Estimating inductance of a coil (lack of measuring equipment)

[rbm]

Is there any method to estimate the inductance of a transformer primary for those, like myself, without the means to measure it?  I've read the technique to use a frequency generator to determine the frequency where the amplitude of the injected signal is half and then calculate.  But I don't have access to a frequency generator so can't empirically determine the inductance of the transformer primary.

It's a microwave oven transformer primary so the wire gauge is large and number of windings low.  I'm guessing its inductance is also low.  I'm using this as part of an underdamped LRC tank circuit and am trying to estimate the peak current that would be drawn.

[JimRemington]

The transformer is designed to work at 50 or 60 Hz, so you should be able measure the impedance using a line AC source, a multimeter and a series resistor.

[Benta]

Do you have a 'scope? Then it's quite easy.

[rbm]

I have a 60MHz scope.  What is the process using the scope?  Is it using the 1kHz 1Vp-p reference from the scope?

[Benta]

I have a 60MHz scope.  What is the process using the scope?  Is it using the 1kHz 1Vp-p reference from the scope?

Yep. Let the 1 kHz output drive the coil. insert a series resistor in the circuit and measure the voltage over the resistor with the scope.
You should get a nice triangle waveform, and from this you can compute the current through the coil, and from this the inductance.

[Jay_Diddy_B]

Hi,
I can do a quick estimate without doing any measurements.

The magnetizing inductance of a line frequency transformer so that the magnetizing current is small. Assume that the microwave is 700W

So the load current will be 700W/120V = 5.8A

Let us estimate the magnetizing current is 1/10 of the load current so 0.6A

(If you connect an Amp meter in series with the primary and measure the primary current with no load and the transformer powered from a wall outlet. If you have a measured value, you can use the measured value instead of 0.6A estimated)

Impedance = 120V / 0.6A = 200

Impedance = 2 x pi x F x L = 200

L = 200 / (60Hz x 2 x pi) = 0.53 Henries


The inductance will be quite high.

Regards,

Jay_Diddy_B