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Re: Current Limiting and Impedence

Original poster: "Gerald  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>

Hi Mark,

Glad you are making progress.

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


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.

Hope you remember that capacitance loads on an ideal transformer's secondary are reflected back on to its primary by the turns ratio squared (the capacitance looks N^2 times larger, XC looks N^2 times smaller). The input impedance of your MOT may very well be capacitive and capacitive reactance subtracts from inductive reactance. You may want to go thru the numbers to see if it is significant and take this into account when you size your ballast (depends on where you are at relative to Cres) or alternatively, once you size your ballast, take the effect Cp has on Cres and the limiting effect of the ballast to estimate the maximum current. SRSG's (because of the inductive kick) can raise the output voltage and result in a further increase in the maximum current. So its good to have plenty of margin for saturation.

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.


It is easy to test what you build to see where it saturates with a variac. Just plot current vs voltage across the inductor. To keep from saturating watch the volts per turn and the cross sectional area. I have seen recommendations in the group to not exceed 1-2 volts per turn and have at least 2.5 sq in of core material per 1KVA. I think the 1-2 volts per turn depends on the material so maybe this means, without more data, to not exceed 1 volt per turn. Once these saturation parameters are set, you can tweek the inductance independently by changing the gap. Larger gap gives smaller inductance. Smaller gap gives larger inductance.

Gerry R.