# Lagging or Leading - Other than PF, why do we care?

#### Duffsam

Joined Jun 10, 2015
3
Hello,

I'm currently studying Electrical/Electronics Engineering at my local college in the UK. I'm rather curious, and I'm trying to read into/ahead of the modules I'im studying.
I believe that I am competent in calculating the phase angle of an AC RLC circuit, whether it be in series or parallel.
I also understand that phase angle is related to power factor, whereby if a circuit has a lagging phase angle, its power factor will be low, causing the circuit to use its supplied electrical energy inefficiently.

My question is, why else should I [or we, as EE engineers] care about phase angle? Are there any other electrical principles it affects?

Sam Duffield

#### WBahn

Joined Mar 31, 2012
26,398
MOD NOTE: Moved to Homework Help.

#### WBahn

Joined Mar 31, 2012
26,398
The biggest is simply that we can't properly analyze the behavior of an AC circuit without taking the phase angle between the voltage and the current into account.

Suppose in the circuit above I know that the magnitude of I1 is 1000 A and that the magnitude of I2 is 1001 A. What is the magnitude of Io?

#### JoeJester

Joined Apr 26, 2005
4,390
look up power correction factor and power correction penalties.

#### WBahn

Joined Mar 31, 2012
26,398
In the TS's defense, I thought at first he was asking why EE's should care AT ALL about phase angle, but I see that he is asking WHY ELSE they should care. I actually had a pretty pointed response all typed out when I spotted that easy to miss distinction.

#### Duffsam

Joined Jun 10, 2015
3
The biggest is simply that we can't properly analyze the behavior of an AC circuit without taking the phase angle between the voltage and the current into account.

View attachment 87236
Suppose in the circuit above I know that the magnitude of I1 is 1000 A and that the magnitude of I2 is 1001 A. What is the magnitude of Io?
Thank you, WBahn.
It appears that I've overlooked the fact that the phase angle of currents in a circuit grants the ability to analyse a circuit, not just the ability to calculate a circuit's power factor.

I guess I hoped there would be more to it, for example, relays that can distinguish between phase angles, or transmitters/receivers that require a resonant RLC circuit?

#### WBahn

Joined Mar 31, 2012
26,398
Oh, there are plenty of reasons and phase angles play a role in a great many things in a variety of ways.

#### MrAl

Joined Jun 17, 2014
8,476
Hello there,

Another view is that there are TWO PARTS to an AC signal, the amplitude and the phase angle, and they are both needed to completely specify any signal, similar how we need two parts to describe any complex number such as a+b*j where 'a' is the real part and 'b' is the imaginary part.
In fact, an AC quantity can be represented in either form, complex or polar, where complex consists of the real and and imaginary parts and the polar form consists of the magnitude and phase angle, and we can convert between the two when needed.

This is different than when we look at a DC circuit where we usually get just one number like 12 volts, 4 volts, 3 volts, etc. With AC we might get 5v at angle of 30 degrees for example, or just something like 10+7*j volts.

The conversion is as:
Magnitude=sqrt(real^2+imaginary^2), in whatever units you're working in, and

#### MrAl

Joined Jun 17, 2014
8,476
Hi again,

Nothing says it better than "Three Phase System", where if we did not have anything like 'phase' we would not be able to tell the three different phases apart from each other where phases A, B, and C are separated by 120 degrees apart from each other. There are other multi phase systems too with up to maybe 48 different phases. The most i have ever worked with on paper was 48, but in real life the most i have ever worked with was 3 phases (120 degrees apart), for example a line tied solar power converter.

#### mxg2579

Joined Jun 10, 2015
15
Like MrAl said, three phase systems are the main ticket here. In the field of Power and Energy, phase angle plays such a big role in just about everything: AC Machinery, Power delivery, etc. If we wouldn't observe phase angle, many things would be near impossible to understand. There are so many applications and uses that demonstrate the importance of phase angle.