Re: [TCML] mot ballast

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In a message dated 1/10/08 8:38:40 A.M. Eastern Standard Time,
Mddeming@xxxxxxx writes:

>Something has been nagging at the back of my mind for   years:

>If a short on the HV side of a transformer is reflected back to  the
primary

>as a short, then how does a mot with a shorted secondary act as  a  ballast?

>Is it purely the leakage inductance that makes it not appear  as  a dead
short?

>Is there a way to calculate this value, or is it just cut and   try?

Good question. Also pertains to folks using stick  welders as ballasts,
but we know that the stick welders have a lot of leakage  inductance by design.

My guess is that the "dead short" isn't as dead as  you night think.
What's the DC resistance of the MOT secondary windings? OK, I  opened my mouth so
I'll go measure some:

Four different MOT's, biggest (90 Ohms) to  smallest (150 Ohms) of
secondary winding DC resistance.

Obviously, when the thing goes into hard  saturation, further current
through the primary is gonna behave like it's  through an air-core winding. But
then that's just like leakage inductance, isn't  it? ;)

I think the answer also lies in the "percent  impedance" of the MOT. By
definition, that's the percent of name-plate  current in the primary to cause
the nameplate current to flow in the secondary  when the secondary is shorted.
It's also a way of describing  reactance (usually inductive reactance when
we're talking about transformers and  reactors). Line and load reactors (or even
DC link reactors) for industrial  motor drives are rated in "% impedance", and
the impedance of the  supply is an important design consideration for the
drive and the motor hooked  up to it. Also the basis of short-circuit
coordination studies to make sure a  short in a piece of equipment doesn't cause a
current high enough to destroy the  equipment supplying it Sounds just like a
"ballast" issue, eh? ;)
It seems that very "good" transformers  (high-quality, high-efficiency)
have very low percent impedance ratings. Thus  they "leak" little and reflect
dead shorts accordingly. I think the % impedance  of my pole pig is 1.6%, which
is ridiculously low. I suspect MOT's are much  higher - after all, we're
almost shorting them out when we use them to cook  things.

Hope this helps!

-Phil LaBudde
Center for the Advanced Study of Ballistic  Improbabilities

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