Hi
Took home a HERAEUS hardening (type KR 260) furnace for 100 EUR. Inside dimensions: 260 * 150 * 450 mm. Type-plate says 20 Amps.
Only problem is, one of the heating elements is burnt. I thought that it will be a piece of cake to repair it. Dumb assumption!
Now I tried to find out by some "logic" what the right resistance would be. It has 5 heating elements: Bottom: infinite (burnt) Top: 24.2 Ohms Side left: 12.2 Ohms Side right 1: 8.2 Ohms Side right 2: 4.5 Ohms
Side right 1 and 2 is quite strange. Adding the two, I get 12.7 Ohms, so this would be equal to side left. But why two separate elements? Seems the
4.5 Ohms are going into the temperature-switch and is heating it. Uh!? OK, maybe to fool the sensor for pre-heating?Now as the thing is 3-phase, it is making sense that they switch together left and right (-> 24 Ohms) and have it on one phase, top on one phase and bottom on one phase and thus I guess bottom should be 24 Ohms too.
But it ain't that simple! There is a row of connectors that go to the elements, has some bridges and receives 4 lines from a magic switch / relays combination.
Drawing the elements as resistors, I have:
bott left right 1 right 2 top +----+ +----+ +-----+ +-----+ +----+ +--| xx |----| 12 |--+--| 8.2 |--+--| 4.5 |----| 24 |----+ | +----+ +----+ | +-----+ | +-----+ +----+ | | | | | c1 c2 c3 c4
c1 .. c4 are the wires coming from that "magic switch and relays". Interesting (maybe) to note is, that c1's cable has a different color than the rest. xx is the unknown resistor. Calculating from the remains of the broken resistor, I get 30 Ohm for the bottom-element.
From the drawing, there is no way to get an even load on the phases.
The magic switch has 5 positions: Off pre-heating
600°C 800°C 1150°CMaybe this switch can be ignored for now, because I assume it works. Also, it's hard to find out how he switches, because there is that relay in between that doesn't work without power (that I don't want to try out right now).
Anyhow, the -for me- un-understandable grouping of resistors and the completely uneven load on the phases (OK, it doesn't matter; but it is not desireable) gives me the impression, that someone rewired the oven (with some abuse on beer-tokens?)
OK, now to the core question: Does anyone have some insights how resistors (or better: heating-capacity) is distributed in furnaces. I wouldn't wonder if most heating capacity is on the ground and least on the top.
HERAEUS quit the production of heat-treating furnaces and doesn't seem to answer. Yes, I googled but only found that this type is sold for about 1500 EUR used.
Nick