magnetic flux and mean path length

Thread Starter

PG1995

Joined Apr 15, 2011
832
I agree with Kermit.

PG, I also see your point about the average being zero over the span indicated by the author. This is misleading as you say. Normally average value means the average value of one half cycle, but it is the average of the positive half cycle, not the half cycle that has values both positive and negative.
Hi

I have another question related to this posting. Please help me with the Q2 in the attachment (ignore Q1). Thank you
 

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steveb

Joined Jul 3, 2008
2,436
Hi

I have another question related to this posting. Please help me with the Q2 in the attachment (ignore Q1). Thank you
I'll look at this question in more detail when I get home from work.

However, I'm noticing something that I missed before. The author is considering rate of change of flux. Since rate of change is just slope or derivative, this explains why he is considering the half cycle where the slope is always the same sign.

Anyway, I need to look at this more carefully, but I thought I would mention that important fact. The description given is a little cryptic.
 
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steveb

Joined Jul 3, 2008
2,436
I can't really understand your question Q2. All I can say is that the book is confusing. The average and RMS values are being related to the rate of change of flux, not flux itself. Yet the graphs are shown in terms of flux, leaving to reader to imagine just a little too much. Then the 4 and 4.44 scale factors are introduced without explanation. These numbers can be derived if you go through it carefully. To do this properly, average value and RMS value can be calculated as integrals. If you don't want to do the integrals, then just remember the numbers 0.636 and 0.707. However, frequency is also involved which brings the 2pi f scale factor when you take rate of change.

I recommend carefully writing out the equations and graphs for the rate of change of flux. Rate of change is derivative and higher frequency results in higher rate of change. The factors of 4 and 4.44 are related to 2pi*0.637 and 2pi*0.707.

I'm sorry I can't think of a clear way to explain this all, but your question is too vague, the book is too confusing and the available communication format here is not conducive to a useful conversation that should be properly done at the chalkboard.
 

Thread Starter

PG1995

Joined Apr 15, 2011
832
Hi Steve

I can understand how difficult it can be sometimes to help someone with such stuff especially when the book is also confusing. You are right about the book. I have just read some of the sections on transformer and it has confused me very much. The instructor uses two books: Chapman, Hughes. The Chapman one is good and understandable as far as the first chapter goes. Thank you for the reply. I will come back to this problem later.

Best regards
PG
 

Thread Starter

PG1995

Joined Apr 15, 2011
832
Hi :)

Please have a look on the attachment and please help me with the query. Thank you.

Regards
PG
You should keep two facts in your mind. First, the method suggested here is approximate; and, second, the two answers you mentioned are very close to each other in value (< 3 % error).

What this means is that your alternate method is not really wrong because is tries to approximately consider the path length. However, the method suggested is considered the proper way and it is based on the logical idea that the cross sectional area that is perpendicular to the path is the best estimate to use. So, if you want to apply that method, with the understanding that it also is not a perfect method, you should obey the rules setforth.
Hi

I don't expect a reply to this post. I'm making it for my own future reference and for someone like me who can find it useful. If you have something to correct or something to add then you are always welcome!

Regards
PG
 

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Kermit2

Joined Feb 5, 2010
4,162
A teacher once told me to think of the electric constants and the magnetic constants as two loops each one passing through the center of the other and at right angles to it.
Picture two wedding rings, and then remember they are ALWAYS together.

Now, physicists want to get them divorced and take them each out to the bar by themselves and get them drunk and ply them for secrets about each other. ;)

Designers and mechanics just want them to get along and be happy with each other.

Philosophers want it all to mean something profound and amazing!

Techs just want everyone else to leave them alone!

Good luck with the test.
 

Thread Starter

PG1995

Joined Apr 15, 2011
832
Hi

From post #14 onwards till the post #23 excluding posts #19 and #20, I discuss these questions. I think now I'm able to make sense out of all this. Please see the attachment. I don't expect a reply for this reply. But if you have ever anything to add, you are always welcome! Thank you.

Regards
PG
 

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Thread Starter

PG1995

Joined Apr 15, 2011
832
Hi

Could you please help me with the Q1 and Q2, and Q3? For the Q1 and Q2 you can refer to these posts #12 and #13. Please note that in the attachment for the questions, Q1 and Q2, 2 Wb/s is the rate of change of flux at some particular time.

The Q4 and Q5 are about what I and steveb were discussing in posts #19 and #20. It seems like what I was saying in post #19 wasn't incorrect if the voltage source is an alternating one but the reply by Steve assumes that we are dealing with DC and his reply is correct too. Just by humble opinion. For Q4 and Q5 you need this higher resolution copy of the attachment. If you can't access the image, then please use the following details here: username: imgshack4every1, password: imgshack4every1.

Regards
PG
 

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