Re: Bumping above 40%

On Mon, 1 Apr 1996 12:00:44 +0700, tesla-at-grendel.objinc-dot-com, you

>>From richard.quick-at-slug-dot-org Mon Apr  1 01:14 MST 1996
[big snip]
> Messing around with this
>in the last few weeks has led me to believe that there very well
>may be some 60 cycle resonance establishing under certian
>conditions in my control circuits. 
>One inductor that people have consistantly ignored while dis-
>cussing this problem is the distribution transformer outside that
>is supplying the lab with 120/240. In my case the supply trans-
>former is a hefty pad mount puppie with no visible rating plate,
>but it looks like it weighs a couple of tons if the hard points
>for lifting it are any reliable gauge. I would be curious to know
>more about this unit, but it is unrealistic to think I could ever
>get it isolated for measurement and diagnostic work. Suffice to
>say simply that this inductor is large, and should not be ignored
>when considering problems such as bumping in the coil control
	In the March 1996 issue of PCIM (Power Conversion &
Intelligent Motion) (www.pcim-dot-com) page 20 is an article _Power Supply
Magnetics - Part 3_ bye Donald E. Pauly. On page 24 he states:

"Contrary to popular belief, line distortion is primarily caused by
leakage inductance of the utility company power transformer but not
its resistance.  The line voltage drop in this resistance from the
rectifier pulses (23A in his case. jf) is only about 280mV.  This is
invisible on an oscilloscope without special techniques.

	Leakage inductance of a utility transformer is on the order of
60uH.  When the rectifier pulse starts, the rapid current increase
through that inductance lowers the line voltage near its peak by 3.0V.
at the end of the pulse, the slower current decrease raises the line
voltage just after the peak by 1.8V."

You are running much higher current, on a larger transformer tho.This
may be responsible for the bumping that occurs on the variac.