# Sat an exam yesterday - impedances in mesh analysis

#### Raikiri

Joined Mar 22, 2009
16
I sat a power electronics exam yesterday and I was kind of banking on a mesh analysis question, if I get this Q wrong I screwed up the exam basically. The usual Qs I've done with mesh have only has resistors and sources without phases but the exam Q had impedances so I'm wondering if I did it right or not...

The question asked to calculate the two mesh currents, the way I was taught to do this, at least when resistors are the only components was to establish a set of simultaneous eqns, so thats what I did. After a bit of working out, I'm pretty sure you can't do this but I got rid of the j's in the equations and I was left with only I1's and I2's.

Final answer was I1 = 10A ; I2 = 5A ...... no phases.

Could someone put me out of my misery and let me know if this is correct?

thanks #### The Electrician

Joined Oct 9, 2007
2,866
The question asked to calculate the two mesh currents, the way I was taught to do this, at least when resistors are the only components was to establish a set of simultaneous eqns, so thats what I did. After a bit of working out, I'm pretty sure you can't do this
You most certainly can "...establish a set of simultaneous eqns..."; that's what you should do.

If you take the currents in the meshes to be clockwise, and denote the left mesh as I1 and the right as I2, you should get these two equations:

(1+j)I1 + (-j)I1 - (-j)I2 - 5 = 0

-(-j)I1 + (1+j2)I2 + (-j)I2 - 5 = 0

Collecting terms, they become:

(1)I1 + (j)I2 = 5

(j)I1 + (1+j)I2 = 5

They are in suitable form for solving with a matrix solver. The solution is:

I1 = 2-j
I2 = 1-3j

#### Raikiri

Joined Mar 22, 2009
16
Thanks electrician but I knew you could use simultaneous eqns, I was referring to the part where I removed the j's from the eqns so I was left with only I1's and I2's. Can you do this or does this not work with complex numbers?

My final answer gave me I1=10A ; I2=5A

#### The Electrician

Joined Oct 9, 2007
2,866
Thanks electrician but I knew you could use simultaneous eqns, I was referring to the part where I removed the j's from the eqns so I was left with only I1's and I2's. Can you do this or does this not work with complex numbers?

My final answer gave me I1=10A ; I2=5A