# Calculate MMF in order to achieve a flux density in the specimen plate of 0.5T - Help Please

#### phil_ndt

Joined Nov 17, 2017
5
Hi all,

New to the forum, however i have been finding it very useful and insightful with the amount of helpful information on here. I am studying electronic principles and need some assistance with the following question as I am having some troubles nailing it down:

The electromagnetic steel yoke shown in the figure below
10 mm wide and is used as part of an inspection system to scan plates in a large power plant to detect wall thinning and other flaws.

Calculate the magnetic moving force (MMF) in each part of path of the circuit in order to achieve a flux density in the specimen plate of 0.5T assuming no leakage flux (30 marks) (A magnetisation curve is provided below)

I could really use some help and some insight on how to tackle this please as I am having a lot of trouble finding the required information in the course literature.

Phil.

#### topple1992

Joined Nov 28, 2017
3
Im also struggling with this, although this exact question is given on the TMA on the eddy current module
if you have any luck getting an answer on this please give me a shout

#### phil_ndt

Joined Nov 17, 2017
5
Hi all,

New to the forum, however i have been finding it very useful and insightful with the amount of helpful information on here. I am studying electronic principles and need some assistance with the following question as I am having some troubles nailing it down:

The electromagnetic steel yoke shown in the figure below
10 mm wide and is used as part of an inspection system to scan plates in a large power plant to detect wall thinning and other flaws.
View attachment 139515
Calculate the magnetic moving force (MMF) in each part of path of the circuit in order to achieve a flux density in the specimen plate of 0.5T assuming no leakage flux (30 marks) (A magnetisation curve is provided below)

View attachment 139516

I could really use some help and some insight on how to tackle this please as I am having a lot of trouble finding the required information in the course literature.

Phil.
I Have done a bit more on this.
I haven't received any help or guidance from any one so i don't know if its right or not but I think it's on the right path... Have you had a crack at the question? Maybe we can compare results?

#### phil_ndt

Joined Nov 17, 2017
5
Heres what i came up with... but its wrong as the questions actually specifies MMF in each part of path of the circuit to achieve a flux density in the specimen plate of 0.5T. Which means the air gap is not relevant and i calculated the circuit with the air gap (#facepalm).
Can anyone with some know how review what i have and help rectify my issues?

Flux Density (B) = 0.5T
Using the magnetisation curve we get a field intensity (H) of 350 At/m in the specimen

I know this is wrong but its best i could come up with from the terrible learning module information which is basically gibberish.

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#### topple1992

Joined Nov 28, 2017
3
Heres what i came up with... but its wrong as the questions actually specifies MMF in each part of path of the circuit to achieve a flux density in the specimen plate of 0.5T. Which means the air gap is not relevant and i calculated the circuit with the air gap (#facepalm).
Can anyone with some know how review what i have and help rectify my issues?

Flux Density (B) = 0.5T
Using the magnetisation curve we get a field intensity (H) of 350 At/m in the specimen
View attachment 140927
View attachment 140928

I know this is wrong but its best i could come up with from the terrible learning module information which is basically gibberish.

Ive had a quick look over your workings. The information provided in my learning module is very basic however i think the permeability values need to be stated in the question. That way you can calculate the reluctance of each material.
The reluctance will differ in the yoke leg and in the material being magnetised

Also not sure if im reading into it too much but a sheet of material 1mm thick wouldn’t be cast steel. Ive based my magnetisation value on the curve for silicon sheet steel.

Sorry i can’t be of more help

#### phil_ndt

Joined Nov 17, 2017
5
Yeah I see what you are saying. The question is difficult to decipher.
Ive exhausted all options so I think I've hit a wall with this question. Don't think any more amount of reading and research is going to better help me understand!

#### MrAl

Joined Jun 17, 2014
11,734
Hi,

I'd like to help but i have to look up a bunch of stuff as i havent been doing these kinds of problems for a while now. My workload for last month and this month is much higher than usual too so i am very preoccupied.

One thing i can tell you if this helps is that the reluctance increases when you have a cross sectional slot in the base plate because that makes the plate more narrow and that increases the resistance to the flux flow.

#### phil_ndt

Joined Nov 17, 2017
5
Oh yes of course that does make sense!
Thanks for for the feedback!

#### JoBenton

Joined Jan 11, 2018
1
Hi,

Did anyone manage to answer this question?

#### Cameron2507

Joined Jan 13, 2018
1
Hi,

I have also struggled with this whole TMA of eddy current, the learning packages seem to offer absolutely no support on how to go about answering any of the first few questions other than some ridiculous equations which are not explained in any way.

#### topple1992

Joined Nov 28, 2017
3
I got an answer down for this particular question but it was only about 75% of what i think was required. I didnt find the eddy current literature helpful in the slightest, this resulted in a ‘best guess’. Has anyone had any luck getting what they think is a complete answer?