Kiln continually breaks the relay

[Aurea]

I have a new small kiln:

https://www.technicalsupermarket.com/component/option,com_virtuemart/Itemid,4/page,shop.product_details/flypage,shop.flypage/product_id,2326/category_id,424/manufacturer_id,0/vmcchk,1/

It works fine for a while (sometimes one full firing, sometimes a few hours) and then the relays are gone and the kiln only heats full blast or doesn’t heat at all, so sometimes the relay is badly burned or sometimes looks like new, either way won’t work anymore.

The factory in London doesn’t have answers, engineers and electricians here (Mexico) don’t find anything wrong with the connections or the electrical installation. (The kiln has its own line and breaker). 

The kiln is made for one phase 230v and electricity here is biphasic 220v, but the manufacturer says that’s not the issue, that they have sold lots of ovens to Mexico and USA and never had a problem. 

It’s a very small kiln with a very simple box, but nobody can tell me how to fix it. If somebody here has answers I’ll be very grateful. Thank you in advance. 
 

* I have only done bisque firings (max temp 1000 degrees Centigrade). 

Edited March 10 by Aurea
Added pictures and answering questions.

[Bill Kielb]

Yes, you likely match the 240v split phase grid standard in North America and 3000 watts ought to be just short of 13 amps so most relays ought to perform just fine. Please post pictures of the relay (s) making sure the part number and ratings printed on the relay are visible in the picture. Post any picture of a failed relay you may have as well. Post a picture of the relay mounted in place as well. Relays fail from contact overload, and also often because of overheating. So one thing folks will look at is where are these mounted in the cabinet and how are they kept cool. Blocked air inlet / discharge louvers sometimes contribute to the overheating. So context and clarity of the photos you post will be important.

Edited March 10 by Bill Kielb

[PeterH]

What cone/temperature do you fire to?

PS I'm a little confused about the small print in the kiln spec.
Continuously working time on 1240 C (2264): 10 minutes

... and in the manual https://www.technicalsupermarket.com/(EN) RH-16L KILN INSTRUCTION MANUAL.pdf
Using this kiln at temperatures higher than 1,240°C/2,264°F or for longer than 20 minutes at these temperatures may cause problems with heating element(s) and/or with the kiln’s metal  body that are NOT under warranty.

 

[Bill Kielb]

@PeterH The English translation throughout the manual has a familiarity to it 

Edited March 10 by Bill Kielb

[Bill Kielb]

Thanks for the pictures! Best guess here is this is overheating. Solid state relays (SSR) depend on cooling. So the thing I notice most is a fan guard but no fan. If installing an SSR without a heat sink then the device must be derated significantly. I have added some design curves below to show that once these things exceed 40c (104f - pretty much body temperature) they need to be significantly derated. Most of them heat about 1 degree C per amp which almost always means for this size device which depends on the backplate for all cooling they can only tolerate 7-8 amps without being mounted to a real heat sink. Cooling them always depends on the temperature of the ambient air as well so using cool air will help this significantly.

The 40 amp SSR appears to have lasted longer than the 25 amp SSR but still heated beyond its derated value. The extra rating is great but since the backplate is a certain size it only failed at a higher temperature. It looks like this is designed to be fan cooled so I think I would install a new 40 amp and set up a fan temporarily to cool the SSR and retest while measuring the temperature of the SSR with a non contact infrared thermometer to confirm.

My best guess for now, but minimal cooling appears designed into this to save money. The time limits in the literature are probably closer to being correct - just thermal restrictions on the SSR.

Some charts below and a quick blurb of how this can be designed just as an FYI of how severely these derate without real cooling. A guess would be you are currently operating around the yellow line on the graph. Set up a temp fan, confirm with infrared and once confirmed get a decent fan installed behind the guard. Blow air into the cabinet rather than suck it out. More air molecules, more cooling. All an educated guess though from the pictures.

Edited March 10 by Bill Kielb

[neilestrick]

Is there a fan on the outside of the case that we can't see in the photos? Definitely need a fan to keep the SSR cool.

[Aurea]

That’s great information thank you so much!!!
I will try this theory tomorrow. Will this set up in the attached image work? 
 

Also I’m in Yucatán Mexico, so temperature here is around 30 degrees centigrade. (25 degrees centigrade when cool weather and 40 degrees centigrade when hot weather). 

