I have some working knowledge of electronics, but there are a couple concepts in these tutorials which are leaving me quite stumped. The main one right now is the capacitor tank. Both of these tutorials are quite insistent that the quality of the tank capacitor be high. To quote the first one:
The tank capacitor: is very very important. It handles tremendous amounts of reactive power at very high frequencies. It is absolutely essential that this part be selected appropriately. It must be a high-quality polypropylene or mica capacitor. I use giant snubber capacitors made by Eurofarad; alternatively, a series/parallel array of smaller capacitors (such as the Tesla coiler's beloved CDE942 series) should work. The ultimate capacitor, of course, is a water or conduction-cooled unit made by Celem, but such caps will run you about $150 for a 2 uF unit. You want enough capacitance to resonate with your work coil at no more than 70 kHz.
What I don't understand is how to go about finding such a tank of capacitors (i'm thinking multiple may be better). How does one go about calculating how much reactive power is handled by the caps, and how much reactive power a given capacitor bank can handle?
Forget about reactive, you need capacitors that are very low loss at the frequency of
operation (because they are in a resonant LC circuit) and able to take enough current
without overheating (here the bank of capacitors takes 100's of amps collectively).
A "tank circuit" is just an LC resonator.
rytcd:
I have some working knowledge of electronics,
I hope you have a bit more than "some". This project can be very dangerous if you don't know what you are doing. It uses lethal voltages.
"Reactive power" - what's that? Power is power and is real. Reactive current, yes and as it flows through a resistance it generates power but that power is real not reactive.
MarkT:
Forget about reactive, you need capacitors that are very low loss at the frequency of
operation (because they are in a resonant LC circuit) and able to take enough current
without overheating (here the bank of capacitors takes 100's of amps collectively).
A "tank circuit" is just an LC resonator.
How would one go about calculating this? Is it merely a question of taking the equivalent series resistance of the capacitor, adding up the heat losses therein, and then just making something which will not burn up with that extra heat?
@russell Well, I'd never call myself an expert, but I do know enough to be safe and wary during the course of my project, enough to be familiar with your Smith chart, for instance, and hopefully enough to avoid copious amounts of magic smoke. That said, there are certainly elements of the design portion of this circuit which I do not fully understand. For instance, I would like to get some idea of just what kind of amperage to expect going through my tank circuit. How could this be calculated?
rytcd:
I would like to get some idea of just what kind of amperage to expect going through my tank circuit. How could this be calculated?
At resonance the current flowing round the tank circuit is given simply by the applied voltage divided by the total resistance in the circuit as the reactance of the capacitor cancels out the reactance of the inductor:
I = V / ( capacitorESR + inductorESR + sourceR )
Your difficulty will be determining those three values.
Looks cool, but this stuff is ultra dangerous. 600V and those currents. One unfortunate move and you're gone.
But couldn't you measure them with an esr meter? Datasheet?
As for what capacitors...i'd also say go for multiple. IMO the system would be more consistent , but probably would need replacement often.
In either case. Proceed with caution, this is not a toy project.
yeah, I haven't had much luck finding an affordable single cap that looks like it might do the job. I would greatly appreciate any suggestions on any multiples which could be assembled into something suitable
russellz:
"Reactive power" - what's that? Power is power and is real. Reactive current, yes and as it flows through a resistance it generates power but that power is real not reactive.
Russell.
Reactive power -- The product of, in the case of a series LCR circuit, system current times reactive voltage or in the case of a parallel circuit the product of system voltage times reactive current. In a "simple" CR or LR circuit the reactive variable is at right angles to the "real" variable.
MarkT:
And the resistance of the inductor depends on the frequency due to the skin effect too...
Yes. Let's take a guess at the coil size, say 5 turns of ¼ in copper pipe on a 3 in diameter former and with ¼ in spacing between turns. By my calculations that gives an inductance of 1.8 µH and a Q of 70 at 60 kHz. That corresponds to an esr of 9.7 mΩ.
Hi,
Forget the first induction furnace you cited, it is an Instructables project and the guy has not responded to any questions in the more than 12 months that it has been there, also schematic is not a full schematic, no diagrams for power supplies.
The second looks better, the guy has put a lot of theory and thought into design and has provided a decent schematic, although you have to wade through tons of small web pages to get there.
He has done a fair bit of the maths too.
