[GreenKeys] Measuring Electromagnet Saturation?

[Randy or Sherry Guttery]

WA5CAB at cs.com wrote:

> Oh, good Lord.  You can't measure inductance with a DC source, regardless of 
> source impedance.  

Who said anything about measuring inductance with a DC source? The 
question was how to determine the amount of DC current to saturate a 
coil inductor with a iron (steel) core.  Since the AC impedance (not 
inductance - impedance) of such a coil drops like a rock when the core 
saturates - that is one way to determine the DC current required.

> And the solenoid coil impedance is just the DC coil 
> resistance.

Which is why you use AC riding on the DC "offset".

Again - look up Magamps and Saturable reactors to learn about this.

Here is one place to start:

http://en.wikipedia.org/wiki/Saturable_reactor

Note that the "usual" saturable reactor has a separate winding for the 
AC load control and another for the DC controlling current. In this 
instance - we don't have both windings - so the one winding will have to 
do for both - and that's done by riding an AC current on a DC offset. 
The DC offset is increased until the core saturates - at which time the 
AC IMPEDANCE of the reactor drops towards zero.  Using a relatively high 
AC frequency (same 1K) the impedance is going to be significant UNTIL 
the core saturates. 60 cycle will still show it - but obviously - not as 
dramatically. Watching a resistor in series with the reactor - the ac 
component will go from very low - to suddenly nearly all of the 
generator's output.

Depending on the frequency, the filtering in the DC supply, etc. - it 
might be helpful to decouple the AC from the power using a really big 
inductor (one that isn't going to saturate anywhere near the test 
current) -- but then again - an appropriate sized resistor would likely 
do an adequate job- and would only be needed if the power supply is 
heavily filtered (or very active filtering). A little experimenting with 
the test circuit would soon show if that's issue...

best regards...
-- 
randy guttery

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