Electric Oven Design Questions

M

Mike Fennessy

Joined
Dec 19, 2005
Messages
326
OK, I've read every thread I can find on building an electric ht oven a couple of times and I think I've got it. Here's the overall plan and I'm hoping to get some feedback on whether it makes sense or not. I've also got a couple of questions about size. I can't imagine it not working but I'd love any input as it's not something I wanna screw up.

My main concern is the oven in the britishblades pdf uses two 13 amp, 3.1KW 240V, which seems like way overkill for a 6x4x14 oven.

From what I've read here, many people use one 14-15 amp for ovens that are bigger (hard for me to follow the ohms and length calculations though). I'm having a hard time imagining fitting 5' of element into a box that small much less 10'.

My main question is this: I'm roughly planning on an oven size of 4x4x15” or 18”. Is one 240 Vac, 13 Amps, 3120 Watts element enough power for 18” or even 15"? I have no problem keeping it small or even going smaller but I'd really like to keep it to one SSR and one heating element if possible for simplicity, but I'll go with two if that's what it takes to make it right.

Below is my understanding of how the overall thing works. If anybody that knows about these things could give me some feedback on whether I've got it right I'd really appreciate it.

High Temperature Thermocouple for Kiln: Starting at: $26.95 Model: TC-K-KLN (borrowing this from my gas forge for now)
Product Description: This thermocouple is for kilns used for firing ceramics as it has the highest temperature rating for a K type thermocouple. It is designed for operating continuously at 2300 °F (1300 °C), or the cone 6-temperature.

Which connects to PID :

Universal 1/32 DIN PID Temperature Controller $36.95 Model: SYL-1512A (also from my gas forge update to programmable later if needed/wanted)
Power supply voltage rating 85~260VAC/50~60Hz -or- 85-360 VDC
Output mode (Relay contact: 3A at 240 VAC) (SSR: 8VDC, 40 mA)

Which connects to and controls the SSR with an 8VDC signal:

New Solid State Relay SSR-25DA 25A /250V 3-32VDC (on it's way)
(very cheap off ebay. Should I be worried enough to get a second as backup?)

Which turns one leg of the heating element off and on :

#7103: 240 Vac Kanthal Heating Element - 2300 °F Max $34.00
Material: Kanthal A1 Wire
Maximum Temperature: 2300°F
Power: 240 Vac, 13 Amps, 3120 Watts
Wire Size: .046", (17 AWG)
Unstretched Length: 41"
Minimum Operational (Stretched) Length: 60"
Maximum Stretched Lenght: 140"
Coil Outer Diameter: .280"
Pig Tail Length: 1-1/4"

OR:

Resistance wire, Kanthal A-1, 16 ga, 50ft $25
from ebay description: To make a 3400w element for use with 230v you will need about 50ft

I don't care about saving $9 or going through the extra step of winding it myself, I'm just trying to figure out if one's better than the other.

Misc stuff:
I plan on running the PID and SSR cooling fan from a 110 outlet and plugging the heating element into an outlet with a dedicated breaker box right next to it so I can completely kill the power to the elements when putting blades in and out without risk of shock, but keep the PID and fan running.

Grounding all matal boxes and skin of oven including door to 220 ground.

Is there a problem with the element being at minimal operational length? Or is it fine as long as I can keep the coils from touching?

Oh yeah, I'm using 2.5 inch brick with isowool between the brick and the steel skin. I'm adding the isowool cause I have a bunch left over from when I built my gas forge.

Sorry so rambling and thanks for any input.
Mike
 
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HI Mike. I have not built my own HT furnace but there are a few things that I would want to incorporate. One would be a safety interlock switch for the door, that would stop power to the heating element. This would remove the threat of electrocution if your design has exposed elements that might be touched. Another item would be 'L' or 'J' cuts in the insulation to to accept the elements. This would further reduce shock hazard, and also reduce the amount of IR that the workpiece is exposed to. I did not look up the controller that you referenced, but I would like a controller that had both ramp and soak capability - and accept multiple programs (I don't believe your controller does). Lastly, I would speak to an applications engineer concerning the element sizing. You want an element that is sized so that it can reliably heat the mass of the oven and the workpiece, but not have too much excess capacity. Excess capacity can cause difficulty in controlling temperatures. Lastly, and this is beyond what most folks do (because of cost), I would design using a controller with an analog voltage or current output, and couple it to an SCR. This provides much more finite control than using a SSR, and has the added benefit of extending element service life.
BTW, you can get temperature control and design basics books from many of the big component makers like Omega, Watlow, and Partlow. There is a lot of information that is pertinent to both your forge and your furnace.
Best
Steve
 
Two main things:
1) You will need a heat sink for the SSRs.

2) One vs two SSRs - While one SSR will cut the power to a 220VAC coil, it does not remove the power. Remember that the other end is still hot. Any contact between the coil and ground will result is a circuit - usually through your body. Installing an interlock won't prevent this, as it only shuts off the 12V supply power to the single SSR, not the 220VAC line. I know that there are many units made with one SSR, but the risk of bumping a coil while removing a blade from a 4" chamber is pretty high. I feel that the extra $$ will be worth the peace of mind. The interlock switch cuts the control power to both SSRs, thus completely removing all power from the coils.

You don't need separate plugs for the 110VAC and 220VAC circuits in the control box. Just bring in the 220 and tap the 110 off one leg. This is done just after the main power switch (dpst), and before the SSR. This 110VAC leg runs the PID and fan ( and from the PID to the SSR control voltage). After this you should have a second switch (dpst) for "Element on-off". The wires from this switch go to the SSR power connections.

