# Homework Help: Ac circuits

1. Jul 28, 2014

### ranju

f we are given a single phase load supplied by single phase voltage source..& a current 10<-1500 & the volge at load terminal is 100<600, then what type of power (real or reactive ) will the load absorb & supply??
Its clear here that the load is inductive one..so the power consumed should be zero..!!! then why the concept of consuming power is being arised??
and what about the type of power being delievered??

2. Jul 28, 2014

### donpacino

complex power is defined as V*I.

Why do you say the load is inductive?
When you say the power consumed should be zero, are you talking about zero volt-amps or zero watts? there is a difference.

walk us through your thought process so we can point out where you are going wrong.

3. Jul 28, 2014

### ranju

its inductive because the current lags the voltage by 90 degrees..I am saying that since its an inductive one so it should absorb zero real power... this is what I think..I am not concerned about what type of power it delievers whether real or reactive one...

4. Jul 28, 2014

### ranju

sorry..the phase difference I stated is wrong..so its obvious that it is also not an inductive load..now I am stuck.. can you please help out..how to proceed..??

5. Jul 29, 2014

### Staff: Mentor

Note that in this instance both the current and voltage at the load are "measured" to have phase angles. This implies that the actual reference for the phase angles is associated with some circuit location or source that isn't specified. But that's not a problem! It's their relative relationship that counts.

Make a rough sketch of the two phasors on the complex plane (don't worry too much about scale for their magnitudes since they have different units: voltage vs current). Now, if you rotate the axes of the plot (or equivalently, both phasors together) their relative angular relationship remains the same. Since it's common to take a voltage as a reference, taking its angle to be zero, rotate the axes to align the real axis with the voltage phasor. What then is the relative angle of the current with respect to the voltage phasor?

6. Jul 29, 2014

### donpacino

power is V*I.
multiplying the voltage and current together you get

100<-90

Because the phase of the power is -90 degrees, is the power real or reactive?

7. Jul 29, 2014

### Staff: Mentor

Careful, the complex power is given by P = VI*, where I* is the complex conjugate of the current I.

A difference in phase between the current and voltage tells you at the very least that there is some reactance in the load. The 90° tells you more, and whether it's + or - 90° tells you even more

8. Jul 29, 2014

### donpacino

wow. 1.5 years out of school and I'm already forgetting simple equations :(
thanks for the correction

9. Jul 30, 2014

### psparky

Aren't the voltage and current out of phase by more than 90 degrees? Is that even possible with the "j' factor in inductors and capacitors?

10. Jul 30, 2014

### donpacino

It does not state anywhere that the load is a single component, so yes it is possible.

Also I want to point out that the question does not ask to determine what the load is, but simply determine the type of power delivered.

11. Jul 30, 2014

### psparky

f we are given a single phase load supplied by single phase voltage source..& a current 10<-150 & the volge at load terminal is 100<60, then what type of power (real or reactive ) will the load absorb & supply??

Are you sure? I would agree that the original description is weak, but still it says a single phase load fed by a single phase voltage source.

But thinking about it, phase changes of 180 happen easily in double pole filters and what not.

12. Jul 30, 2014

### donpacino

If the voltage across the load is 100<60 and the current is 10<-150, then the load must be 10<-150.
Z=V/I

Therefore the load cannot be comprised of a single ideal element.

The term single phase refers to one AC channel existing in the network. In a 3-phase system there are 3 channels in which the voltages on the 3 phases are 120 degrees away from each other. Each one of the three phases connects to their own load. The term single phase does not refer to a system having a single pole or zero

note: I realize many 3 phase systems are typically connected in a delta configuration.

13. Jul 30, 2014

### Staff: Mentor

The question doesn't say anything about what might be between the single supply and the load, nor does it say what reference is used for the phase angle (who knows, it could be some voltage measured elsewhere in another part of the circuitry). All that's given is the current and voltage as interpreted ("measured") at the load. So in this case it's possible to have a single ideal component as the "load".

14. Jul 30, 2014

### donpacino

We are given the voltage across the load and the current through the load.

If we know both of those values we can calculate the impedance of the load.

The impedance of the load is calculated to be 10<-150. I have never heard of a single ideal component having a complex impedance not at an angle of -90 or +90.

That being said the purpose of the problem is to discuss power dissipation, not determine the load

15. Jul 30, 2014

### Staff: Mentor

So it would appear that the load is not comprised of a single standard component. Indeed, it may even contain sources of its own! What does the value of the complex power at the "load" suggest to you?

16. Jul 30, 2014

### Staff: Mentor

A generator with an impedance counts as at least two.
EDIT

Last edited: Jul 30, 2014
17. Jul 30, 2014

### Staff: Mentor

I would have to agree. For example a generator comprising a coil rotating in a magnetic field might be represented by two ideal components.

18. Jul 31, 2014

### Staff: Mentor

You are probably thinking of the phase shift of output voltage relative to input voltage. Big difference. I doubt you have closely investigated how input current relates to input voltage in ordinary filter applications.

19. Jul 31, 2014

### psparky

lol...just making sure someone is paying attention!!! I knew it changed the phase of something....been a while, sorry!!!