# Voltage across inductor

Discussion in 'Homework Help' started by victorhugo289, Sep 13, 2010.

1. ### victorhugo289 Thread Starter Member

Aug 24, 2010
49
3
Hi, why do I keep getting a voltage of 60mV across the inductor in the following series RL circuit:

Resitor: 2.2k
Inductor: 8.2mH
AC source: 13 volts at 0 degrees

The voltage across inductor is supposed to be 18.27mV at an angle of 89.92, with a circuit current of 5.91mA at -0.08051 deg.
However, when I connect my 10meg digital voltmeter I keeep getting about 60mV.

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I have a theory, but I'm not sure. I think the voltage might be unmeasurable with voltmeter due to the fact that it is 89.92 degrees ahead of the current....
Also what I´m probably getting is the current multiplied by 10: 5.91 x 10 = 59.1mA, which is what the voltmeter is probably picking up as voltage across inductor.

2. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
What is the frequency of the voltage source?

3. ### Ghar Active Member

Mar 8, 2010
655
73
What is the resistance of the inductor and how accurate is the number 8.2 mH?

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
784
Looks like the source frequency is 60Hz - based on the OP's calcs.

I think Ghar has it. The inductor will have a resistance value which the OP has not included with the calculation.

victorhugo289,

Your theory unfortunately doesn't make much sense. The voltmeter will simply respond to the voltage magnitude across the test leads. On AC range a DVM is usually calibrated for RMS value indication of a sinusoidal waveform - unless it's a true RMS responding meter, in which case the waveform doesn't matter. The meter won't respond to or be subject to the phase relationships in the circuit.

5. ### victorhugo289 Thread Starter Member

Aug 24, 2010
49
3
--The frequency is 60Hz. (It comes from the secondary winding tap of a 24 volt transformer connected to 120 v) Reactance is about 3.08 ohm.
As to how accurate the inductor value might be, well I don´t know, I took it from an old CRT monitor, it explicitly labeled "8.2mH" which was a surprise to me, it is the size of half an inch but it has very fine wire wrapped around.
I hope this helps, and you can tell me why I keep getting the 60mV.
My voltmeter is not true RMS, it is a normal voltmeter.

As a side note, I've made a test with another resistor and I think I got too the value of current multiplied by 10 as in the first circuit!

6. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
Use your voltmeter to measure the resistance of the inductor. Do this just as you would have done in a usual resistor. Just take the measurement at the two endpoints of the wire, but wait a little for the indication to stabilize, as the inductor will resist the measurement at first.

Tell us what you measured.

7. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
784
Does your meter have AC current range?

If not, how are you attempting to measure the current? It would be a very unusual meter if you had to divide / multiply indicated readings to obtain the true current value.

Perhaps you might consider submitting a schematic diagram of how you are doing the experiment.

I'm also puzzled by this statement ...

You state that you are using a 120:24V step down transformer as your supply (from secondary) but only have 13V actually measured as your source voltage?
As stated in post#1...

The 11V difference in expected and measured (?) secondary voltage is a significant discrepancy.