# reluctance of a iron ring

Discussion in 'Homework Help' started by djstar, Apr 18, 2008.

1. ### djstar Thread Starter Active Member

Jan 26, 2008
37
0
hi, Im currently studying a open learning diploma in electronic and so far found the course very taxing but Ive manage to get through most of the theory in the end . But there is one question that Im totally baffled with. An iron ring has 800 turns of wire wound it. Cross sectional area is 3cm2. Calculate reluctance if the flux density is 0.8 Teslas with 5A current flowing in the wire." any help with what direction I should go in or any formulae would be much appreciated.

2. ### Caveman Active Member

Apr 15, 2008
471
0
Check out wikipedia for Magnetic Reluctance, then Magnetomotive Force (MMF).

The only thing they don't really tell you is how to calculate magnetic flux. I'll give you that one. Since it is a ring, the flux is the same all throughout the center.
Magnetic flux density * Cross sectional area = Magnetic Flux.
Units are:
Magnetic Flux Density = Tesla = Weber/m^2
Area = m^2
Magnetic Flux = Weber.

Good Luck.

3. ### djstar Thread Starter Active Member

Jan 26, 2008
37
0
Many thanks caveman for the quick reply. ive managed to come up with an answer . 166666.6 Wb , does this sound any where near right ?

4. ### Caveman Active Member

Apr 15, 2008
471
0
You're off by a bit.

MMF = N*I = 800turns*5A = 4000 Amp-turns

Flux = Flux Density * Area =
0.8Teslas * 3cm^2 * (1/100 m/cm)^2 = 240e-6 Webers

Reluctance = MMF/Flux = 4000/240e-6 = 16.67e6 Amp-turns per Weber.

You probably messed up on your unit conversion. Remember there are 10000 cm^2 in a m^2. And the final units were wrong.