The task says:

Determine the positive maximum power p (t), for the load connected to the voltage 230 V. The power consumption was 1350 W and the power factor was 0.85.

People have put this on hold because they said I did not try to solve it myself, I did but I did not write what I was doing because It led me to the wrong answer.

So I am gonna try again: P = 1350 W

I know that power is a sine function so my initial thought is that giving me voltage and power factor is nothing more than trying to trick me.

I tried to solve it like this:

I know that \$p(t) = P\sqrt2sin(wt+φ)\$ so positive maximum should be \$P\sqrt2\$ which gives me 1909 but that answer is wrong.

after that I tried to find apparent power \$ S = \frac{1350}{0.85} = 1588\$ and multiply that with \$\sqrt2\$ and that would give me 2246 VA. (But that is also incorrect). The correct solution is 2938 W, and don't know why is that solution and not what i firstly tried

Any help is greatly appreciated

  • 1
    \$\begingroup\$ I think the task quoted is pretty strange, but I have an interpretation to try. I will be back later. \$\endgroup\$
    – user80875
    Commented Aug 4, 2019 at 23:16
  • \$\begingroup\$ @CharlesCowie Okay I'll be waiting :) \$\endgroup\$ Commented Aug 4, 2019 at 23:55
  • 1
    \$\begingroup\$ @HarrySvensson When current and voltage are not in phase, power goes negative for part of the cycle. Therefore to get the required average power the peak positive power must be higher. In this example the average power is specified as 1350W, so 'lower power at lower power factor' doesn't apply. \$\endgroup\$ Commented Aug 5, 2019 at 0:05

1 Answer 1


The quoted task seems to be about P(t), power as a function of time or instantaneous power. The instantaneous voltage, real current, reactive current, total current, and power are shown below. Power has been divided by 100 to make a more readable graph. The "positive maximum power" of the task is the positive peak of the power waveform. I determined that graphically as 2938 watts. I will leave it to the student to determine it analytically.

enter image description here


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