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Re: electromagnetism



sorry but who is harnwell and bleaney and bleaney? i have tried to do some
of it but as you can see, this is why i got stuck.:

firstly, i tried to calculate the total reluctance using S=flux lenght/mu*A
= 5*10^-3/ 1200 * mu0 * 16*10^-3
=207.2 At/Wb
i then calculated the self inductance using L=N^2/S
=400^2/207.2
= 772.2 H
this is where i got confused and started e-mailing physics lecturers. i
noticed in my notes, for a similar shaped circuit as the one i have got, i
have writen beside it " series/paralell circuit, total S= centre+1/2 side
reluctances(assuming symetrical)." i did not understand what this meant and
have looked throught books but cannot seem to find an explanation.
if that means what i think it does then the total reluctance would be
(S=flux lenght/mu*A) + (2*S=flux lenght/mu*A)
=(2.5^-3/1200*mu0*16*10^-3) + (2*( 5*10^-3/ 1200*mu0*10*10^-3))
i am not sure however and genuinly cannot find anything in any books. i am
not lazy, looking for someone to do it for me. i am just confused as to what
to do.
today however, i had an idea after seeing a different question in a book.
what i did was :
S1= core length/Ur*U0*A = 0.025/(1200 *U0 * 0.16) =103.6 at/wb
S2= 2*(flux path lenght / Ur*U0*A) = 2* (0.05/ 1200 *U0 *0.1) =2* 331.57
at/wb
= 663.15 at/wb
therefore total reluctance = 663.15+766.75 at/wb
for S2, i multiplied this by 2 because i treated them as two seperate
shapes. you may not be able to see what i mean as you do not have a diagram,
which is with the question, and i am not allowed to send attachments.
then i worked out the self inductance by using the formula L= N^2/S, however
my answer for this depends on the answer for reluctance being correct.
paul

----- Original Message -----
From: "Bernard Cleyet" <anngeorg@PACBELL.NET>
To: <PHYS-L@lists.nau.edu>
Sent: Tuesday, April 02, 2002 8:08 PM
Subject: Re: electromagnetism


this (at least upon a quick reading) is a simple plug in the formulae
question.
I suggest you visit (again) your library, as you're not getting much
co-operation
from us. [This was permitted (even encouraged) when I took take home
exams at
UCSB.]

hint: Harnwell has a section on the magnetic circuit and another on the
iron
core transformer. I checked Bleaney and Bleaney -- has one page on MMF,
reluctance, etc.

bc

P.s. I see I miss-read you post, as did Ludwik. If you still can't find a
worked
e.g., I may help you. However, this is sucha common problem, I am
surprised you
haven't found help in your text or another in the library. (Reitz and
Milford
also discuss this problem.)

Ludwik Kowalski wrote:

Suppose I do remember how to solve such problems (I do not)
and post a solution. Would you present it to your teacher as
your own or would you give me credit for doing this for you?
Ludwik Kowalski

paul giusti wrote:

i am from Hamilton, near Glasgow and am at Strathclyde Uni doing
Electronic
and Electrical Engineering. i have an exam on electromagnetism when i
return
from my holidays,hence why i cannot get in contact with my lecturers.
i have
been doing past papers, however i do not have any worked solutions.
the
question i have put on this e-mail is one that has appeared in
previous
exams however i cannot seem to do it or find anything in any books or
my
notes to help me. sending the question to physics lecturers is my only
hope.

thanks in advance

kind regards
paul

a transformer is constructed from E- and I-shaped cores with coils
wound
around the center limb. the middle part of the core has length 25mm
and
cross-sectional area 160mm^2, with a mean flux path length of 50mm on
each
side.

The relative permeability of the core is 1200. the primary coil has
400
turns, while the secondary has 100 turns.

a) calculate the total reluctance, and the self-inductance of the
primary.

b) if 240 V is applied to the primary and a load of 50ohms is
connected
across the secondary, determine the flux and flux density in each limb
of
the transformer.

c) determine the percentage drop in the flux through the centre limb
if a
manufacturing fault leaves a 0.1mm gap between it and the I plate.

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