Which SSR for electric oven?

E

ecos

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Jul 2, 2006
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I'm in the process of ordering supplies for my oven. I have 2 3,000watt Kanthal heating elements, and high temp wire to connect the elements to the SSR, on the way from Budget Casting Supply, and I am now ordering the PID (ramp soak), thermocouple (K type 2300), and SSR's from Aubers.

My question is.....is there any negative to buying a oversized SSR's? For example using 40amp when your oven may only draw 20? I'm not sure if it would interfere with accurate temp control or some such. It will be run on 220v if that makes any difference.

Also out of curiosity why is ceramic fiber board never used in making ovens? I've read it is even more insulating than the insulating firebricks. I assume there is a reason since everyone uses insulating firebricks, I just don't know what it is? Longevity? Price? Or are the firebricks just the best balance of insulation and heat sink?

Thank you in advance.
 
ecos said:
I'm in the process of ordering supplies for my oven. I have 2 3,000watt Kanthal heating elements, and high temp wire to connect the elements to the SSR, on the way from Budget Casting Supply, and I am now ordering the PID (ramp soak), thermocouple (K type 2300), and SSR's from Aubers.

My question is.....is there any negative to buying a oversized SSR's? For example using 40amp when your oven may only draw 20? I'm not sure if it would interfere with accurate temp control or some such. It will be run on 220v if that makes any difference.

Also out of curiosity why is ceramic fiber board never used in making ovens? I've read it is even more insulating than the insulating firebricks. I assume there is a reason since everyone uses insulating firebricks, I just don't know what it is? Longevity? Price? Or are the firebricks just the best balance of insulation and heat sink?

Thank you in advance.
Click to expand...

No downside

In 220V operation, I would use two SSR's , one for each hot line of the 220.

Don't forget the heatsinks for them.
 
what brand are u getting? the omron's we use at work sometimes short closed at the input side. Have u considered a mercury contactor? most of them have a 1 million cycle life or more. just a suggestion.
 
In picking an SSR, go with 50% to 100% oversize. A 40 amp SSR works great for 20 amp load. Also, the rated capacity is with a good heat sink. It is half that without one. Over about 10 amps, I would not trust it to last the thousands of cycles it will run without burning out unless you use a heat sink. Dual SSRs on 220VAC are a good practice.

You say your oven will only draw 20 amps, but you also said you are installing two 3000 watt elements and have 220VAC. If my math is right, that is more than 20 amps. I get 27 amps, which is 13.5 amps per leg. Still fine for two 40 amp SSRs and heat sink. With the continuous draw, I would use 10 gauge wiring from the power panel and a twin 20 amp breaker in the box. 12 gauge will meet code, but may get warm after a long HT. Also, make sure your power cord and receptacle is heavy enough for a 6000 watt load. A 40 amp range cord will work.

Why such a huge oven? 6000 watts is the size of a big machine shop HT oven. Most knifemakers use one with half that wattage or less. Are you sure it isn't two coils that total 3000 watts??????
 
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Thanks for the responses guys.

Stacy, I just used the 20amp draw/40amp SSR as an example. The price difference in SSR's was minimal so I thought to build it on the stronger side to be safe. As for the heating elements it looks like you are right and its overkill. I've been doing research on making an oven this week and for some reason I had it in my head I needed 2 3,000watt elements. Looks like I got confused...I just ordered a cnc mill and I'm trying to figure out CAD/CAM/tooling, etc for it, I'm building an oven, I'm building a rolling mill, etc all at the same time...so my thought process is a bit frazzled at the moment.

Eventually I plan on 3 sizes of ovens. One decent sized for heat treating batches of blades, one small one for mokume, and one long one for lengthy blades. For the first oven I was thinking something along the lines of this http://www.paragonweb.com/PMT-18.cfm . That model is supposed to be 3,600 watt so I should be ok if I make the dimensions slightly smaller(13" height is higher than we need) and use a single 3,000 watt element and save the other as a backup? The maximum stretched length for these elements is 140", so it sounds like it would fit physically.

Typically I do more thorough research prior to asking questions and shopping but the sea freight shipment I use only goes out once a month and I'm trying to get everything I need in time...then I get to wait 2months for it to arrive (that kills me).
 
There's no real negative to overspeccing the SSR, but the heat sink you use probably makes much more difference than the rating of the SSR in the real world.

It's worth checking the switching current on the controller and SSRs you intend to use. If you may be switching two SSRs off the controller output, it's wise to ensure the controller can cope.

The last oven I built was 42" long and used 2-off 230V elements of 3000W each (it works out at 13A per element, the rating of UK 230V domestic power outlets). I used Fotek 25A SSRs, one for each element, on heatsinks rated for 15A. The controller (an AutomationDirect Solo 4848VR) is rated to give 40 mA of output, enough to switch the 2 SSRs in parallel. The specs said the SSRs would each draw up to 20 mA switching current. If I was doing it again with an extra 10 or 20 bucks in the budget, I'd spend it on better heatsinks before I'd up the SSR rating.

6 kW on an oven with a chamber 9" wide, 6" high and 42" long is plenty. I had the elements already and needed to decide between one element or two. I chose two. One would probably have been enough: I'm sure it would have been enough for carbon steel. Maybe marginal for stainless, but, as I don't like the idea of 42" stainless blades very much, I don't feel that would be a problem.

For the 18" ovens, I use 2 elements, rated for 13A at 110V, connected in series for operation on 230V and run one element on each side of the oven. The series connection is done on the inside back of the oven, so only the uncoiled tails provide heat from the back face, to try to keep the heating as even as possible throughout the length of the oven. It seems to work pretty well.

If you are looking at several ovens/furnaces, I'd suggest you build one good control box and arrange things so that you can just plug in whichever piece of equipment you want to run at the time. It's what I've done.
 
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