Three Phase Power Question
Three Phase Power Question
(OP)
Hello guys,
It's been a while since I last dealt with generators and 3-phase - so need a refresher (and an electrical engineer that I've been trying to hire!!!).
I have current demands for each phase of a load on the generator (say, a motor). If I were to work-out the individual phase power, it is = V(line) * I (line).
If I am confirming the size of the 3-phase generator for the load, how would I calculate the 3-phase equivalent power?
Is it
a) P = sqrt(3) * V(line) * I(line); or
b) P = sqrt(3) * V(line-line) * I(line)
line voltage is 220, and line-line voltage is 440V. I(line) would be the calculated maximum demand any one of the lines.
Sorry, such a high-school question, but as they say, don't use it, you loose it.
It's been a while since I last dealt with generators and 3-phase - so need a refresher (and an electrical engineer that I've been trying to hire!!!).
I have current demands for each phase of a load on the generator (say, a motor). If I were to work-out the individual phase power, it is = V(line) * I (line).
If I am confirming the size of the 3-phase generator for the load, how would I calculate the 3-phase equivalent power?
Is it
a) P = sqrt(3) * V(line) * I(line); or
b) P = sqrt(3) * V(line-line) * I(line)
line voltage is 220, and line-line voltage is 440V. I(line) would be the calculated maximum demand any one of the lines.
Sorry, such a high-school question, but as they say, don't use it, you loose it.






RE: Three Phase Power Question
b)P = sqrt(3) * V(line-line) * I(line)
a) is wrong and should be:
P = 3 * V(line) * I(line)
V(line-line) = sqrt(3) * V(line)
So V(Line-line) would be more like 381V (not 440V) if V(line) is 220V.
RE: Three Phase Power Question
P = sqrt(3)*U*I*PF. U is line-line voltage.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Three Phase Power Question
Since you also hinted you'll be starting a motor, you also have to consider the generator overloading capability vs the motor starting method so you can purchase the most practical (read cheapest) generator that will still do the job required.
RE: Three Phase Power Question
If your power factor at full load is greater than 80% you must calculate both KVA and KW. The KVA of the generator must be equal to or greater than the KVA of the load and the KW of the generator must be equal to or greater than the KW of the load.
BUT, if you have motor loads, you should consider the largest motor at 300%. When the generator is sized for any large motor loads the KW will generally be adequate regardless of the power factor.
Bill
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"Why not the best?"
Jimmy Carter
RE: Three Phase Power Question
it's slowly coming back.
Sorry, VL-N is 230 not 220, and VL is 400 not 440.
Going back to sizing, the demand data I have seems to assume resistive (or near) loads as the pf is indicated around 1.
The generator capacity is specified in KVA (100) and KW (80), indicating rated power factor: 0.80
Therefore, if I had 90 KVA (or KW) of resistive loads, would this generator be undersized?
RE: Three Phase Power Question
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(2B)+(2B)' ?
RE: Three Phase Power Question
Rafiq Bulsara
http://www.srengineersct.com
RE: Three Phase Power Question
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Three Phase Power Question
An overloaded generator runs slower and the voltage drops almost proportionally to the frequency. You often CAN NOT live with this.
Bill
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"Why not the best?"
Jimmy Carter
RE: Three Phase Power Question