current leading or lagging the voltage

[PG1995]

Hi PG,

Well the blue wave lags the green wave by 60 degrees, or if you call the blue crossing zero then the green lags the blue by 300 degrees.

I'll try to get back later with some circuits to illustrate much better.

Thank you, Ratch, MrAl.

I'm studying phasors which deals with the angle at t=0; i.e. if v(t)=cos(wt+20), then phasor is V=1<20. So, I think I need to need to know how to determine which wave is leading or lagging at zero crossing. Please now a look on the attachment and help me with it. Thank you.

 

[Ratchit]

PG,

As the diagram shows, phasors are assumed by convention to be rotating in a CCW direction. Phasors are drawn relative to each other, usually at the 0° line so they can be easily measured. Therefore, if a particular voltage or current phasor is more CCW than another phasor, that phasor is designated to be "leading". And remember, voltages and currents stay within their 180° limit. Its direction of rotation determines whether it is lagging or leading. It cannot be more concise than that.

Ratch

 

[PG1995]

Thank you, Ratch.

Okay. One last question. If the phasor V is at 30 degree angle and phasor I is 200 degree from the +ve x-axis. Then, which one is leading? Notice that 200-30 is less than 180 degree, i.e. 170 deg.

Now if the phasor V is at 20 deg and I is at 330 deg, which one leading?

I'm so sorry for taking this long to understand this. I hope you don't mind. Thanks.

Best wishes
PG

 

[Ratchit]

PG,

Okay. One last question. If the phasor V is at 30 degree angle and phasor I is 200 degree from the +ve x-axis. Then, which one is leading? Notice that 200-30 is less than 180 degree, i.e. 170 deg.

Back up the V phasor to the 0° line and notice the I phasor is at the 170° line. Since the I phasor reaches 170° before the V phasor does, the I phasor is leading. You will never find that situation in a series or parallel circuit because the V and I are never more that 90° from each other.

Now if the phasor V is at 20 deg and I is at 330 deg, which one leading?

Remember the phase difference is never more than 90° difference between V & I, or 180° between like voltages or like currents in a series or parallel circuit. So I is at -30° and V is at 20°. Therefore, since V has reached 20° before I does, V is the leading phasor in this case.

Ratch

 

[PG1995]

PG,



Back up the V phasor to the 0° line and notice the I phasor is at the 170° line. Since the I phasor reaches 170° before the V phasor does, the I phasor is leading. You will never find that situation in a series or parallel circuit because the V and I are never more that 90° from each other.




Remember the phase difference is never more than 90° difference between V & I, or 180° between like voltages or like currents in a series or parallel circuit. So I is at -30° and V is at 20°. Therefore, since V has reached 20° before I does, V is the leading phasor in this case.

Ratch

Thank you very much.

But what if I back up the V phasor to the 0 deg line and then I is at 310 deg. Now the I phasor has reached 310 deg mark before the V phasor, therefore, isn't I the leading one? Please let me know and I have almost understood it.

Best wishes
PG

 

[Ratchit]

PG,

But what if I back up the V phasor to the 0 deg line and then I is at 310 deg. Now the I phasor has reached 310 deg mark before the V phasor, therefore, isn't I the leading one? Please let me know and I have almost understood it.

No, the I phasor is at -50°. We keep the angles at less than 90° for I and V. You cannot build a circuit that exceeds that limit, remember?

Ratch