# calculate inductance from measurements

#### MrSoftware

Joined Oct 29, 2013
2,144
Can someone please double check my math here, I'm rusty and I'm sure I'm screwing something up.

I'm wiring a transformer to give me 240V output from 120V source, and I noticed that it's drawing 2.7A from the 120V side when there is no load attached. So that got me thinking, what's the inductance of this thing? My Kill-A-Watt meter says input is 120V, 60Hz, 2.7A and pF of 0.20.

Using this formula:
L = V / (ωI)
Where
- L is the inductance in Henrys
- V is the peak-to-peak voltage across the inductor
- I is the current flowing through the inductor in amps
- ω is the angular plot frequency of the waveform in radians per second.

So this is where I think I'm missing something.

V = 340V (120V RMS is 170V peak, but V is peak-to-peak which would be 340)
ω = 2*(pie)*60Hz = ~377
I = 2.7 RMS (I suspect I should use peak here, not RMS? Ipp = 7.6)

Plug all this in, and I get 0.334H using RMS current, or 0.119H using peak-to-peak current. How far off am I?

#### Veracohr

Joined Jan 3, 2011
771
If the meter reports rms current then use rms voltage. The result will be the same as long as you’re consistent with the measurements. Mix rms and peak values and you’ll get the wrong answer.

#### Ian0

Joined Aug 7, 2020
8,939
If I=2.7A then Impedance Z=120/2.7A = 44.4Ω
Resistance is the real part: R=0.2Z = 8.89Ω
Reactance is the imaginary part: X=√(44.4^2-8.89^2) = 43.5Ω
Reactance is 2πf x inductance
X=2πfL
L=115mH

It won't be quite right, because the core loss appears as a parallel resistance, which is different from the winding resistance.

• DickCappels

#### crutschow

Joined Mar 14, 2008
33,331
Seems like a rather large magnetizing current.
What is the transformer and how large is it?

#### Ian0

Joined Aug 7, 2020
8,939
Seems like a rather large magnetizing current.
That's what I thought.