Transformer with attached load: required source voltage

AI Thread Summary
A large power transformer with a turns ratio of 1:5 has a primary voltage of 400V and a secondary current of 100A, leading to a secondary voltage of 2000V and a primary current of 20A. The required source resistance for maximum power transfer is calculated as R = Vp/Ip, but initial calculations presented were incorrect. The discussion emphasizes the importance of defining primary and secondary resistances accurately, as transformers alter impedance. Clarification is provided that the source resistance (Rsource) and load resistance (RL) should be considered for proper calculations. Understanding these concepts is crucial for achieving maximum power transfer in transformer circuits.
Kev1n
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1. A large power transformer has a turns ratio N1 / N2 of 1:5. When the primary voltage is 400v and it is connected to a resistive load its secondary current is 100A. Calculate the required value of source resistance of the supply to give a maximum power trnasfer



2. Vp = 400v, Ratio 1:5, Is = 100A



3. Vp = 400v, 1:5 Vs = 2000, Is = 100A, 1:5, Ip = 20A
R = Vp/Ip = 4 ohms.
I am a bit lost here, any pointers
 
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Kev1n said:
2. Vp = 400v, Ratio 1:5, Is = 100A

3. Vp = 400v, 1:5 Vs = 2000, Is = 100A, 1:5, Ip = 20A
R = Vp/Ip = 4 ohms.
I am a bit lost here, any pointers

So far so good. :cool:

What can you say about "maximum power transfer" of a simple circuit?
 


collinsmark said:
So far so good. :cool:

What can you say about "maximum power transfer" of a simple circuit?

The Rp and Rs should be equal?
 


Is that what you meant
 


Kev1n said:
The Rp and Rs should be equal?
[...]
Is that what you meant

I guess it depends on what you mean by 'Rp' and 'Rs'. But if you define Rp as the resistance in the primary and Rs as the resistance in the secondary, then 'No'. (Transformers not only modify voltage and current, but also impedance.)

Try again by putting things in terms of Rsource and RL, where

Rsource is the source resistance of the supply (which is what you are solving for in this problem), and
RL is the load resistance, as seen by the primary source.
 


Kev1n said:
1. A large power transformer has a turns ratio N1 / N2 of 1:5. When the primary voltage is 400v and it is connected to a resistive load its secondary current is 100A. Calculate the required value of source resistance of the supply to give a maximum power trnasfer

2. Vp = 400v, Ratio 1:5, Is = 100A

3. Vp = 400v, 1:5 Vs = 2000, Is = 100A, 1:5, Ip = 20A
R = Vp/Ip = 4 ohms.
I am a bit lost here, any pointers

collinsmark said:
So far so good. :cool:



Hello Kev1n,

Forgive me for speaking too soon in my first response. I may have caused some confusion. I seem to have skimmed over your attempted solution, and I missed a mistake.

Your calculation for Ip is incorrect in your original post. And your calculation of R = Vp/Ip = 4 ohms is also incorrect. Forgive me for the confusion of stating "so far so good." :blushing:

But what I said before about transformers changing impedance is true.
 

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