# RC Circuit Phasor Diagrams Reference Point in AAC AC Circuits Chapter 3

Thread Starter

#### K1JOS

Joined Aug 17, 2014
12
I am studying for my ham radio extra license and AAC Vol II, AC Circuits has been invaluable for me. It is quite clear including the math but I am frequently puzzled how reference points are established.

For example, in the following figure, it makes perfect sense that the AC source Et defines the vector angle of zero: In the next Chapter on Series RL it is explained clearly why inductor current lags behind voltage but the reference point used in the figure is not-intuitive. Since Et = El why isn't the AC source voltage phase still the reference point at zero degrees? If displayed this way then the inductor current $$I_{L}$$ lagging by 90 should be at -90 angle. Leading should be postive anmd lagging should be negative angle with respect to the AC source E and I phases. As I look ahead to more complex circuits, I want to avoid rote memorization and have a more intuitive feel to understanding how mixed circuits work. I must be missing something obvious if these are the teaching standards used? Can anyone offer some advice?

thanks
Jerry K1JOS

#### WBahn

Joined Mar 31, 2012
26,398
The phase reference is completely arbitrary and you can pick whatever you want. In this case the phase angle of the current in the inductor was chosen as the phase reference. It would be perfectly acceptable to chose the phase angle of the source voltage instead. All that matters are the relative angles between items.

Thread Starter

#### K1JOS

Joined Aug 17, 2014
12
The phase reference is completely arbitrary and you can pick whatever you want. In this case the phase angle of the current in the inductor was chosen as the phase reference. It would be perfectly acceptable to chose the phase angle of the source voltage instead. All that matters are the relative angles between items.
Yes, i understand that, and I did a few example calculations with El <90 and I <0 versus El <0 and I< -90 and they are mathematically equivalent. I was just trying to understand why AAC and many textbooks change the reference point around. Its not for making the math easier.

Where I got totally confused is in next chapter on mixed RL Since both a resistor and/or an inductor do not change the source E phase - only the current phases why does this figure show different phase relationships between ET, ER and EL ? The phase changes should only be for current not voltage? I guess this is where I am confused still

Jerry

PS I tried to use LaTex coding but cant seem to get the hang of it.

#### Jony130

Joined Feb 17, 2009
5,230
But how can you can have a phase shift is current in series circuit? Do not forget that the current is the same everywhere in a series circuit ( only one current is flowing).

Thread Starter

#### K1JOS

Joined Aug 17, 2014
12
But how can you can have a phase shift is current in series circuit? Do not forget that the current is the same everywhere in a series circuit ( only one current is flowing).
Ahhh, I think I misunderstood a basic concept. Looking at earlier figure of a simple series inductor: I assumed (incorrectly it seems) that the AC source voltage and AC source current were always in phase and that the current lag only was seen across the inductor.

If the AC source was a mechanical AC armature generator - how could the the voltage and current phases change? Aren't= they both dependent on the physical relationship of the spinning armature to magnets in the generator? I guess my "basics" on AC are still not well formed jerry

#### WBahn

Joined Mar 31, 2012
26,398
The phase difference between the voltage and current in an AC source is dictated by the load. The mechanical arrangement dictates the voltage, but the current is dictated by the interaction of that voltage and the load. They do not need to be in phase. In fact, look at that simple generator and inductor circuit -- they are 90° out of phase.

Thread Starter

#### K1JOS

Joined Aug 17, 2014
12
Many thanks, that clears up a lot !