Re: PFC Question

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Ed Phillips <evp@xxxxxxxxxxx>

"It has often been said that the inductance of the variac (that 
changes with setting) causes the phase of the charging waveform to 
change with setting. I have personally experienced this phase change 
with variac setting and assume that it is due to the variac 
inductance vs setting.
What is needed here is to measure the phase of the variac output wrt 
the phase of the input of the variac to put things into 
perspective.  Your estimate of 1mh of leakage inductance is based on 
a coupling of 0.99.
This coupling seems high to me.  If the coupling is say 0.95, the 
leakage inductance may be significant enough to cause the output 
phase shift.  I have disassembled a variac of old design and found 
the the area of actual core to be much less than the area of the 
winding.  It would not surprise me to find the coupling to be less than 0.99."

   I have never understood why saturation isn't a factor in using an 
ungapped variac in series with a load.  When the brush is set to full 
voltage position the series reactance would be equal to the 
magnetizing reactance and would be quite high.  Since the core is 
sized for the nominal operating voltage at any brush setting much 
less than 100% that I would expect to see saturation currents 
becoming significant.  An easy check on the leakage impedance would 
be to short the two ends of the winding and run say about 1 amp (100 
watt bulb in series across 120V line) and measure the voltage drop 
between the brush and the shorted ends for different brush 
positions.  Why don't you do that and publish the results here?  Only 
a few minutes work.

Ed