Being a luddite, I rather like 'relays that go clunk' - but what experience do you all have of this kind of kit - and what about the reliability of solid-state switching.
As the controller aims for a specific 'ramp / hold' profile during each firing, the relays may be switched some twenty to fifty times each firing cycle.
There will be another main 'safety' relay - which will be capable of breaking the power to both elements - probably controlled by an independent 'kiln-sitter' - just to prevent 'meltdown' in case the main controller goes out to lunch, or the element relays fail 'closed'.
Suggestions on the best relay technology, please ?
Solid state is fine and would only need to be rated for a resitive load. As would a relay/contactor or even an proportional SCR system. I've not seen mercury wetted for a long time?
Dad has a 10kW(ish) kiln, the controller for that uses solid state relays to do ramps and holds, the recommended fuses to protect the relays cost more than the relays themselves - at one time it was blowing quite a few fuses, I think it turned out to be an issue with the wrong heating elements.
Opto coupled zero voltage switching SSRs will be best.
3-4kW is NOT 10 amps at 240v - 4kW is 16.7 amps at 240v
Pick a SSR that is rated at double or more than the running current to allow margin for inrush currents into cold elements. Don't even think about using them in parallel for extra current handling
- one always comes on first!
Mount on a heatsink, fins vertical - check the data sheet but you possibly need something like 2 degrees C per watt.
The mercury ones are mecury in glass bottles that tilt so that the mercury covers the contacts inside to switch on. They were usually used in explosive atmospheres (eg coal mines, petrol pumps etc) in day s of yore. Expensive & haven't seen any for a long time.
I've been involved with control panels for more years than I care to remember now... lol
You must NEVER EVER rely on a SSR to isolate the supply or cut things off in an emergency - it may not turn off. Don't share one fuse as you won't be able to get close enough protection.
How I'd do it:
1) Mains in via 2-pole switch disconnector.
2) Fuse (2A gG) off to feed temperature controller & contactor coil circuit.
3) Two separate fuses to two poles of 3-pole conventional contactor with stop/start buttons & third contact used as holding contact. Can be dropped out by the "kiln sitter".
4) Each of the two fused contacts feeds a SSR, which feeds it's own element. The SSRs are, of course, controlled by the temperature controller.
It may be an idea to delay one element slightly on initial switch-on, allowing the first one to get fairly hot first (with consequent increase in resistance and reduction in total startup current). Don't delay too long unless there is no danger from uneven heating. After startup the SSRs can switch together as the thermal inertia will keep the heater resistance up. Sometimes the delay is done using another "kiln sitter" type unit in reverse, to enable the second heater only when the kiln temperature is above a certain point.
It would be good to use a "kiln sitter" as you described, killing the supply to the conventional contactor (which should use "stop" & "start" buttons with a holding contact to give failsafe operation).
I was usually on the low-power side - let somebody else worry about the 'big amps'...
No - I was planning on the 'safety' being an electro-mechanical device
Don't share one fuse as you won't
Good plan
OK - this is my 'safety' relay ? - and the 'stop' button is an 'emergency stop' - in series with the kiln sitter ? Do you have an example of how this would be wired ?
You'd prefer SSR's over a mechanical relay ? I was leaning towards electro-mechanical... but then, I'm a dinosaur!
Hmm - if I use the controller that I have 'spare' then it's only got a single output - so I'd need to look at some extra delay circuitry.
Assuming 3kw per element, I guess it's only the same as two electric fires switched on at the same time.. so maybe running two relays (rather than a single one with twice the rating) is making life complicated.
I think there could be issues with uneven heating - depending on the physical layout of the elements. Kiln is planned to be about 3ft 6" x 2 ft - haven't got as far as drawing element layouts yet.
I have various books & dvd's on order from people who have been this way before.... so I'll not be completely in the dark.
I'm looking at 'build-your-own' due to frustration with my existing commercial kiln, which tends to shed bits of firebrick onto the glass as it fuses... As I discovered the hard way, the controller is also rather 'dumb' - as a fault on the thermocouple resulted in the kiln trying to fire 'forever', as it waited for the temperature to register an increase.... Luckily, I caught it somewhere round-about the 980c mark - normal process temperatures are under 800 - so that's why I'm thinking about kiln-sitters as a second line of safety
The other motivation is to build a large kiln for round-about 1000 -
1500 euro - commercial kit costs 4 time that.
If you can point me at a circuit diagram then that would be great.
I'm planning on mounting the control gear remote from the 'hot bits' - so I guess conventional contactors would work OK ?
I was planning on keeping the control gear remote from the 'hot bits' - probably in a wall-mounted cabinet, with cabling to the heating elements and thermocouples - so I can keep the heat away from the controls
Fairly pricey, though the SSR. The same people do various 30A relays for under a tenner...
Zero voltage switching and more suitable for the number of high power on/off operations that you propose Also any further control you might want to do is done a low voltage and low power. A few milliamps will control 10s of amps with ease.
I'd use SSRs. Ten or more years ago I built a heater controller for a brewery with four octal base relays each controlling a 3 kW immersion heater. They didn't last particularly long, and I replaced them with SSRs which have proved much more reliable. The ones I used were about 2 inches square, from RS (because they had an account). They need a heat sink.
The other thing you might consider for your kiln is a "policeman", i.e. a second thermocouple and temperature controller set a bit higher than your normal target temperature and arranged to trip the supplies in the event of a control system failure.
Yes - they're known as 'kiln-sitters' - and they're designed to do just what you say - preventing the whole lot from 'meltdown' in the event of a controller / power 'relay' failure. Having a background in software control systems - I was a bit surprised to see that the (commercial) controller on my current kiln didn't have any logic to handle 'heat on but temperature not increasing'. When the thermocouple failed, soon after installation, the thing just went on heating - luckily I spotted it and pulled the plug before it got much above the high 900 c's....
The Big Kiln will have an independent 'kiln sitter' to avoid that - might even end up with a pic-based controller with a little bit more intelligence too!
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