B/H Curves relating to Magnetomotive Force 4

[dcampbellpc]

I design solenoids, and my questions is relatively simple. When comparing 2 materials B/H curves, how do I tell which material will provide me with the better magnetomotive force in my solenoid application? I have a magnetic FEA program and can model different materials, but my results are often counterintuitive. The material with the higher B/H curve provides less magnetomotive force, thus causing my confusion. Are the inductance(B) and coercivity(H) proportional to the magnetomotive force?

 

[hacksaw]

you want a combination of high moment density (B) and low coercivity (threshold for magnetization and ac losses); this translates into high permeability which is also desired.

 

[UKpete]

Regarding your final question, the mmf is the force which drives flux around the magnetic circuit:

flux = mmf / reluctance

Coercivity is the value of H where the magnet characteristic crosses the horizontal axis - it is a fixed value for a magnet material regardless of the dimensions of the magnet or condition of the magnetic circuit (although it's value does vary with magnet temperature). The mmf is proportional to the value of H at the operating point and is given for a simple magnetic circuit by:

mmf = H x Lm (Lm = magnet length in direction of flux)

 

[electricpete]

I assume your geometry and voltage are fixed, and the only thing you are changing is the core material.
Voltage applied,, number of turns, cross section etc all remain the same.

Compare the case of two different materials with different permeabilities (mu)
(Higher mu means higher B vs H curve)

Volts per turn is constant, established by the supply voltage and the number of turns.
That means flux is the same in either case.

Reluctance R = Area*PathLength/mu.
The only thing you changed is me. So reluctance R is lower for the higher mu

NOW, we go to UKPete's magnetic circuit equation:
flux = mmf / reluctance
We said flux is constant and reluctance is lower (for the higher mu). That means mmf is lower (for the higher mu.) In physical terms, the higher mu material does a better job at limiting exciting current.

In the context of your whole design, the reduced exciting current may give you additional thermal margin which allows you to wrap more turns which gives you higher flux and force. But if you change only material and not turns, voltage etc you don’t a higher force (neglecting resistive losses etc).

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[electricpete]

In summary, if the only thing you change is the material,
the material with the higher B/H curve WILL provide cause lower current and lower magnetomotive force (amp-turns).

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