But be careful, the furnace is high frequency and any electrical burns from it are very bad as they burn under the skin.
jackrae:
Reactive power -- The product of, in the case of a series LCR circuit, system current times reactive voltage or in the case of a parallel circuit the product of system voltage times reactive current. In a "simple" CR or LR circuit the reactive variable is at right angles to the "real" variable.
Thanks but I was just being a bit pedantic. The scientific definition of power is the rate at which work is done or energy converted. When the current phasor is perpendicular to the voltage phasor no work is done and there is no power. Thus "reactive power could be considered an oxymoron.
TomGeorge:
But be careful, the furnace is high frequency and any electrical burns from it are very bad as they burn under the skin.
Tom....
In the radio spectrum its technically low frequency (30kHz to 300kHz), which is why the
burning would be deep as lower frequencies penetrate further, but even microwaves penetrate
enough!
You need to check the regulations to see what frequency bands are free for induction
heaters in your territory too...
60kHz time reference now located at Anthorn, on the Solway coast. This location is approximately the "latitude centre" of the UK so maybe better placed than Rugby for country-wide coverage
well, I am a licensed ham, but I'm no expert on RF burns either. Although within radio, it's typically the smaller wavelengths like 70cm which pose a greater risk of burns at a given power. The wavelength of 65khz is on the order of 4.7km. it also does not have much going on in regards to an antenna, as the coil is MUCH smaller than the wavelength, so I would be hopeful that real RF interference would be minimal in range. Still, certainly valid points which I will be sure to test and account for if I ever get the thing working :).
In regard to that last part, I still feel a bit uneducated in regards to choosing suitable tank capacitors, but I think I've managed to find two kinds which might be suitable. Additional input gratefully accepted, they are: the larger one and the smaller one. shooting for 3.34uF, the larger one would cost 97.68 for 10*.33uf=3.3 uF, the smaller would cost $122.76 for 22*.15uF=3.3uF
There are two things which leave me hesitant to get the less expensive option. One is that the smaller is labeled for High pulse dv/dt, and the larger is labeled as a snubber cap, although these two functions sound quite similar to me.
The other is that, I believe where ESR is concerned, one would calculate these simply as parallel resistors, in which case, 2 of the smaller ones in parallel have an esr of 8mOhm/2=4mOhm, whereas one larger one is 5.4, slightly higher. This, if correct, I'd imagine would lead to more heat produced and (less importantly here) greater inefficiency. still, the larger snubbers seem like they would be easier to mount, and more importantly, the flat casing I think would be easier to make good thermal contact with the heat sink I will be using.
At the moment I am thinking in terms of going for the larger one, but I would appreciate feedback on this before I blow a large chunk of my project budget on it
Microwaves ( as in microwave oven ), is the wavelength which excites water molecules in your food.
An "induction cooktop" doesn't heat the water in your food. It creates currents in your pan, which are dissipated by the resistance of the metal of your pan, which makes your pan hot.
I suppose an induction furnace for melting metal works the same way. In principle, you could heat either the metal you want to melt, directly, or heat a metal crucible containing the metal you want to melt.
I don't think it will make much difference which capacitors you use. Heat sinking won't make much difference with the plastic casing. The esr you get will be a good order of magnitude lower than that of the coil if my guess of its dimensions is about right.
michinyon:
Microwaves ( as in microwave oven ), is the wavelength which excites water molecules in your food.
An "induction cooktop" doesn't heat the water in your food. It creates currents in your pan, which are dissipated by the resistance of the metal of your pan, which makes your pan hot.
I suppose an induction furnace for melting metal works the same way. In principle, you could heat either the metal you want to melt, directly, or heat a metal crucible containing the metal you want to melt.
You are correct. Microwaves are a subdivision of radio waves which are on the high frequency end of the spectrum, and have a small wavelength (relatively, it is not actually on the micro scale). There isn't one definite definition of what constitutes a wave as being "micro", so it seems like more of an informal term to me. They are used in other tings from cooking, such as communications and industrial applications.
and yes, an induction furnace functions by the same mechanism as an induction cooktop, albeit with more power. It seems to me that, if used in conjunction with a metal crucible, can be used to melt just about anything, be it metal, vlass, or ice cubes. If it works, I may even try casting Titanium in my vacuum chamber with it, which I have not been able to find recod of anyone ever doing in a home foundry.
