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Induction melting benefits


#1

Hello All,

I have a question about induction melting. I have always used
torches to cast, but really wanted to get an induction melting
furnace later this year. Not that torches are bad, they work okay
for many situations. All I need is something to melt up to 2 or 3
troy ounces of metal. However, many of the items I will need to melt
are at platinum’s melting point and higher. I have a scientific
background, but I definitely wanted to ask someone who knows these
things before I purchased something. For small scale melting, will a
small table top 120 volt 1.5 - 2.5 kW medium frequency (25 - 100
kHz) system work? Unless it screws up the casting, I am not worried
if it takes more than 1 minute to melt. I heard that using the
induction systems are really easy, but it seems like the choices are
not so simple. You have the skin effect from the frequency, the
output power, number of flux lines per unit volume, magnetic
susceptibility, and other things! Does any of that matter?

What seems appealing to me is that you could use cover gases and
vacuums much easier, control the input power better, and the
stirring mechanism from the medium frequency eddy currents. Are
these safe assumptions to consider versus the torch?

Thank you very much.
Seech


#2
For small scale melting, will a small table top 120 volt 1.5 - 2.5
kW medium frequency (25 - 100 kHz) system work? Unless it screws up
the casting, I am not worried if it takes more than 1 minute to
melt. I heard that using the induction systems are really easy, but
it seems like the choices are not so simple. You have the skin
effect from the frequency, the output power, number of flux lines
per unit volume, magnetic susceptibility, and other things! Does
any of that matter? 
What seems appealing to me is that you could use cover gases and
vacuums much easier, control the input power better, and the
stirring mechanism from the medium frequency eddy currents. Are
these safe assumptions to consider versus the torch? 

No not really. Yes you can use cover gas or vacuum with an induction
heater. But you would need more power than 1-2.5Kw at least 5 kw
minimum. Small induction casting machines use the graphite crucible
as the susceptor (http://en.wikipedia.org/wiki/Susceptor) to capture
the energy from the induction PS. The energy from the coil falls off
with the square of the distance from the coil so by using a susceptor
crucible you keep the distance between the coil and the load
constant and at a minimum to get maximum energy transfer. Because the
energy is being dumped into the crucible rather than the metal in it
you will get little if any stirring especially at those frequencies.
You will need lots lower frequency (60 or 120 Hz induction systems
are great for stirring) to get much stirring action. You will also
have a difficult time of melting high temperature metals as most will
react with graphite and even if no reaction graphite is not suitable
for those high temperatures except in vacuum. Yes there are machines
that use zirconia crucibles as suseptors but it gets tricky as it is
not a conductor at low temperatures so you need to preheat it to 615C
before trying to heat it with the induction furnace. It is not so
easy to melt metal in a non conductive crucible as then you need to
try to couple the energy to the metal charge and if it is not ferro
magnetic it is really hard to get much current into it. Add to that
the issue that as the charge melts the load it presents to the
induction coil changes so you need a machine that can change its
output frequency dynamically to track the load changes to maintain
maximum energy transfer. This is another reason that a suseptor
crucible is preferred for these small furnaces.

Induction furnaces are great tools but much more complex to utilize
than a torch especially if you are trying to create your own system
by just purchasing a power supply. A lot of the cost of the induction
casting machines is the engineering that goes into delivering an
integrated functional system rather than just a power supply.

Jim

James Binnion
James Binnion Metal Arts