1
\$\begingroup\$

I noticed that unlike the single phase ac power sources, the power in three phase is constant which means the instantaneous power is the same as the real power which is $$3V_{rms}I_{rms}\cos{\theta}$$

But there is also a reactive power present.

$$3V_{rms}I_{rms}\sin{\theta}$$

Clearly, the complex power would just be the VI for each of the branches leading to the complex power 3VI but the reactive power is clearly missing in the instantaneous power which for single phase sources would have been $$s(t)=cos(θ)P+sin(θ)Q$$ (as shown in this link S = VI*/2 derivation)

But in this case only the real power is present in the instantaneous power. At first I thought I didn't understand something, but now I'm wondering if the reactive power is present but just flows between each load. Am I correct or is there something I don't understand?

\$\endgroup\$
2
\$\begingroup\$

In a balanced three phase system, the power in each of the three phases behaves in the same way as it does in single-phase systems. When you add together the power in the three phases, the result is constant rather than pulsating. The reactive energy flows back and forth between storage elements in each of the three phases of the load and storage elements in the source or distribution system just as it does in a single phase system.

|improve this answer|||||
\$\endgroup\$
  • \$\begingroup\$ I'm still confused about the reason for lack of mathematical presence for reactive power from the source. Is the reactive power supplied from source exactly equal the reactive power coming back from the load at any given time when considering all three branches together? \$\endgroup\$ – Colin Hicks Nov 1 '18 at 2:12
  • 1
    \$\begingroup\$ Yes it is. The reactive VA behaves similarly to the real power with regard to adding together the three phases. \$\endgroup\$ – Charles Cowie Nov 1 '18 at 2:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.