# find inductive reactance to find inductance??? answers on a postage

Discussion in 'Homework Help' started by ninjaman, Dec 11, 2014.

1. ### ninjaman Thread Starter Member

May 18, 2013
307
1
hello

I have to find the value of an unknown inductor. I researched a number of methods some requiring a current source. I didn't use those as I don't understand that too much. I chose an option that required a scope, function generator and resistor in series with the inductor.
my instructor said that I could find the inductive reactance by putting a resistor in series with the inductor. putting an AC voltage across them and find the output voltage. this voltage across the known resistor should give current. this current goes through the inductor. I should be able to find the resistance using ohms law. voltage over unknown inductor over current. I tried this a few different ways and couldn't find the correct answer. I used an lcr meter to find the inductance.
is this method, the voltage divider method, correct? does it work?

thanks
simon

2. ### crutschow Expert

Mar 14, 2008
16,540
4,458
You apply a sinewave voltage and measure the voltage across the resistor to determine the current. Then measure the voltage across the inductor from which you can calculate the inductive reactance. From that reactance value and the frequency of the applied voltage, you can calculate the inductance.
Is that what you did?

3. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,071
Use a sine-wave generator to drive a 1K resistor in series with the unknown inductor. Vary the frequency of the generator until the voltage across the resistor (measured with a high-input impedance AC voltmeter or scope) is equal to the voltage across the inductor. At this one frequency, the inductive reactance equals the resistance.

Last edited: Dec 11, 2014
4. ### Papabravo Expert

Feb 24, 2006
11,155
2,180
The current flowing in this circuit is an AC current. AC currents have both a magnitude and a phase, unlike DC currents which have only a magnitude. If the AC current is known for the resistor and the inductor, then the voltage across the inductor is given by:

$v_L(t) = L \frac{di(t)}{dt}$
The derivative in the above equation guarantees that there will be a phase difference between the voltage and the current for any sinusoidal waveform.

Using the nmenonic ELI the ICE man, helps us remember that for an inductor the Voltage will lead the current for an inductor.

5. ### crutschow Expert

Mar 14, 2008
16,540
4,458
You don't need to worry about the phase if you simply measure the magnitude of the voltage across the resistor to get the current and then across the inductor to derive the inductive reactance..

6. ### Papabravo Expert

Feb 24, 2006
11,155
2,180
Measure it how? That is, what are you actually measuring with various types of meters and or scope displays?

7. ### crutschow Expert

Mar 14, 2008
16,540
4,458
If the frequency is low enough, you can measure it with a DVM.
You measure the voltage across the resistor to calculate the current.
Then you measure the voltage across the inductor and use that to calculate the inductive reactance from the calculated current.
From that you can calculate the inductance from the calculated reactance and the applied frequency.