I'll add a schematic.
 

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Thanks for the replies. Second SSR is on it's way. Does anyone have an example of an interlock switch that would work for this? There seems to be a bunch of different types.

Thanks for the schematic Stacy, that's perfect.
 
I used a simple momentary on push button switch mounted on the top of the door as an interlock and had the wire coming from the PID to the SCR going thru it. Mounted the switch on the oven body and made a bracket with a bolt that was adjustable so the heaad pressed the switch button enough when oven door is latched tight
 
Bldsmth is 100% correct about the use of 2 ssr's for safety, and the bussing of power which I omitted in my haste (btw most controllers will run off of the 208v as well, either by dip switch or automatically).
 
It is called a "Limit Switch" or "Microswitch". The type with a roller will make installation easy. just mount the switch under the door, so the roller is pushed down when the door closes. The type you want is a Normally Open switch. That means it will be off (open) when the door is open, and on (closed) when the door depresses the arm ( called the whisker).
If you get one without a roller, just bend down the last 1/4" of the whisker at about 45 degrees. That will allow the door to push it down when closing.
http://www.ebay.com/itm/2pcs-Micro-...208?pt=LH_DefaultDomain_0&hash=item43ae58b238
http://www.ebay.com/itm/2pcs-Micro-...673?pt=LH_DefaultDomain_0&hash=item4602285259
http://www.ebay.com/itm/2pcs-Micro-...384?pt=LH_DefaultDomain_0&hash=item43ae58b2e8

To describe the installation so it makes sense in words:
The switch is mounted on the bottom side of the shell at the door opening side.( Left or right opening door)
The whisker sticks out and sits against the side of the door.
The switch is mounted in the position that allows the door side to depress the whisker as it closes.


You can put a simple cover over the switch, but the voltage is low, so there is no worry about not covering it.
Use high temperature wire, as the shell can get hot.
 
Sorry for late input, but I finally am on a PC (instead of phone) and could read the Auberins' manual and also better see the hand drawn schematic. Perhaps I am misunderstanding your controls instructions and/or the schematic, but there are some things in question.

1) It appears that the neutral is tied to ground. This is a safety hazard if performed anywhere other than the power mains to the building.
2) The Auberins controller in question is designed to provide an internally generated 8vdc signal out of its SSR output, there is no 12vdc power supply present or needed.
3) Fusing for the main power/elements is recommended, for personal and equipment safety.

Also, the whole device can be run off of 208/220/240vac power. The controller can run off of this voltage, and instrument fans are readily available for this voltage. Six of one, half dozen of another.

Best,
Steve
 
There was some confusion over the elements that Andy Gascoigne clarified in a recent BB thread.

His original build used a pair of 110V, 14A elements connected in series to run on 230V UK mains. BCS only listed the 110V elements at the time, but later added a 220V, 14A element.

I've built 4 knife-sized HT furnaces loosely based on Andy's design so far. All use 16ga Kanthal A1 elements wound for 14A at 110V and connected in series for 230V UK mains. I've also built one 42" long, using a pair of 14A, 220V elements.

The small ones use a single SSR. The big one uses 2. If you use 2, make sure the specs on the controller show it has enough output current to switch both SSRs in parallel. From memory, the Fotek SSRs I used needed 20 mA each and my controller could provide 40 mA

http://www.automationdirect.com/adc...ers/1-z-16_DIN_Size_(SL4848_Series)/SL4848-VR

Regards

Tim
 
Yes, I over-simplified the schematic to make fewer lines, as his concern was how to wire L/1 and L/2 and not need to have separate 110VAC lines. The ground and neutral are separate wires. The ground symbols at the fan and PID should be replaced with NEUTRAL. I had started to redraw it, but didn't, sorry if it confused anyone. I'll fix it tonight.
 
Bldsmth,
Thanks for clarifying, and I figured that was the case ~but asked for the clarity of newbies reading. Much like the concern we have over junior members playing with fire, I have the same concern for folks dabbling with electricity: a little knowledge can be dangerous. Many folks don't have a solid base in the fundamentals, and might take the sketch "verbatim"; whereas you and I would look at it more as relaying just a general idea.
Best,
Steve
 
For the main power switches (DPST) is this really big enough for 13A?
(Amazon Link) 250V 15A 30A Electrical DPST 4-Terminals Black Rocker Switch

It seems like 15A isn't enough leeway for a 13A circuit but I don't understand what 15A 30A means. Does it mean 30A starting surge and 15A sustained or something like that? I've see others that were 15A 250/20A125 but this doesn't mention 125V anywhere.

If not, does anyone have a link to a switch that would work?


Thanks again everyone for your help, it's slowly coming together :D:thumbup:
 
Could be rated at 30A for resistive load, 15A for inductive load? It's not at all clear.

Also, switches are often rated lower on DC than on AC, so it could be that? On DC, the spark as the switch breaks will always be in the same direction, so one contact will build up as the other pits. On AC, things average out and there should be minimal net transfer of contact material oner multiple operations.

When I've built them, my ovens have had a no-volt-release circuit using a contactor. I use a normally-open doorswitch contact in the hold-in circuit, that breaks when the door is opened. It means there is no problem with switching the full heating current through the switches, particularly the door switch. The downside is that you have to press the start button every time you close the door. The upside is that it's a pretty safe system.
 
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