doubt about magnetic permeabilty

goodluckchuck

i made core out of Mu metal strips (μ=0.01) to make a transformer, but surprisingly instead of giving higher emf due to its higher permeability it gave poorer results than a laminated iron core....what might be the reason..please help.

Meir Achuz

My guess is that there were eddy currents without the lamination.

goodluckchuck

Hey hi, i made a laminated core out of Mu metal.

goodluckchuck

Guys plz help me out..that core costed me..i need to kno y its not giving the result.

Meir Achuz

I have some questions:
1. Doesn't the no load EMF of a transformer depend on the turn ratio,
and not the mu of the core?
2. How does the mu of the mu metal compare with that of the iron?
3. How does the hysteresis curve of the mu metal compare with the iron?
If it is fatter, it would have less efficiency.

marcusl

The skin depth is
[tex]\delta=\sqrt{\frac{2}{\omega\sigma\mu}}[/tex]

Since the permeability of mu metal is 100 times or more larger than that of iron, the skin depth is 10 times or more smaller. Your laminations need to be at least 10 times thinner.

Also be careful not to overdrive your winding, or you will drive the core into the saturated part of its hysteresis curve where the relative permeability = 1. Calculate the vacuum field produced by your winding at the intended drive current, and make sure it is a fraction of an Oe (<<1e-4 T). Mu metal cores are often used when signal levels are microvolts.

marcusl

BTW, please use proper English here and not text speak.

goodluckchuck

thickness of the laminates is 0.35 mm, crosssectional area = 15 cm sq, i have one more question, by using the formula to calculate the magnetic field inside the core of a solenoid the value of magnetic field is coming around 28T, is it really possible ? because while experimenting with the iron core i used to get similiar values of magnetic intensities, but using these values in calculations to find the emf i got close results with the practical values obtained...im confused some say iron gets saturated at 1.6T but i got the calculated values of magnetic field to be in the ranges of 10 to 14T which gave me the correct emf as compared to the practical value.

goodluckchuck

1. it does, but higher permeability should give higher magntic intensity thereby higher flux followed by higher emf for the same number of primary and secondary turns, B ( For a solenoid )= (μ.N.I)/L
2. μ of mu = 100 x μ of iron.
3.has high initial permeability and maximum permeability with minimum hysteresis loss.

goodluckchuck

hello guys...? any more suggestions ?

marcusl

Did you follow up on the good advice you already received? Did you calculate skin depth at your frequency of operation and verify that your lamination thickness is smaller? Do you understand M.A.'s comment about flux, cores and emf? Did you compute hysteresis losses as he suggested? Please do these before asking for more help.

Regarding ferromagnetic permeability, it is a known fact that B saturates. If your calculations don't agree, then you probably a) are not using a proper model, b) made improper assumptions or c) made a calculational error.

One further point: you must insulate the laminations. If they are in electrical contact then your effective laminate thickness is that of the full stack.