0
\$\begingroup\$

I have been given the following question:

A single-phase load is supplied by a single-phase voltage source. If the current flowing from the load to the source is 15∠-135° A and if the voltage at the load terminals is 220∠45°, V then the

  1. load absorbs real power and delivers reactive power.
  2. load absorbs real power and absorbs reactive power.
  3. load delivers real power and delivers reactive power.
  4. load delivers real power and absorbs reactive power

I have done my best and I am fully confused which one is the right answer. I think 4 might be correct because total angle is -90 degrees and current is flowing from load to source. Please give little explanation also.

\$\endgroup\$
  • \$\begingroup\$ I believe that phasor angles are normally measured from the positive real axis with a positive angle being counterclockwise and a negative angle being clockwise. It appears that the angle difference between the voltage and current is 180 degrees. Real power is delivered by the load to the source. There is no reactive power. Reactive power is not "delivered," but I believe there is a convention that uses that term to describe whether the source or load is capacitive or inductive. Are you sure the signs are correct in the question? \$\endgroup\$ – Charles Cowie Jan 21 '17 at 11:32
2
\$\begingroup\$

Let's do the canonic complex power calculation - all numbers and directions as you say and assuming you obey the commonly used conventions

enter image description here

The result: The load outputs negative real power, total output = -3300 W . That's equivalently: The load takes 3300 W real power. No reactive power in either direction.

The Answer: None of your options is right.

Useful to know: As already commented, outside the theoretical textbooks it's not common to talk about how much reactive power a load absorbs or delivers. These words are ok for real power, but practical electricians rather say how much reactive power is supplied into the load and was it inductive or capacitive. In power distribution systems there are sometimes synchronous generators or other reactive power compensation elements that are said to deliver or absorb reactive power.

\$\endgroup\$
  • \$\begingroup\$ "... , the amount is 3,3 kVAr and it's inductive. " - are you sure. I'm currently learning this topic, but since on capacitive loads the current leads the voltage (negative phase) it should be a capacitive load. Am I wrong? \$\endgroup\$ – try-catch-finally Apr 22 '17 at 16:31
  • \$\begingroup\$ @try-catch-finally actually I'm wrong. I have got a bigger screen and the numbers that you have asked are different than in my calculation. Thanks for the note. I fix the answer to be in accordance with the question asap. \$\endgroup\$ – user287001 Apr 22 '17 at 17:21
  • \$\begingroup\$ @try-catch-finally The fix is done. Without any complex numbers you should see that the phase difference between U and I is 180 degrees. That comes 0 degrees if the I is reversed. \$\endgroup\$ – user287001 Apr 22 '17 at 18:15

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.