Re: Current Limiting and Impedence

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Mark Dunn" <mdunn@xxxxxxxxxxxx>


Gerry:

I completed my testing of the circuit and components and am a bit
embarrassed to present the data.  Seems my math was right all along, but
my measured inputs for the math were bad.

The current limiter is clearly saturated.  This is demonstrated by both
its non-linear behavior in physical measurements and the math analysis.
Note: I was using a clamp on amprobe that was supposed to be highly
accurate in the 100 to 1000 mA range, but was way off(It was reading a
low value near proportional to voltage and thus suggested a constant Z &
constant L over the range of voltages tested - Appears I need to return
this device to the mfg).  Upon switching to my DVM the problem was
revealed.  The material for my current limiter is 3C85 Ferrite which I
don't have exact specs on, but 3C81 has a Bsat = 450 mT and a
recommended Bpk = 320 mT.  For my application I am pushing 865 mT which
is obviously too high.

A second problem was that I assumed that I could use primary Z data with
shorted secondaries on the MOT transformers to approximate the primary Z
value during operation.  In fact, it is now clear that the primary
impedence is much higher during operation than the shorted condition.
My operating data proves it.  99 volts and 17.5 amps means my
transformers have a primary impedence of 5.65 ohms(shorted it was only
1.2 ohms).

The funny thing that caused all the confusion was that the operating
data showed the transformer impedence to be 5.65 and the current limiter
impedence to be 21/17.5 = 1.2 ohms.  These values are coincidently
nearly exactly reverse from what I had predicted originally using the
error based methods above.  This coincidence was so hard to believe that
it caused me to believe that I was missing something and initiate the
question.  This is why we thought the voltage drops were reversed.

Simple problem.  Sorry about all the confusion to get it solved.  Thank
you very much for the help.

I am going to build a new current limiter out of an old steel E-core
that I have.  This way I can gap it to vary the L and current limiting(I
don't have a Variac).  Most of my work is with Ferrite, so I have lots
of data on that.  Do you have a reference for properties of steel
E-cores?  I have a vague reference to Bsat = 750 mT, but I need AL, Ui,
and Bpk.  Ferrite data is readily available from a number of
manufactures because they are selling ferrite cores.  Steel cores are
laminated and I think specified by the transformer manufacturers to the
sheet metal shop so data is hard to come by for the properties of a
specific geometry.  Obviously, I can wind a coil and run some tests to
get baseline data, but I'd like to confirm with published.

Thanks.
Mark