Re: [ATM] Microwave heating of glass

[David Weinshenker <daze39@earthlink.net>]

Dominic-Luc Webb wrote:
> I think this microwave technique is more than anything cute and
> would make a nice conversation piece. I once pieced together a
> small 230VAC 2.3 kW heater with a homemade dimmer. I do not know
> if a dimmer is an appropriate circuit to control energy, but I was
> able to fire up kanthal elements that can provide a lot more heat
> than most hobby kilns:

By "dimmer" do you mean an SCR or TRIAC phase control type of circuit?
That should work well for any resistance electric heater or incandescent
lamp.

A magnetron typically wants to be operated at its nominal voltage - if
you tried to run it at a lower voltage, it might not oscillate correctly.
This is why the microwave cooking ovens control the power level by
switching on and off at intervals of a few seconds.

I think the limiting factor in how fast the ovens can cycle is the
time for the magnetron filament to warm up - in the standard oven
design, this is powered from the same transformer that feeds the
high-voltage rectifier for the tube's anode-cathode voltage. If
a separate supply were used to keep the filament continuously
heated, the power output might be started and stopped at more
frequent intervals (a few times a second, perhaps) - this might
provide a steady enough temperature in practice to be useful as
a control in a glass-melting system based, for example, on magnetrons
salvaged from home ovens. (Is it possible to put power from several
independent magnetrons into the same absorber? Do they naturally
phase-lock with each other, or just interfere destructively?).

But I do indeed wonder how much of a practical point there is
to the scheme - what can one do with microwaves that can't be
done with conventional electric or gas heat? Gas burners with
the required thermal power are inexpensive and easily modulated,
if multi-KW electrical sources are not readily available for
electrical resistance heaters (or for microwave sources, for
that matter)...

 From the point of view of practical amateur glass-melting,
would microwave heating perhaps not be a diversion from
attention more productively spent on the art of cooking
and forming the glass (rather than putting a project in
High-Frequency Power Electronics in the way of getting
started on that part)?

-dave w
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