Calculating three phase power
Calculating three phase power
(OP)
How can I calculate the output power from a 3 phase UPS with unbalanced loads?
Load Voltage(Vrms)R-Y 409.8 Y-B 414.3 B-R 414.1
Load Voltage R-N 237.4 Y-N 238 B-N 239.7
Load Current(Arms) R 79 Y 27.7 B 41.3 N 68.4
Our engineer attached a Fluke scope meter to each phase voltage and current and then added them together. This came out at approximately 60kVA. But if you use P=V*I*1.73 for each phase and then sum these results this adds up to 105.6kVA.
Anyone help?
Load Voltage(Vrms)R-Y 409.8 Y-B 414.3 B-R 414.1
Load Voltage R-N 237.4 Y-N 238 B-N 239.7
Load Current(Arms) R 79 Y 27.7 B 41.3 N 68.4
Our engineer attached a Fluke scope meter to each phase voltage and current and then added them together. This came out at approximately 60kVA. But if you use P=V*I*1.73 for each phase and then sum these results this adds up to 105.6kVA.
Anyone help?
UPS engineer http://www.calibrepower.co.uk






RE: Calculating three phase power
Bill
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"Why not the best?"
Jimmy Carter
RE: Calculating three phase power
For unbalanced system, calculate per phase values and add them up.
For each phase power, the formula is V*I where V is line to neutral or (Vll/1.732).
Rafiq Bulsara
http://www.srengineersct.com
RE: Calculating three phase power
RE: Calculating three phase power
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If we learn from our mistakes I'm getting a great education!
RE: Calculating three phase power
Rafiq Bulsara
http://www.srengineersct.com
RE: Calculating three phase power
How was that connected and measured? Given your numbers, the only way I can get close to 60 kVA is by using the peak instead of RMS L-N voltages.
SUM( Vln * Il ) = 41.7 kVA
Vln = line-neutral voltages (Vr-n, etc.)
Il = line currents
Using the line to line voltages, the proper formula is
SUM( Vll * Il )/ 1.73 = 72.3/ 1.73 = 41.6 kVA
Vll = line to line voltage
which is close enough.
But you don't. That will work for a balanced situation where V is Vll and V and I are the balanced line to line voltage, line current. But then you don't sum them.
RE: Calculating three phase power
My post did not state clearly how my engineer took the readings. He attached a Fluke scopemeter to each phase and set the scope to kVA power and then added these results together.
UPS engineer http://www.calibrepower.co.uk
RE: Calculating three phase power
kW is power.
KVA is apparent power.
You are calculating apparent power, not true power. {rant off}
Yes, I get 35.2 KVA
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Calculating three phase power
Thanks but I realise that and will be applying 0.8pf for this UPS. Didn't want to confuse things any more!
UPS engineer http://www.calibrepower.co.uk
RE: Calculating three phase power
UPS engineer http://www.calibrepower.co.uk
RE: Calculating three phase power
Rafiq Bulsara
http://www.srengineersct.com
RE: Calculating three phase power
Your system is a 4-wire, so 3-element metering is adequate.
So, 35.2 kVA is the correct answer!