T
tobeeepe
Someone posted this question to the "the other board" website:
Three 2000 horsepower synchronous motors of 92% efficiency and 80 % power factor are to be added to an existing 10000 kva load with lagging 82% power factor. If the motors are operating fully loaded, what will be the new kva and power factor?
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For Single motor the appeerent power = (2000*746)/.92 = 1621739W
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My quick answer was this:
Both are lagging loads (motors are lagging), so you add the reactive parts, and real parts.
Total kW = 10,000*0.82+3x1621.7 =13065.22
Total kVar = 10,000*sin (cos-1 0.82)+3x1216.31 = 9372.57
Total kVA = sqrt(kW**2+kVAr**2) = 16079 kVA
pf = kW/kVA = 0.81
**********
But we could also argue that the motors would be overexcited, to make up for the reactive consuption of the lagging load connected with them. In that case, the answer would be:
Total kW = 10,000*0.82+3x1621.7 =13065.22
Total kVar = 10,000*sin (cos-1 0.82)-3x1216.31 = 2075
Total kVA = sqrt(kW**2+kVAr**2) = 13229 kVA
pf = kW/kVA = 0.99
**********
Any comment on these approaches and which is correct?
Thanks!
Three 2000 horsepower synchronous motors of 92% efficiency and 80 % power factor are to be added to an existing 10000 kva load with lagging 82% power factor. If the motors are operating fully loaded, what will be the new kva and power factor?
------------------------------------------------------------------------------
For Single motor the appeerent power = (2000*746)/.92 = 1621739W
**********
My quick answer was this:
Both are lagging loads (motors are lagging), so you add the reactive parts, and real parts.
Total kW = 10,000*0.82+3x1621.7 =13065.22
Total kVar = 10,000*sin (cos-1 0.82)+3x1216.31 = 9372.57
Total kVA = sqrt(kW**2+kVAr**2) = 16079 kVA
pf = kW/kVA = 0.81
**********
But we could also argue that the motors would be overexcited, to make up for the reactive consuption of the lagging load connected with them. In that case, the answer would be:
Total kW = 10,000*0.82+3x1621.7 =13065.22
Total kVar = 10,000*sin (cos-1 0.82)-3x1216.31 = 2075
Total kVA = sqrt(kW**2+kVAr**2) = 13229 kVA
pf = kW/kVA = 0.99
**********
Any comment on these approaches and which is correct?
Thanks!