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Home Made Variac substitute!
All,
If you cannot obtain a variac it is possible to use a microwave oven
transformer as a kind of saturable reactor power controller.
It is simple!
You need to acquire a large ~150 Watt wirewound ceramic potentiometer that is
somewhere around 2.5-5 K ohms. You wire the pot to the MOT HV winding output.
Place the wiper terminal on the grounded, or core connection of the MOT output
and place the pot in a cabinet so that you cannot accidentally make contact
with it because at the minimum power setting (Max resistance) it will have the
nearly full MOT output voltage of nearly 2 kVAC across it. Make sure the shaft
is grounded and that you also use a good insulating knob. You could also
'float' the entire pot (better) and use an insulating shaft to it as is
frequently done for HV air variable tuning capacitors in transmitters. You
connect the primary (120 volt winding) of the MOT in series with the reactive
or resistive load you wish to control.
The idea is basically that when the secondary winding of the MOT is shorted
(control pot set to lowest R) the primary will reflect the shorted secondary
and have minimum reactance. Max power flow will occur. When the MOT is
operating into a high resistance (or no resistance) the primary of the MOT is
capable of safely absorbing the entire 120 volts of the ac line across it with
little current flow through the load in series with it. Instead of a high
wattage pot which may be hard to find you could use a rotary switch and a bank
of progressive power resistors. To control higher power levels multi MOT's
could be paralleled. You just have to be careful knowing that there is
dangerous HVAC (no not Heating and Ventilating or even air conditioning) on the
control resistance. There seems to be some power amplification achieved through
the transformer based on the turns ratio. In this example a 100 watt control
resistance can control a 1000 watt load. It has been a while since I cobbled
this together on the bench so my recollection of the resistance values required
are foggy. The experimenters out there should take this idea and evaluate it
for themselves.
BTW, if you grab a 'tranny' from a microwave oven you've taken the wrong
component. Most microwave oven 'trannys' have been replaced in modern ovens
with solid state timers requiring no gears.
Robert W. Stephens
Director
AREA31 Research Facility
<http://www.area31-dot-org>www.area31-dot-org
>
> ----- Original Message -----
> From: <mailto:tesla-at-pupman-dot-com>Tesla List
> To: <mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com
> Sent: Thursday, February 03, 2000 18:22
> Subject: Home Made Variac
>
> Original Poster: "Michael Novak"
> <<mailto:Acmnovak-at-execpc-dot-com>Acmnovak-at-execpc-dot-com>
>
>
> I can't afford to buy a variac but I need some way to vary the line
> voltage... So, I decided to build one. I'm pretty sure I can get my hands on
> some 10 guage magnet wire, however, I still need to find a suitable core.
> Isn't
> a variac basically a bifliar-wound 1:1 turns ratio transformer? If so, does
> anyone have some sort of formulas on the subject? What would be the
> disadvantages of having a core which is not laminated such as the kind found
> in
> transformers?
> Any thoughts?
> -Michael
>