# Compass needle

1)does a magnetic compass needle have high remanence or low remanence?
2)in a shell type transformer core required to carry DC in addition to normal AC, what effect does the introduction of an airgap have on AC performance of such transformers.

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Hootenanny
Staff Emeritus
Gold Member
Please show some working and / or initial thoughts...

~H

1:i have plotted absolute permeabilty v/s magnetic field strength(H) of two materials A & B from a given BH curve.material A has high permeability,high remanence and high coercivity while B is complete opposite of A.therfore i need to find out which material is suitable to make a magnetic needle.

2:as for the this question i have to comment on what effect such a arrangement(as mentioned in the question) has on the AC performance of the core.i do know that introducing a airgap increases the reluctance of the core, thereby adding to the losses.

berkeman
Mentor
Good starts.

-1-, think about what it means to be a permanent magnet. What does the material have to be able to do all on its own? What does it mean to still have a Br when the exciting H=0?

-2- Can you comment on what "saturation" is in a transformer? How is it caused, and is it bad or good? How would the DC current component be related to any saturation effects? How can you mitigate saturation effects?

1:permanent magnet has high remanence and equally high coercivity.when H=0 and there is stil some B left that means that the material has not totally demagnetised and therfore some extra force will be required to totally demagnetise it.

2:magnetic saturation means that the material cannot be magnetised further ie further inc of magnetiization force does not produce a significant inc in magnetic flux.saturation causes inductance to drop thereby causing the current to rise faster which inturn increase the magnetic field strenght thereby driving the core into further saturation.it is not good becoz after saturation has been reached giving force to inc the field is just a waste of energy(ie giving force)

i do not comprehend what DC current has to do with saturation. please explain to me what it does.

Hootenanny
Staff Emeritus
Gold Member
copresh1 said:
1:permanent magnet has high remanence and equally high coercivity.when H=0 and there is stil some B left that means that the material has not totally demagnetised and therfore some extra force will be required to totally demagnetise it.
So, do you want a compass needle to be a perminant magnet or not?

~H

yes it shud be a permanent magnet

but what about the second question?

berkeman
Mentor
copresh1 said:
but what about the second question?
You posted some info about saturation, but I'm not sure that you are understanding it (sounds like what the book is telling you, not what you are understanding from what the book is telling you).

Look at the B-H curve for a soft magnetic material ("soft" is the term used for materials that do not have significant Br at H=0). What does the curve do in the upper right quadrant of the plot as H gets positive and large? That is saturation. What do you think happens to the transformer's transfer function as the magnetic material starts to get into saturation? What equations would you use to figure that out?

Now, a transformer is usually used just with an AC excitation. What part of the B-H curve is the transformer's magnetic material operating on in this case? Now add a DC current to the transformer primary coil, in addition to the AC excitation. What part of the B-H curve are you operating on now? What does this shift in operating area have to do with saturation effects, and how will the transfer function of the transformer be affected?

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i think the used with just a ac excitation the the lower part of the curve will come in to play.

berkeman
Mentor
Right. DC excitation will bias you one way or the other toward the outer parts of the B-H curve. What does the curve do out there, and why is that bad?

at that point the there is no further inc in flux density,ie saturation

berkeman
Mentor
Good. Now to the original 2nd question -- what does an airgap in a transformer's magnetic path do?

increases the reluctance of the core.

berkeman
Mentor
But what does that have to do with saturation? What does introducing an air gap into a transformer do to its B-H curve?

flatens it out

it flatens the curve ie its a straight line

berkeman
Mentor
copresh1 said:
it flatens the curve ie its a straight line
At really high values of H, the curve will still bend over (lowering the effectiveness of the transformer). So what are the advantages and disadvantages of using an air gap in the transformer?

advantages:the core can be magnetized further
disadvantages:lowers the efectiveness by inc the reluctance

berkeman
Mentor
That's pretty much it. To withstand the DC current and still act like a transformer, you have two choices -- make the core a lot thicker so that you are still on the linear part of the B-H curve (usually expensive and a problem for the product's size), or introduce an air gap in the transformer design, and live with the fact that the inductance of the transformer is a lot lower (and hence the passband of the transformer is narrowed some, depending on the source and load impedances).