Measuring Inductance

Donald L Burdette says:

Someone recently asked how to measure inductance. Obviously, the best way is to have an inductance bridge or meter. But since I have neither, here's my favorite way:

Get a sine wave oscillator, and put the inductor and a resistor in series across the output. I generally start with about 100 ohms. Adjust the oscillator frequency until the voltage across the inductor and the resistor is equal. Since they are 90 degrees out of phase, each will be 0.707 times the oscillator voltage. At this frequency, the inductor has an impedance of 100 ohms, and the inductance can be calculated from Z = 2 * pi * f * L.

Questions:

• How do you know the voltage at the inductor and the resistor are the same? They are in series not parallel! Why you say the inductive reactance of the inductor is the same as the resistance of the resistor? Is this just your assumption? James Newton of Massmind replies: You measure the voltage with a multimeter.+

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• amolgsharma at gmail.com asks:

  But should the value of Inductivity be different for different frequencies? IF so, then which value of inductivity should be assumed for practical considerations?

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• amolgsharma at gmail.com asks:

  Hello all, I'm a student and want to measure inductance of a solenoid valve. I measured it with L meter and found that it changes with change in frequency of the L meter. Why so and what exact relation inductance has with the frequency?

  James Newton replies: The formula is given on this page. f is the Frequency. As to the reason, it is simply that the higher the frequency, the more the signal is transmitted. It is transmitted into the air from each point in the wire, and so recieved back into different points along the wire.+

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• spdaylight at comcast.net asks: " What do the terms in the equasion Z = 2 * pi * f * L. mean." James Newton replies: 2 * pi * f * L is the formula for inductive reactance: The resistance presented by an inductor at a specific frequency. Since the voltage at the inductor and the resistor are the same, we know the inductive reactance of the inductor is the same as the resistance of the resistor. If the resistor was 100 Ohms, then Z is 100 Ohms. We can now use algebra to solve for L: L = Z / (2 * pi * f) or for this example: L = 100 / (2 * pi * f).+

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Comments:

• L=inductrance in H (henry)
  Z=impedance in ohm
  f=frequency in Hz+
• Hello I am student and I want to find the inductance of an inductor. However I was hoping I could to do it with Time constants as I do not quite understand the frequency argument. Any help would be appreciated. James Newton of James Newton's Massmind replies: Time is the inverse of frequency.+

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• Not quite true, this method ignores the DC resistance of the inductor.
  
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