solve mesh equation

[MP_11950640]

Can anybody please help. 🙂

i cant seem til solve this one.

• The DC generator in Figure 2 can deliver a maximum current of 21 amperes. rGr_GrG​ and rBr_BrB​ symbolize the internal resistances of the generator windings and the accumulator.

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Task 2.1:

Determine the value of R3​ when the generator delivers maximum current.

Task 2.2:

Determine the power dissipated in the generator's windings when it delivers maximum current.

[ppal]

Not Checked if correct!

But write some node equations to get the summation of currents at the nodes:

[Werner_E]

@ppal 

In your system there always will be  I₃= I_G and R₃ is not used at all and will remain at its guess value.

[Werner_E]

Given your input values, R3 would come out as being infinite! Thats the reason the solve block fails.

Slightly changing one of the input values (in the picture I had chosen to change R1) and increasing the guess value for R3) makes the solve block work and we see that R3 is already pretty large.

You could also solve the system symbolically

The last expression for R3 throws a division by zero error if evaluated with your input values.

BTW, the result given with the slightly altered value for R1 is quite similar to what we get if we use "minerr" instead of "solve".

But I guess that you should rather check your input values and/or the correctness of your equations.

Shouldn't it be I1=I3+I4 and not I1=I3+I5 ???

[LucMeekes]

In this purely resistive system, none of the currents should be larger (in absolute value) than the maximum current that the generator can deliver: 21 A.

Luc

[Werner_E]

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@LucMeekes wrote:

In this purely resistive system, none of the currents should be larger (in absolute value) than the maximum current that the generator can deliver: 21 A.

Luc

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Good argument 😉

[LucMeekes]

Hey Michael,

You can do:

Then:

and:

now you can solve for maximum Ig:

And then calculate:

(Plot:

)

I think you can work out the power loss in the generator yourself.

Success!
Luc

[ppal]

[ttokoro]

[Werner_E]

@ttokoro 

The solution to Task 2.1 is not 0 ohm because the current the generator can deliver is limited to 21 A according to what the poster wrote (the information also seems to be in that Danish text in the picture).
The solution for 2.1 was already given by @LucMeekes  and later also by @ppal .

But we get the same results as yours with the very equations @MP_11950640  had set up (apart from the wrong node equation which I already mentioned and which may be a typo).

Main error was the the OP used a fix constant value of 1A for I_G and solved for R₃ while he rather should have solved for I_G and make the solve block dependable of a variable R₃.

Now that we have a function I_G(R₃) its easy to determine the resistance which yields the 21 A. We can use 'root' to do the job

Prime 10 sheet attached

@MP_11950640   Could you work out a solution to the problem you posted here: Assistance Needed with Adjusting Axes

[Werner_E]

Looks like I was drawn away by this thread and got lost...

Coming back to the original question we can say that just two errors have to be corrected:

1) wrong index 5 in one of the node equations

2) wrong value for I_G was provided - it must be 21 A and not 1 A

Fixing these two makes the solve block work and yields the correct result for R₃. No other changes necessary.