(10-05-2017, 12:35 AM)Nowhere-Man Wrote: Apparent power would be 500 watts
Ah. There is a nest of vipers in Apparent Power. If you rigorously derive the current flows in a three phase system, which is inherently balanced, you get 3EI for the Apparent Power. So far so good, straight text book stuff.
But if you then set two phase currents equal to zero, in other words a single phase supply, you get, wait for it, root(2)EI. In other words not the text book EI. It is all down to the fact that a single phase supply is inherently imbalanced.
You needn't invoke a three-phase trick - you can derive the root(2)EI from first principles.
I got heavily into this about a decade ago when we were assisting in setting up a new startup with an Electrical Engineering Professor. He said that all the text books have single phase apparent power wrong, and it should be root(2)EI. We were all super skeptical until he derived it. This was all to do with there being no accepted IEC method for defining Apparent power in the general system of a three-phase supply with non-linear loads and distorted waveforms. His method produced more realistic numbers in those situations - somewhat important for power-hungry companies when it comes to pay the electricity bill.
The uncomfortable, but entirely correct corollary was the root(2) factor in single phase Apparent Power. This manifests itself directly when considering rotating machines like a motor. A single phase motor is always less efficient that an otherwise identical motor but wired for three phase - because of the root(2) in Apparent Power.
To prevent confusion the Prof in question re-named his method as deriving Universal Apparent Power (UAP) rather than the incorrect text book AP.
