Please help me find a reference to solve this NCEES practice test problem

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Sparky Bill PE

Sparky Bill PE

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Funny part is I got it right. I "counted the windings" on leg A which was 4 loops and did 3.0/4 = .75 and got the correct answer. After looking at solutions I realized that wasn't AT ALL how I was supposed to solve it. I couldn't find anything in my references (zach stones/ Engr pro guides / wildi/ camera). I'm sure I over looked it. 

image.png

 
I used Electric Machinery and Power System Fundamentals by Chapman for this. I imagine most motors books should cover flux.

Since leg B is shorter than leg C, we know by inspection the flux in B will be greater than C.

 
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SparkyBill said:
I"m not connecting the dots of connecting what you just said to this problem. Is A and C area the same? 
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I was just making a note when dealing with flux, φ = BA is a good formula to have on hand. It's not needed for this problem in particular since flux density is not given. However, you may come across a problem requiring use of this equation.

When dealing with flux type questions, a good machinery book will be of value.

In regards to the question you posted, it's really just engineer's intuition that the total flux through leg A is split between legs B and C. And since leg C is 3x the length of Leg B, the flux through C will be 1/4 of the total flux, and the flux through leg B will be 3/4 of the total flux.

You probably won't find this in any reference material. If anything, it might be in a physics book since magnetic flux is usually covered in Physics II for most college curriculums.

 
Chattaneer PE said:
I was just making a note when dealing with flux, φ = BA is a good formula to have on hand. It's not needed for this problem in particular since flux density is not given. However, you may come across a problem requiring use of this equation.

When dealing with flux type questions, a good machinery book will be of value.

In regards to the question you posted, it's really just engineer's intuition that the total flux through leg A is split between legs B and C. And since leg C is 3x the length of Leg B, the flux through C will be 1/4 of the total flux, and the flux through leg B will be 3/4 of the total flux.

You probably won't find this in any reference material. If anything, it might be in a physics book since magnetic flux is usually covered in Physics II for most college curriculums.
Click to expand...
Yeah I looked in my Wildi book and couldn't find anything, I forgot I even had my Chapman book with me, I'll check it tomorrow. 

I'm missing how C is 3x the length of leg B.

If B is "d distance away" from a, why wouldn't C be "2d distance away" ? 

 
SparkyBill said:
Yeah I looked in my Wildi book and couldn't find anything, I forgot I even had my Chapman book with me, I'll check it tomorrow. 

I'm missing how C is 3x the length of leg B.

If B is "d distance away" from a, why wouldn't C be "2d distance away" ? 
Click to expand...
Please excuse the crude drawing, but the length of leg B (blue) is d, and the length of leg C (green) is 3d. The physical length of the core path, not the distance from A.

Screenshot_20200125-185934.png

 
So if C is 3/4 and B is 1/4, does the flux in B+C always = A?

I remember covering this in college, but apparently I don't remember this at all. I tried to look it up in my machines book I'm sorry for the questions. 

 
SparkyBill said:
So if C is 3/4 and B is 1/4, does the flux in B+C always = A?

I remember covering this in college, but apparently I don't remember this at all. I tried to look it up in my machines book I'm sorry for the questions. 
Click to expand...
No problem.

Yes the total flux (e.g. flux through A) is B+C.

φA = φB + φC

φB = (3/4)φA

φC = (1/4)φA

φB is greater than φC. Just imagine it's because there is more "resistance" in leg C because it's longer.

 
Chattaneer PE said:
No problem.

Yes the total flux (e.g. flux through A) is B+C.

φA = φB + φC

φB = (3/4)φA

φC = (1/4)φA

φB is greater than φC. Just imagine it's because there is more "resistance" in leg C because it's longer.
Click to expand...
Thanks for explaining that!

Now that I get that, it's hard for me to imagine how A doesn't equal C. All I can see is your green part being "mirrored over" and have A=C. I am digging through Wildi/Chapman books right now to see if I can find a section to read up on . 

 

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