# Three phase power

Discussion in 'Homework Help' started by andy24691, Feb 19, 2011.

1. ### andy24691 Thread Starter Member

Nov 25, 2010
42
0
I had a lab session where I did the two watt metre test on a 3 phase, 4 wire star system. I have values for each line voltage and line current. I also have a total power measurement. Should the total power be equal to the sum of each line voltage multiplied by their respective currents? When I do this the power works out about 0.6kW larger then the total power reading I read off. I was wondering whether you need to take phase angles and all that stuff into consideration?

2. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790

Indeed you do. Any phase displacement between phase current and voltage means that the load is partly reactive and a non-unity power factor case.

Unless you measured the phase relationship between current and voltage per phase it's going to be hard to reconcile any differences. If the loads you used were notionally purely resistive, then the discrepancies may be due to measurement errors.

If 0.6kW is not a significant error in the total power measurement then the latter point may be relevant. If it is a significant error then I would lean to the non-unity power factor explanation.