Inductance calculator

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akis

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#1
I am trying to work out the inductance of a coil wound around a ferrite core.

I am using the formula: L = u0 * u1 * N^2 * A / d

where u0 is permeability of space ( 4 * pi * 10^-7)
u1 is permeability of ferrite ( EPCOS N97 = 2300)
N is turns ( 2.5 )
A is cross sectional area = 1.767 cm^2
l is coil length = 5 mm

The formula says 638 uH but I am measuring 9uH.

If I wind about 12 turns and on a slightly larger diameter I measure 739 uH.

Is there a way to be able to calculate the inductance without having to wind up and then measure?


PS. I realise that the wire is wound on a former and there is a gap between that and the ferrite - may be that has something to do with it ?
 
#4
There's a useful lab saying, "test it the way you use it". It's especially important with ferrite transformers and inductors because there are so many types, optimized for different frequency bands and for different purposes. Sometimes, if you can't test at the frequencies and power levels anticipated, the calculations will be more accurate than the tests!
 
#7
akis said:
I am trying to work out the inductance of a coil wound around a ferrite core.

I am using the formula: L = u0 * u1 * N^2 * A / d

where u0 is permeability of space ( 4 * pi * 10^-7)
u1 is permeability of ferrite ( EPCOS N97 = 2300)
N is turns ( 2.5 )
A is cross sectional area = 1.767 cm^2
l is coil length = 5 mm

The formula says 638 uH but I am measuring 9uH.

If I wind about 12 turns and on a slightly larger diameter I measure 739 uH.

Is there a way to be able to calculate the inductance without having to wind up and then measure?


PS. I realise that the wire is wound on a former and there is a gap between that and the ferrite - may be that has something to do with it ?
Click to expand...
What shape is your ferrite core? Closed magnetic circuit like a U-core, open like a cylinder? Dimensional quantities, datasheet of the core?
Form and dimensions of the winding?
 
#8
I use EPCOS RM10 and RM12 cores and bobbins. I have ordered a RM14 (bigger) to try and get better regulation. But my problem is the inaccuracy of my inductance meter. I have ordered a better one and will test again.
 
#9
akis said:
I use EPCOS RM10 and RM12 cores and bobbins. I have ordered a RM14 (bigger) to try and get better regulation. But my problem is the inaccuracy of my inductance meter. I have ordered a better one and will test again.
Click to expand...

Then you do not use the correct formula.

In the case of a ready-made core, the manufacturer gives you directly the AL, in nanohenry/turn². All you have to do is to multiply this figure by the number of turns squared to get your inductance.

For example, the ungapped RM12 in N97 material has AL=5300.
http://www.farnell.com/datasheets/486640.pdf
Thus, with 2.5 turn, your inductance should be 2.5²*5300=33125nH, or 33.125µH.
 
#10
My inductance meter uses frequencies up to 800 KHz. Does the secondary coil affect the inductance of the primary? If I short the secondary coil I read different values. I think I read somewhere this is called "leaked inductance"?
 
#11
akis said:
My inductance meter uses frequencies up to 800 KHz. Does the secondary coil affect the inductance of the primary?
Click to expand...
If the turn ratio between the secondary and the primary is large, the secondary stray capacitance reflected to the primary will be significant, and if you make the measurement at a high frequency, it will affect the accuracy of the measurement.

If I short the secondary coil I read different values. I think I read somewhere this is called "leaked inductance"?
Click to expand...
What you measure is the leakage inductance as seen from the primary.

To make this measurement accurately, your inductance-meter must have a good vectorial discrimination, otherwise the displayed value will be corrupted by the windings resistance.
 
#12
akis;3054813 The formula says 638 uH but I am measuring 9uH. If I wind about 12 turns and on a slightly larger diameter I measure 739 uH. Is there a way to be able to calculate the inductance without having to wind up and then measure? [/QUOTE said:
Your calculation shouldn't be so far out. Perhaps the test signal is saturating the core? If that's the case, then inductance will fall dramatically for part of the cycle and put the meter off.
Click to expand...
 
#13
akis said:
My inductance meter uses frequencies up to 800 KHz. Does the secondary coil affect the inductance of the primary? If I short the secondary coil I read different values. I think I read somewhere this is called "leaked inductance"?
Click to expand...

You should have said it's a transformer, rather than an inductor, at the start.

Shorting the secondary coil effectively places a resistance across most of the primary inductance, equal to the secondary winding resistance multiplied by square of the turns ratio. Leakage inductance is that part of primary inductance that is not linked to the secondary, so it remains in series.

Basically, your meter is driving current through the secondary and it may not be able to distinguish that from the current through the primary inductance. A transformer is a network of resistors, capacitors and inductors. For accurate results you would need to measure at several frequencies and do some simultaneous equations to eliminate some of the variables.

Leave the secondary open. It then only contributes its winding capacitance.

Current through the secondary makes saturation less likely, by the way.
 
#15
akis said:
That means that I can wind just the primary and measure it and there will be no secondary to alter the results with capacitance or leakage inductance ?
Click to expand...
Yes, if you add the secondary, at best it will change nothing to the measured value, but if the meter is upset by the capacitance, it could display an incorrect value.
Besides, it is easier to correct the number of turns if you don't have to unwind the secondary first.
 
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