×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*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.

Students Click Here

Jobs

Calculating flux in a permanent magnet circuit

Calculating flux in a permanent magnet circuit

Calculating flux in a permanent magnet circuit

(OP)
I am hoping someone here has experience with permanent magnets.  Since I haven't been able to find any published techniques (that arent really expensive FEA sim programs) I have a theory on how to size a permanent magnet for an alternator I am designing, and I want someone to confirm that my technique will yield a result that is useful enough to go through the expense of building a prototype.  My goal is to find the minimum thickness of Neodymium magnet that will produce the needed voltage in an alternator at the RPM I plan to run.

First, permanent magnets as I understand them: Permanent magnets have a 'Br' rating, which is the amount of flux that would flow if this magnet were part of a magnetic circuit with 0 Reluctance.  They also have an Hc rating, which defines the opposing magnetic field intensity that would result in 0 flux in that same magnetic circuit.  If Br is plotted as a point on the vertical axis, and Hc as a point on the negative horizontal axis, then the curve between them is the B-H curve.  

The B-H curve (most of it) is drawn by reducing the net flux in the circuit by A) Adding an air gap that will store potential energy as a magnetic field, or B) Using an electromagnet to create an H-field that opposes the one from the magnet.  It is my understanding that a magnetic circuit is roughly analogous to an electric circuit, in that Kirchoff's loop law can be applied to both.  

****If I add an air gap that has a reluctance of 10 Ampturn/Tesla, and there is 1 Tesla flowing through the circuit from the permanent magnet, then is that air gap the equivalent to using a coil to supply 10 amp turns opposing the permanent magnet flux? If the air gap and coil were swapped, would the total flux in the circuit remain the same: 1 Tesla?****

By choosing my air gap and core material, I can know the reluctance of my magnetic circuit.  I will take that reluctance, multiply it with the chosen flux density (flux needed to produce rated voltage@rpm) to get H opposing.  Br is constant, no matter how thick or thin the magnet is, and since Hc is rated per unit length, I should be able to scale the H axis of the B-H curve to find the correct magnet thickness.  It should be similar to how I would do per-unit calculations or normalized filter design in other EE calculations.
 

RE: Calculating flux in a permanent magnet circuit

A few thoughts:
1 - There are probably a lot more answerers hiding in another forum:
forum340: Magnetic engineering
2 - Strictly speaking, the units of reluctance would not be Amp-turn per Tesla, it would be Amp-turn per (Tesla-meter^2) or Amp-turn per Weber.  But I gather you are assuming constant cross section of the path so the distinction might not be important if you account for the area correctly in your calculations.
3 - The equivalence between the 10*Amp-turn/Wb reluctance, creating 10amp-turns MMF drop at 1Wb and the 10 Amp-turn opposing coil (presumed to have negligible reluctance) holds only at that single operating point.  i.e. the 10 Amp-turn opposing coil will remain 10 Amp-turn regardless of other changes in the circuit while the 10*Amp-turn/Wb reluctance will not create 10 Amp-turn mmf drop for any operating point other than 1 Weber.

=====================================
(2B)+(2B)'  ?

RE: Calculating flux in a permanent magnet circuit

Quote:

If the air gap and coil were swapped, would the total flux in the circuit remain the same: 1 Tesla?
Yes. I'll assume for simplicity area of 1m^2. If the operating point of the original circuit had average flux density 1T in a cross section 1m^2 = 1Wb flowing through series elements including a reluctance of 10 Amp-turn/Wb, and if you could replace that single reluctance with 10 Amp-turn negligible-reluctance coil of polarity opposing the PM, the flux and flux density would remain 1Wb and 1T.

=====================================
(2B)+(2B)'  ?

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.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources