Complex RLC circuit

Thread Starter

john williamsXT

Joined Jul 14, 2009
10
This is really a mathematics question.
I have obtained the following differential equations for a complex RLC circuit.
\(
(L_1D^2+RD+\frac{1}{C})i_1+(RD+\frac{1}{C})i_2=-5000\sin{100t}
\)
\(
L_1D^2i_1-(R_2D+\frac{1}{C_2})i_2=0
\)
Eliminating either i1 or i2 leads to a third order differential equation, which looks nasty!
Anyone got any tips for solving?
Best regards
John
 

millwood

Joined Dec 31, 1969
0
eq 1 + eq 2 gives you

(2L1D2 + RD+1/c)i1=-5000sin100t, from which you can solve for i1.

once you have i1, you have i2 from the 2nd equation.
 

Ratch

Joined Mar 20, 2007
1,070
john williamsXT,

Anyone got any tips for solving?
Rearranging the second equation gives L1*D^2*i1 = R2*D*i2+i2/C2 .
Substituting into the first term of the first equation and rearranging gives, [(R+R2)*D+1/C +1/C2]*i2 = -5000*sin(100*t) . This is a linear equation of order one, from which an integrating factor can be found and i2 solved. Once you find i2, can i1 be far behind? Looks like lots of algebra.

Ratch
 

Thread Starter

john williamsXT

Joined Jul 14, 2009
10
Hi there
After substituting into the first equation the first equation becomes.
\(
[(R+R_2)D+\frac{1}{C}+\frac{1}{C_2}]i_2+(RD+\frac{1}{C})i_1=-5000\sin{100t}
\)
So you still have a term in i1.
Regards
John
 

Thread Starter

john williamsXT

Joined Jul 14, 2009
10
Actually I have progressed further, as follows.
As indicated in my original post elimination of i1 or 12 leads to a third order differential equation.
The homogeneous equation is as follows.
\(
[D^3+(\frac{RR_2CC_2+L_1C+L_1C_2}{CC_2L_1(R+R_2})D^2+(\frac{R_2C_2+RC}{CC_2L_1(R+R_2)})D+\frac{1}{CC_2L_1(R+R_2)}]i_1=0
\)
Now the circuit components have the following values.
R=2,R2=5,C=2.10(-4),C2=3.10(-4),L1=2
After substitution of these values I get the following(approximate) equation for i1.
\(
(D^3+1191D+2262D+1190476)i_1=0
\)
This will give a characteristic equation in m of.
\(
m^3+1191m^2+2262m+1190476=0
\)
If correct so far is this now best solved by iteration method?
Cheers
John
 

Ratch

Joined Mar 20, 2007
1,070
john williamsXT,

So you still have a term in i1.
You are correct. Sorry for the mistake in algebra.

The roots of your equation in 'm' are -1189.939821, -.5300895-31.62545812i, -.5300895+31.62545812i .

Ratch
 

Thread Starter

john williamsXT

Joined Jul 14, 2009
10
Thanks for that Ratch.
How did you find those roots, by an iteration method?
Do you think these values are realistic?
And what about the nonhomogeneous equation?
Best regards
John
 

Ratch

Joined Mar 20, 2007
1,070
john williamsXT,

How did you find those roots, by an iteration method?
I used a computer program. There are zillions of them around than can find the roots of polynomials. So can many hand calculators. http://www.savetman.com/roots.html

Do you think these values are realistic?
Plug in the roots and see if they satisfy the equation.

And what about the nonhomogeneous equation?
I did not write it, so I cannot vouch for its veracity.

Ratch
 
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