The efficiency of a transformer

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Homework Help Overview

The discussion revolves around a problem involving a step-down transformer with given efficiency, primary and secondary voltages, currents, and turns. The original poster attempts to calculate the number of turns in the primary coil, expressing confusion over differing results from two methods.

Discussion Character

  • Exploratory, Assumption checking, Conceptual clarification

Approaches and Questions Raised

  • The original poster presents two methods for solving the problem but questions why one method yields a different result. Some participants question the validity of the equations used, particularly in the context of non-ideal transformers.

Discussion Status

Participants are exploring the implications of transformer efficiency on the equations used. There is a focus on understanding the relationship between current and turns, with some guidance provided regarding the concept of magnetomotive force (MMF) balance in transformers.

Contextual Notes

Participants note that the transformer is not ideal, which raises questions about the applicability of certain equations. The original poster also mentions a lack of understanding regarding MMF balance as presented in their textbook.

Asmaa Mohammad
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Homework Statement


A step-down transformer has an efficiency of 90%, the voltage of it primary coil is 200V and its current is 0.5A, and its secondary coil has a voltage of 9V and a current of 10A and its number of turns is 90 turns.
Calculate the number of turns of the primary coil.

Homework Equations


η =VsIs/VpIp * 100

Vs/Vp = Ns/Np

Is/Ip = Np/Ns

The Attempt at a Solution



I solved the problem in two slightly different methods, the first one:

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And the second one:

Y69fn.jpg


The solution in the first method (Np = 1800) is the correct one, but I don't understand why the second one is wrong. I thought that both will lead me to the same answer, but apparently they didn't, could you explain it for me, please?
 
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The transformer is not ideal.
Hence,
Asmaa Mohammad said:
Vs/Vp = Ns/Np
this is not true.
 
cnh1995 said:
The transformer is not ideal.
Hence,

this is not true.
OK, but why (Is/Ip = Np/Ns) is true?
 
Asmaa Mohammad said:
OK, but why (Is/Ip = Np/Ns) is true?
It is because of the mmf balance in the core. (Magetizing current is neglected as it is very small compared to the load current).
 
cnh1995 said:
It is because of the mmf balance in the core. (Magetizing current is neglected as it is very small compared to the load current).
I didn't know about the MMF balance in my textbook, but I googled it and understood it. Many thanks!:smile:
 
Asmaa Mohammad said:
I didn't know about the MMF balance in my textbook, but I googled it and understood it.
Good!

Asmaa Mohammad said:
Many thanks!:smile:
Thank google:wink:!
 

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