Edited March 11 by Aurea
Added data.

[Bill Kielb]

Yep! If you have an infrared handy takes some readings and figure where it stabilizes above ambient temp with the fan. Just to mention - the SSR should be bolted to the metal with thermal paste between it and the metal. The fan will cool the SSR and the metal. That’s a nice fan and nice test to verify it works. My hope was the graphs will give you a better perspective of the actual heat sensitivity leading to a decent solution.

[Aurea]

It worked Bill Kielb thank you so much!! 
 

I took the temperature through all the bisque cicle and with the kiln off it started at 32.3°C, when the kiln was at 500°C the Relay was 34°C …  when the oven reached 1000°C the relay was 37.4°C.  So with the fan blowing on it the relay survived and finished the bisque cicle just fine. 

So now I know I have to cool the relay, I don’t have a clue about how to do it … installing  a fan in the electrical box of the kiln…

…will an Xbox fan be enough? I guess I have to hire an electrician but nobody in my city knows about Kilns, and the factory in London clearly can’t help… maybe some computer expert? Or an electrical repairman for AC and ovens? Who will have the kind of knowledge that I need to keep the relay cool and running fine? Any suggestions will be appreciated. 

[Bill Kielb]

@Aurea The good news is we likely don’t need to do massive calcs and I can make some assumptions from your ambient air temps. The also good news is your 40 amp SSR likely easily operates at 60 c or more which is usually minimum design terminal temps. The somewhat hard part probably is pick a right sized computer fan that is 240v and maybe for simplicity it operates whenever plugged in. That kiln has no low voltage or neutral so 240v computer fan is likely easiest. Will chug some number later and add here to confirm reasonable size fan in CFM. Not likely to be gigantic.  (Break time at work - pretty busy morning) meanwhile if you can measure center to center distance between the existing fan screws that would help. It likely was designed for an off the shelf fan.

@Aurea ok approximate (brute force) design on a napkin, here we go: The junction voltage drop will be about 1.2 volts, we don’t know the junction thermal resistance so we will use 1.4 volt drop to be conservative. Further, let’s assume 15 amps max load and the fan contributes 10 watts (fan needs to cool itself controller produces heat this is a very conservative high guess) and performance is derated by 60% of free air blowing into a case. So lots of very conservative assumptions for our napkin calc.

Total wattage = (1.4 x 15 A)+ 10w = 31 watts, convert to btuh => 31 x 3.41 = 105 btuh.  So if we assume an expected 20 degree temperature difference, derate that by 5 degrees for fan heat  (super conservative here and double dipping) we get:

cfm = 105 btuh/ (1.08x15) ~ 7 cfm. Very small!  So the picture looks like a 60 mm fan comparing it to the size of the SSR, the fan below seems to be more than enough, is ball bearing and 240v. Install inside blowing in, wire to the terminals where power comes in so whenever plugged in the fan turns on.

The important part will be the SSR to metal connection. A bit of thermal grease and a snug fit to the metal case will remove heat from the solid state junction best. 

Ok I think we were super conservative with our loads and assumptions maybe someone here can check my math but this ought to be double the air necessary.

If I guessed wrong at the spacing of those existing fan screws, you will need to order the right size. At this airflow requirement most should work. Double ball bearings a nice feature for longevity and most important operates on 240volts ac

Edited March 12 by Bill Kielb

[neilestrick]

Computer fans work great for this application. There's already a grill in place on the box, so measure that and get a fan that fits. This is a small enough box that the output of the fan won't matter. Amazon carries a bunch for very low prices.

[Aurea]

@Bill Kielb @neilestrick

Thank you so much!!  

Bill I’m so impressed and grateful for the time and energy you took to help me out! I have been trying to fix this since last year when I received the kiln but nobody had a clue …and in just one day you fixed it!!!  It took a long time and effort for me to get a kiln and I was devastated  because I couldn’t use it.  I can’t express how happy and relieved I feel!! God bless you!! After all this time,  I am about to finish my first glaze firing with the temporary set up, the kiln is cooling now and everything is going great! 

I will follow the advice and update with pictures and results so more people like me can find this post in the future and fix similar issues easily. 

This is a great community and you guys are amazing, thank you so much!