OK, I tried the Per Lundahl method of just using a variac off the power line to measure the inductance of a transformer winding.
It works! But please don't put 100 volts on a mic input tranny like I mentioned.
We did have a little of a language barrier at the show, when Per mentioned using 100 volts to get a good reading, he was probably talking about a big output transformer. It would burn up a mic input as the current would soar as the transformer reached saturation.
I did a graph of inductance vs voltage level. It is neat in that it shows where the transformer has a fairly stable inductance, where it saturates, and where there isn't enough excitation current to get the max inductance.
I checked a Cinemag mic input primary on the Sencore and Gen Rad meters and got about 8 henries on the sencore and ten henries on the Gen Rad. But the Gen Rad measures inductance at 1 khz.
I got about 24 henries with Per's test at 60 hertz, which makes sense, as the core is working better down there.
The most linear region was from about 1 to 4 volts ac rms.
You can see the transformer going into saturation as the voltages increases.
Here is the chart:
To use this method, and please be careful as you are on the power line, hook up the variac to the winding you want to check. Use an ammeter in series with one of the ac wires going to the transformer. Bring up the voltage very slowly while watching the ammeter. Get about 5 ma going thru the winding and measure the voltage across the primary with an ac volt meter. Keep the current steady when you do this. Take down the voltage and current measurements. Divide the voltage by the current. This is your ohms, or reactance. Then use the formula for inductive reactance to get the henries.
Sample calculation:
Xc=2 Pi F L , Since we are using 60 hertz, we will have a constant of 6.28 times 60 = about 377.
So Xc=377 L for our test. So lets say you got a reading of 5 volts when drawing 5 ma.
5/.005=1000 ohms reactance.
So 1000=2 Pi F L , or 1000=377 L. L equals 1000/377=2.65 henries.
Just keep plotting values til you get some consistent inductances across a set of voltages.
I think I like this method better than any I have tried as it tells you so much more about the transformer than just a henries number.
Thank's Per!
cj
It works! But please don't put 100 volts on a mic input tranny like I mentioned.
We did have a little of a language barrier at the show, when Per mentioned using 100 volts to get a good reading, he was probably talking about a big output transformer. It would burn up a mic input as the current would soar as the transformer reached saturation.
I did a graph of inductance vs voltage level. It is neat in that it shows where the transformer has a fairly stable inductance, where it saturates, and where there isn't enough excitation current to get the max inductance.
I checked a Cinemag mic input primary on the Sencore and Gen Rad meters and got about 8 henries on the sencore and ten henries on the Gen Rad. But the Gen Rad measures inductance at 1 khz.
I got about 24 henries with Per's test at 60 hertz, which makes sense, as the core is working better down there.
The most linear region was from about 1 to 4 volts ac rms.
You can see the transformer going into saturation as the voltages increases.
Here is the chart:
To use this method, and please be careful as you are on the power line, hook up the variac to the winding you want to check. Use an ammeter in series with one of the ac wires going to the transformer. Bring up the voltage very slowly while watching the ammeter. Get about 5 ma going thru the winding and measure the voltage across the primary with an ac volt meter. Keep the current steady when you do this. Take down the voltage and current measurements. Divide the voltage by the current. This is your ohms, or reactance. Then use the formula for inductive reactance to get the henries.
Sample calculation:
Xc=2 Pi F L , Since we are using 60 hertz, we will have a constant of 6.28 times 60 = about 377.
So Xc=377 L for our test. So lets say you got a reading of 5 volts when drawing 5 ma.
5/.005=1000 ohms reactance.
So 1000=2 Pi F L , or 1000=377 L. L equals 1000/377=2.65 henries.
Just keep plotting values til you get some consistent inductances across a set of voltages.
I think I like this method better than any I have tried as it tells you so much more about the transformer than just a henries number.
Thank's Per!
cj