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Old 21st June 2012, 10:38 AM   #11
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by akis View Post
My inductance meter uses frequencies up to 800 KHz. Does the secondary coil affect the inductance of the primary?
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.

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If I short the secondary coil I read different values. I think I read somewhere this is called "leaked inductance"?
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.
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Old 21st June 2012, 11:36 AM   #12
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[QUOTE=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]

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.
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Old 21st June 2012, 12:18 PM   #13
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Originally Posted by akis View Post
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"?
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.
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Old 22nd June 2012, 06:09 AM   #14
akis is offline akis  United Kingdom
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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 ?
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Old 22nd June 2012, 07:00 AM   #15
Elvee is offline Elvee  Belgium
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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 ?
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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