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Re: Current Limiting and Impedence

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

Hi Mark,

The transformer is a PIG if I remember correctly. How are you measuring the transformer input impedance and what are the transformer's HV load conditions during the measurement (HV output shorted or with some other load)??? Also, how are you making the resistance measurement of the transformer LV input??? What is the HV load conditions during your measurements described below??? A simple check of the ballast performance is to remove the transformer from the circuit and short the output of the ballast inductor to ground. Bring up the voltage feeding into the ballast slowly and measure the current vs voltage, plot it at various points and see if it is linear. I still think the ballast is saturating. Make sure you have proper over current protection upstream of the ballast (like a fuse in the variac or a proper sized circuit breaker somewhere in the power delivery. If the ballast starts to saturate, the inductance will go down and current could become excessive. I'm feeling like there are too many unknowns and if we keep things simple, we can eliminate some of them. Also measure with a battery powered DVM the voltage out of the variac, the voltage across the ballast, the voltage across the transformer, and the current thru those devices.

Something else to think about. If you take an ideal ballast that is sized to limit the short circuit current to say 20 amps, imagine what kind of load on the ballast could result in more than 20 amps. I'll let you think about this for next time.

Gerry R

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

Thanks for all your help.  Now if you can bear with me and help me solve
the original problem with the math(BTW <> is "not equal to")....

I have (2) inductors in series.  My current limiter and my transformer.
Mains is 120V/60 Hz/1 ph.

The current limiter is 15 mH, X = 5.65 Ohms, R = 0.3 Ohms.
The transformer is Z = 1.2 Ohms, R = 0.3 Ohms, so X = 1.16 Ohms

So System Impedence Z = 6.82 Ohms

This gives I = 120/6.82 = 17.6 Amps and is near exactly what I am

The measured voltage between the inductor and transformer is about 90 to
95 volts.  Have gotten as high a 100 volts.  This is the voltage drop
across the transformer.  This is inconsistent with the above values and
in fact seems reversed from what the math suggests.  I have double
checked it.