# Ideal transformer

Discussion in 'Homework Help' started by full, Nov 1, 2014.

1. ### full Thread Starter Member

May 3, 2014
225
2
Hello everyone

I do this problem in Ideal transformer

please check where is my wrong ,I check a lot but I not see any wrong in loop

thanks

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Last edited: Nov 1, 2014
2. ### WBahn Moderator

Mar 31, 2012
20,221
5,746
Your mesh equation is ignoring the voltage across the two terminals of the transformer. Remember, a mesh must for a complete loop.

3. ### full Thread Starter Member

May 3, 2014
225
2
thanks sir
is mean same this ?

4. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,173
1,188
Finally you mash 3 has complete loop.
After some thoughts it seems to me that your mash 3 should look like this

Last edited: Nov 2, 2014
5. ### panic mode Senior Member

Oct 10, 2011
1,420
347
problem is that you did not mark circuit correctly:
- you did not replicate circuit fully (missing connection between primary and secondary at the bottom)
- you did not draw full loop of your mesh
- you did not mark all voltages and all currents

without this you will never get reliable and consistent results.

then you have mesh3 problem: you simply ignored voltage difference at the top side of the transformer.
this is NOT a short circuit. you cannot do that.

since the primary and secondary are connected at the bottom side (which is convenient place to consider a reference point, specially since also connected to one side of 120v supply), voltages at transformer are V1 and V2.
difference is V1-V2=V1-(-2V1)=3V1 which is missing in your mesh3 equation

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6. ### full Thread Starter Member

May 3, 2014
225
2
I same your the idea ,but the solution from book is different in loop 3

(this is another problem )

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7. ### Jony130 AAC Fanatic!

Feb 17, 2009
4,173
1,188
Well I'm not a mesh specialist, I prefer a nodal analysis. And I get the correct answer only if I do this mesh like this

Last edited: Nov 2, 2014
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8. ### full Thread Starter Member

May 3, 2014
225
2
I only I want understand why the book solution do to a different loop

thanks sir

Last edited: Nov 2, 2014
9. ### WBahn Moderator

Mar 31, 2012
20,221
5,746
To truly be "meshes" (as opposed to just "loops"), the currents must not overlap and, correspondingly, each component must have either a single mesh current flowing through it or two mesh currents flowing in opposite directions (note that this is for planar circuit topologies, which this is), provided the mesh currents are consistently defined as either all CW or CCW. You meshes are therefore:

But there is nothing that prevents you from using loop currents, of which you have seven to choose from and you must simply choose three that are independent, which can usually be accomplished by making sure that you choose a set that incorporates each component at least once.

In writing your three mesh equations, you will end up with five variables, the three mesh equations and the voltages across the two sides of the transformer. So you need two auxiliary or constraint equations, which are provided by the ideal transformer assumption, namely that the voltages across the transformer and the currents through transformer are scaled by the turns ratio and in the polarities indicated by the dots. Remember, the voltages across the transformer are both positive at the dot (well, technically have the same polarity, you pick) while the currents have opposite polarity. You can remember this last point by keeping in mind that one side of the transformer is acting as a source while the other side is acting as a load.

Once you have your mesh equations, a simple check that should always be performed is to add them all up and see if the result is the loop equation you would get going around the periphery of the circuit, which can usually be seen by inspection and doesn't even need to be written down. If not, then at least one of your mesh equations is wrong.

Last edited: Nov 2, 2014