put, reactance) to the real power (also represented by resistance) --> X/R is the tangent of the power angle theta. You can also calculate theta as the inverse cosine of the power factor. Once you have the value of theta, you can have the value of X/R, which is the tangent of theta. Did you follow?
Click to expand...
OK we know how to calculate the X/R from the PF which is R / Z. But all we have is PF and X / R. we have no R, no X and no Z.
topgone said:
Please, just read back. The calculation was already given to you and you still believe your idea is correct. The ratio of the reactive power ( or simply put, reactance) to the real power (also represented by resistance) --> X/R is the tangent of the power angle theta. You can also calculate theta as the inverse cosine of the power factor. Once you have the value of theta, you can have the value of X/R, which is the tangent of theta. Did you follow?
Click to expand...
We do not know R, or X or Z. and all we have is PF and X/R. and we know how to find X/ R from PF.
But we do not know how to find PF from X/R. Your support is very much appreciated. Thank you.
 
JEFF MILLAR said:
... all we have is PF and X / R. we have no R, no X and no Z.
Click to expand...
You are correct, in that we do not have actual impedance values. All we have is a ratio of X/R and the formulas used to derive this ratio from the relationship of Z, X ,and R.

Are you trying to determine ratios or actual impedances?
 
JEFF MILLAR said:
OK we know how to calculate the X/R from the PF which is R / Z. But all we have is PF and X / R. we have no R, no X and no Z.

We do not know R, or X or Z. and all we have is PF and X/R. and we know how to find X/ R from PF.
But we do not know how to find PF from X/R. Your support is very much appreciated. Thank you.
Click to expand...
Short answer: express Z in terms of X and R. Do we have to compute it for you?
 
JEFF MILLAR said:
OK we know how to calculate the X/R from the PF which is R / Z. But all we have is PF and X / R. we have no R, no X and no Z.

We do not know R, or X or Z. and all we have is PF and X/R. and we know how to find X/ R from PF.
But we do not know how to find PF from X/R. Your support is very much appreciated. Thank you.
Click to expand...

If you are just trying to determine ratios, and you don't care about actual impedance/resistance/reactance, you can simply assume one of the three terms (X, R, and Z) is 1 ohm, and calculate from there. Or if desired, you can go through the algebra to prove that your answer is independent of how many ohms you actually have in these terms.

Whether R=1 ohm, or R=1 k-Ohm, you would still draw the same proportions of the triangle that represents the relationship among X, R, and Z. The PF and X/R ratios are simply trig functions of the phase angle.
 
Carultch said:
If you are just trying to determine ratios, and you don't care about actual impedance/resistance/reactance, you can simply assume one of the three terms (X, R, and Z) is 1 ohm, and calculate from there. Or if desired, you can go through the algebra to prove that your answer is independent of how many ohms you actually have in these terms.

Whether R=1 ohm, or R=1 k-Ohm, you would still draw the same proportions of the triangle that represents the relationship among X, R, and Z. The PF and X/R ratios are simply trig functions of the phase angle.
Click to expand...
Your smarter than i am. Thank you
 
No answer for formula to calculate X/ R ratio from PF = 0.15
All we are given is the X/R and PF
Can calculate the PF in degrees from the X/R ratio.
topgone said:
You got it wrong. X/R is not equal to the tangent of the power factor! X/R is = tan(acos(PF))
Click to expand...
can you calculate 0.15 PF from X/R ratio 6.6. That is my question. Please note that is all the information given.
If you can could you please just post the formula.
 
Does it mean we have to spoon feed you here?
X/R = TAN (THETA) =6.6
therefore THETA = TAN-1 (6.6) = 81.38 degrees (electrical)
PF = cos(THETA) = COS(81.38) = 0.1498 ~ 0.15
Which of the preceding lines do you still not understand?
 
JEFF MILLAR said:
No answer for formula to calculate X/ R ratio from PF = 0.15
All we are given is the X/R and PF
Can calculate the PF in degrees from the X/R ratio.

can you calculate 0.15 PF from X/R ratio 6.6. That is my question. Please note that is all the information given.
If you can could you please just post the formula.
Click to expand...

Power factor is not in degrees. It is a ratio between kW and kVAR. It is the phase angle that has units of degrees or radians, and the cosine of this angle gives power factor. This means, the sign on phase angle is ignored, so power factor alone is a positive number no matter if it is leading or lagging, so if you cared to specify that, you'd need to specify it with words. A negative power factor would mean we switch from considering a load to considering a source, or vice versa, depending on which one we consider positive.

It takes a composition of formulas to solve this problem, and plenty of people have posted those formulas and how to use them, much earlier in this thread. If you still have confusion, please elaborate on it.
 
JEFF MILLAR said:
No answer for formula to calculate X/ R ratio from PF = 0.15
All we are given is the X/R and PF
Can calculate the PF in degrees from the X/R ratio.

can you calculate 0.15 PF from X/R ratio 6.6. That is my question. Please note that is all the information given.
If you can could you please just post the formula.
Click to expand...
I have an interactive visual for this concept, if you'd like to explore it:
 
topgone said:
Does it mean we have to spoon feed you here?
X/R = TAN (THETA) =6.6
therefore THETA = TAN-1 (6.6) = 81.38 degrees (electrical)
PF = cos(THETA) = COS(81.38) = 0.1498 ~ 0.15
Which of the preceding lines do you still not understand?
Click to expand...
WOW, You are the man. Very grateful to you, thank you very much. Seriously.
 
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