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Re: Current Limiting and Impedence

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Mark,

Here is my shot on this one...

At 06:22 AM 4/21/2005, you wrote:

All:

Please follow my math and explain my confusion.

My TC power transformers have an impedence Z = 1.2 Ohms(secondary
shorted).

Ok, so if the output is a dead short, you draw 100 amps. Too high ;-) I think you are using microwave transformers if I remember right.


I built a Current Limiting Inductor(CLI) with L = 15 mH, thus at 60Hz
 X = 2*Pi*60*.015 = 5.65 Ohms. The R for the CLI is negligible. So the
inductor Z = 5.65 Ohms. The system therefore has total impedence of Z =
5.65 + 1.2 = 6.85 Ohms.


It might be more like SQRT (5.65^2 + 1.2^2) = 5.77 ohms.  But close enough....


This limits current to 120 VAC/6.85 Ohms = 17.5 amps.
I have confirmed this through testing.

Ok perfect!! The coil does not always run into a dead short so a 15 amp circuit should be fine. I suppose you could even sneak by with less inductance in that case, but best to start out real safe.


Measuring voltage BETWEEN the inductor and transformer I get around 90
to 100 volts(Mains 120V). So if I break down the circuit and consider
the current the individual components...
Transformer I = 90 VAC /1.2 ohms = 75 amps
CLI   I = 30 VAC /5.65 Ohms = 5.3 amps

That's almost right. It is like a voltage divider. 120 x 5.65 / (5.65 + 1.2) = 99 volts

120 x 5.77 / (5.77 + 1.2) = 99 volts too.

The current is the same in both the inductor and the transformer. I think you got the voltages backwards... It is the voltage across the device divided buy the resistance.

30/1.2 = 25 amps
90/5.65 = 16 amps

Probably some significant phase angle error now in this case.


Obviously, one can't analyze the components this way.  Is it because the
voltage measurements are not accurate due to the phase angle?  Am I not
allowed to analyze individual component impedence?  What don't I
understand.


I think you just got a little confused.  It all seems right to me.

BTW - Here is a $30 meter that can measure real power and power factor.

http://www.themeterguy.com/advertising/Kill%20A%20Watt/killawatt.htm

But don't overload it too much. No relation to "me" even though they use my test report with permission.

Cheers,

        Terry




Thanks.
Mark