The Electrical Contractor Network

ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Books, Tools and Test Equipment for Electrical and Construction Trades

Register Now!

Register Now!

We want your input!


2017 NEC and Related
2017 NEC
Now Available!

Recent Posts
Parking lot pole light swap....
by gfretwell
Yesterday at 08:46 PM
International Wire Colour Codes
by Tjia1981
10/23/16 12:08 PM
Son of Sparky
by HotLine1
10/20/16 07:43 PM
Speaking of Plugmold ...
by gfretwell
10/17/16 02:37 PM
Broken battery charger? Check for cobwebs!
by gfretwell
10/17/16 02:30 PM
New in the Gallery:
12.5A through 0.75mm˛ flex (just out of curiosity)
Shout Box

Top Posters (30 Days)
gfretwell 14
HotLine1 7
ghost307 7
renosteinke 6
Potseal 4
Who's Online
1 registered (gfretwell), 321 Guests and 4 Spiders online.
Key: Admin, Global Mod, Mod
Topic Options
Rate This Topic
#130283 - 03/02/06 01:27 PM superposition theorem
james S Offline

Registered: 05/13/03
Posts: 102
Loc: West England
Am i right in saying that you can't use superposition theorem on ac networks unless j notation is used?
I have a parallel network with the components and supply sources labled in j notation,so to avoid messy equations i have tried to convert these values into impedances and supply voltages (with no phase angles) , and continue like you would in a dc network.
The reason why i think you can't do this is because there are inductive and capacitive currents meeting each other changing the characteristics of the circuit currents.Is this correct?

#130284 - 03/02/06 02:23 PM Re: superposition theorem
Bob Offline

Registered: 02/05/02
Posts: 182
Loc: Mobile, AL, USA
Not sure what you are doing but you can not work with caps and inductors in a DC circuit.

#130285 - 03/02/06 04:55 PM Re: superposition theorem
winnie Offline

Registered: 09/15/03
Posts: 652
Loc: boston, ma
"j" notation is not strictly necessary, but you must do _something_ to account for the energy stored and released from the reactive components. using the concept of 'phase angle' and "j" notation is a method for doing this accounting that works quite well when you are working at a single frequency with fixed value components.

Another technique that you could use to figure out these sort of circuits is to calculate how the current changes with time, using equations that have time components (eg V*sine(t) for voltage, C*dV/dt for the current through the capacitor, etc) . The "V/I characteristic" of components ("resistance" generalized) changes with time, so V = I * V/I (Ohm's law) quickly becomes a pretty serious differential equation. This sort of approach is generally not necessary for 'human' calculations, but is quite often used in computer simulations. Also the basic concepts of how a capacitor works are more easily understood in terms of thinking of change in charge over time, so it is a nifty exercise to go from I=C*dV/dt (and the other associated equations) to phase angle and reactance notation.


#130286 - 03/02/06 05:28 PM Re: superposition theorem
james S Offline

Registered: 05/13/03
Posts: 102
Loc: West England
Thanks jon for your quick reply!
you have just confirmed my thinking of such circuits.Since posting the problem i have worked out the different branch currents using j notation (it took a while),all the text books i have dont venture passed the DC sinario,and say things like this method can be employed for AC circuits changing the resistance for reactance,hence my initial assumption.
thanks again

"and continue like you would in a dc network."
i know you can't work with caps/inductors in dc ,i meant the working out to my problem is treated like a DC circuit.


ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals