×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Are you an
Engineering professional?
Join Eng-Tips Forums!
• Talk With Other Members
• Be Notified Of Responses
• Keyword Search
Favorite Forums
• Automated Signatures
• Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

#### Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

# Determine PE and Heat Tolerance

## Determine PE and Heat Tolerance

(OP)
Can someone shed some light for me on permeance coefficient (PC) for a specific magnet.  We have an application when needs to operate in a fairly warm environment, and I understand that the PE is critical.  My understanding is that even though the manufacture has noted the MaxOperationalTemperature of 80C, given the geometry the PE may cause it to be less?

So after doing searching and searching I came across a couple of PE calculators... I'm not sure how they are getting the information but something is not right..
The magnet is a Sintered NdFeB (Grade N52) ring (0.5"OD x 0.1875"ID x 0.0625" Thick) magnetized through thickness.

1) http://www.intemag.com/calculators/Permeance_Coefficient_Calcs.html
(Yeild a PE answer of 0.354)
2) http://www.magnetsales.com/Design/Calc_filles/PermeanceCoefficient-Ring.asp
(Yeilds a PE answer of 0.35)

These both jive so I assume this is a correct value.

The manufacture has provided two charts...
a) BH Graph (See file below)
http://files.engineering.com/getfile.aspx?folder=75f18df8-d169-4250-aae4-7b75c33c7c05&amp;file=BH-Graph.pdf
b) Demagnetization Chart (see file below)

Questions:
1) These websites seem to be calculating the PC using some type of formula.  What is the formula?
Obviously it is only dependant on the shape.

2) It was my understanding that the PC could also be derivied from the BH graph as PC=Hd/Bd (correct?)
By looking at the graph though that would put the PC at about .925, which isn't close to 0.35

3) Should the BH graph change for different size magnets of the same material/grade?

4) How do I read the Demag chart to determine the max temperature.  The stated Max Temperature is 80C, but we have tried the magnet and it fails far below that.  Its above the knee at 20C and below the knee at 50C.. but I can't image it wouldn't take 50C.. Am I missing something?

Obviously I'm making some wrong assumptions here, but I can't figure it out.

Paul

Replies continue below

### RE: Determine PE and Heat Tolerance

(OP)
Sorry there are several references to PE.. these should all be PC not PE.  Not sure why I put that.  Didn't see any way to edit the post after it was submitted.

Cheers!
Paul

### RE: Determine PE and Heat Tolerance

Ignoring fringing the PC is the length of the magnet divided by the length of the air gap.  If your PC is 0.35 at 80 C for the magnetic material you have chosen you are well into the denag part of the curve [PC is the slope of a line from the orgin, a PC of 1 has a slope of 45 degrees].

There are grades of Neo RE magnets that have a straight line demag curve at 80 C.  Look at the Magnetics website.

### RE: Determine PE and Heat Tolerance

Well, PC has two diff calculation, one for Br, one for Hcj. PC can be diff, according to the raw material. PC means Br/Hcj changes when temperature up/down. Be careful, shape also concerns. What we are talking should be standard blocks and columns, without tumbling and machining. N grade normally used in 80℃,working temperature means Br loss is less than 5% after 2 hours, under 80℃. And loss must be recoverable.

### RE: Determine PE and Heat Tolerance

#### Quote:

4) How do I read the Demag chart to determine the max temperature.  The stated Max Temperature is 80C, but we have tried the magnet and it fails far below that.  Its above the knee at 20C and below the knee at 50C.. but I can't image it wouldn't take 50C.. Am I missing something?

http://www.shinetsu-rare-earth-magnet.jp/e/design/

#### Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

#### Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Close Box

# Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

• Talk To Other Members
• Notification Of Responses To Questions
• Favorite Forums One Click Access
• Keyword Search Of All Posts, And More...

Register now while it's still free!