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To:      grendel!grendel.objinc-dot-com!tesla-at-uucp-1.csn-dot-net 
Subject: Re: variacs

%UNIPLEX
%TO            grendel!grendel.objinc-dot-com!tesla-at-uucp-1.csn-dot-net
%FROM          darl
%SYSTEM        tasa1
%SUBJECT       Re: variacs
%DATE          19/01/96 13:34
%REFERENCE     13379
%ADDRESS       grendel!grendel.objinc-dot-com!tesla-at-uucp-1.csn-dot-net

Regarding the use of a variac designed for 400 Hz on 60 Hz:~

Transformers and chokes designed for higher frequencies have less iron and
will overheat when used at lower frequencies.  They depend upon the iron to
supply the 
amount of counter-inductance to buck the current in the device and thus limit
the amount of current in the supply winding.  Therefore, if there isn't
enough iron, there's not enough opposition to the flow of current since the
small amount of iron doesn't buck the current and the current climbs to a
high value and usually the supply winding burns out in a few seconds.~

I tried a small transformer one time with the secondary open, thinking in
this way to make sure it wasn't "overloaded."  However, in about five seconds
the little devil smoked!  Therefore, my suggestion when testing any
transformer is to feed the supply current through a 60-100 W light bulb so if
there is a load on the line (even with nothing connected to the secondary),
the only thing that happens is that the bulb lights up and limits the current
through the winding.  With lower voltage transformers, smaller bulbs may be
the order of the day.  Even a 10-15 W bulb will limit the current to a safe
level when put in series with the winding and still allow the experimenter to
get an idea of the practicality of using a certain transformer and not to
find the smoke alarms are what are tested.~

Regardless, don't attempt to use 400 Hz devices on 60 Hz.....  However, it is
possible to use 60 Hz devices on 400 Hz but it's a terrible waste of iron and
the efficiency factor cries foul.  Transformer/choke designers just put
enough iron in the device to allow for a 100-degree Centigrade temperature
rise which is very often above boiling.  Ask them sometimes if they use this
amount of iron in transformers they wind for themselves and they'll usually
tell you that they put much more iron in them!  Consumer economics rules.....

Nuff said, again.~

Bob L.~