[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [TCML] mot ballast
In a message dated 1/10/08 8:38:40 A.M. Eastern Standard Time,
>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
>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
>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!
Center for the Advanced Study of Ballistic Improbabilities
**************Start the year off right. Easy ways to stay in shape.
Tesla mailing list