Sent: Sunday, July 27, 1997 3:12 AM
I have heard several comments on building ballasts for non shunted type
transformers. The problem arose for myself several years ago and I did make
several heavy duty ballasts specificaly for Tesla coil useage since I had no
large variac on hand.
The first ballast had its "core" wound with heavy gauge guy wire rolled
into a toroid shape on a suitable form. #10 THHN wire (salvage) was wound on
the toroid core with approx. 25 taps having a max. inductance of 7 mH. A
switch was fabricated using 3/8" bolt heads for contacts & 3 automotive
starter brushes soldered together for the wiper mounted on a peice of
phenolic. It has been used as a varible reactor and varible transformer for
voltage control as well handling over 10kW.
Another ballast was made using 1" steel strap form the junk yard. 4 rolls
were wound and then strapped together for a core deminsion of 3'' x 4" The
wire used on this core was #14 house wiring (romex) having 3-wires inside a
sheath. This wire can be found in building supply centers such as Home
Depot for $25 to $30 for a 250' roll. The romex was wound onto the core
complete with sheath to make winding easier and all taps and both ends had
all 3 wires electricaly connected to act as a trifilar winding (all wires
paralleled) for max. current handling. Its inductance is 9mH with approx.
200' wire winding. Weight of completed ballast came to 95lb. The wire is
wound on flat plywood "shuttles" for winding around the toroid cores. It is
important to remember that the core does not have to be silicont transformer
steel for these cores because of the low duty cycle they endure for powering
Tesla coils on an intermittent basis. If these cores were run in continuous
duty or for long periods of time then they would heat up dut to hysteresis
loss and eddy currents.