relays for kilns

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tob
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Re: relays for kilns

Post by tob »

Bert Weiss wrote:I agree with Brad here, but I do it only slightly differently. I use a mechanical DPDT contactor attached to a hitemp limit circuit with a light switch, and a SPST SSR controlling the kiln. This setup allows you to remove all power to the elements when you want safety, and control the elements the most efficient way. On the last system I put together, I used the high temp feature on the Bartlett RTC 1000 controller instead of a redundant hi temp controller.

BTW, I bought some used Watlow Dinamite SCR's from Brad around 17 or 18 years ago. Not one has failed on me in all that time. They are firing away as I write this.

I once made the mistake of buying 480 volt SCR's, thinking that meant up to 480v. They don't work on 240v. I have a set of 6 if anybody is working with a 480v setup.

We actually do it pretty much the same way, Bert. What I posted above is really only half of my wiring diagram - the part showing the SSR and contactor. I split the original wiring diagram into two parts because I thought a drawing showing everything at once was too cluttered to easily comprehend.

The diagram in this post is the other part that shows the safety circuit wiring, which utilizes both a door switch and an alarm relay of the controller for an overtemp switch. On my annealer I also have a timer switch that I can use to cut the power to the oven manually, or by setting the timer to kill the power. The timer switch I use is a heavy duty switch like this one - http://www.lowes.com/pd_30194-251-T104___?Ntt=t104

Image


A smaller, cheaper timer could be used by wiring it into the safety circuit like you did with your light switch, instead of on the main power lines, as mine is wired. But I like the extra peace of mind of cutting off the main power directly. (And I have a bunch of these switches I picked up as surplus a while back, so why not?) However, 99.9% of the time either way will work equally well.

Image

TOB

P.S. Glad to hear the Watlows are still chugging along for you. :) It's hard to kill them with anything short of direct lightning strike or other disaster.
Last edited by tob on Sat Dec 26, 2015 2:19 am, edited 1 time in total.
Bert Weiss
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Re: relays for kilns

Post by Bert Weiss »

Thanks Brad. That is a useful drawing. I am wracking my brain to remember, but I think I had to put some device between the hitemp alarm output and the contactor. The output from the controller had to be somehow transformed so it could control the contactor.

I used to put a fast acting fuse between the contractor and the SSR, but the fuses cost more than the SSR, so I don't bother to do that.
Bert

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tob
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Re: relays for kilns

Post by tob »

The only two things I can think of that might be necessary between the alarm output and contactor are:

1) A transformer, if your output voltage is different than the coil voltage of the contactor. (Although I find this scenario very unlikely because there are better ways to deal with it.)

2) Far more likely is a snubber or a metal oxide varistor (MOV) to dampen any voltage surges caused by the relay coil or contacts. Some controllers have one installed internally and their manufacturers specifically recommend NOT doing this. Others recommend installing an external one.

Other than those, I can’t think of anything else that might be necessary between the output and contactor.

As far as the fast acting (and expensive), semi-conductor fuses that many SSR manufacturers recommend, I feel the same way you do. The proper fuses are usually just as pricey as a new SSR, so I don’t normally use one on my SSR circuits. This does open things to the possibility of the SSR blowing and locking the power on if there's a mishap, but that’s what the overtemp circuit is for - if things get too hot the system shuts off. Using the alarm circuit for overtemp control may not be as foolproof as a completely separate overtemp controller and thermocouple, but isn’t far behind and is usually a lot cheaper and easier to install.
Mark Wright
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Re: relays for kilns

Post by Mark Wright »

Thanks to everyone that contributed to this topic. Now that I am better educated I am wondering if there is a safety problem with some kilns. Namely kilns that are wired with 240 VAC and a DPST relay.

My 2 kilns, an Olympic & a Skutt, both use the same DPST relay and the incoming power goes directly to the relays.
So – If relays fail in the closed position, which they apparently do, but only one side of the relay fails in this closed position:
1- I probably would not know it, as the kiln would continue to operate normally.
2- The heating coil connected to this bad relay would be electrically “hot” even when the kiln is off, because the income line that is connected to the failing side of the relay is “hot” and therefore the coil is “hot”. (For 240 VAC both incoming lines are hot. This is not true for 120 VAC.)
3- If I touched the heating element with the kiln off and the lid open I would be shocked, if I was grounded! On my kilns the “lid open” switch only breaks the 12 VDC signal line, not the incoming voltage.

This seems to say to me that until I incorporate the above information with SSR’s and switches I should never touch the coils in my kilns unless they are unplugged. Maybe it’s a good practice all the time. I am definitely guilty of adjusting them with the kiln plugged in.

Mark
Bert Weiss
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Re: relays for kilns

Post by Bert Weiss »

Mark, I have only ever worked with single pole single throw relays like in Brad's drawings. One side of 240v single phase power is connected directly to one side of the element and the other side goes through the relay. When the relay is open, the elements do not get hot. However if you touch an element to ground, you get 120v AC hot. I have run my own kilns that way with no safety systems in place. When I build kilns for others, I always do it with safety systems in place. I want a double pole double throw normally open relay in the circuit. Because that relay is normally open, you have to send a powered signal to if for it to close. If you put that power on a switch, throwing that switch will cause that relay to open. When it is open, there is zero power to the elements, and touching ground will do nothing. The high temp limit controller is also attached to that signal. So, if the thermocouple attached to the high temp limit hits it's high set point, that controller will shut down that circuit so the power goes away, stopping the run away caused by the single pole single throw relay sticking closed. Brad's drawing shows this setup.
Bert

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tob
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Re: relays for kilns

Post by tob »

There appears to be a typo in your post, Bert. There's no such thing as a DPDT normally open relay. The terminals on one side are normally open, but those on the other side are normally closed. So the user can choose between them for normally open or normally closed operation. Only a single throw relay comes in exclusively normally open or normally closed flavors. Just thought I would mention it so there's no confusion for those who are not familiar with relay terminology. For those people, here's a more detailed explanation, with diagrams.

A double pole (DP) relay can control two electrical lines, such as the two 120Vac “hot” lines that make up a 240Vac electrical circuit in the USA.

A double throw (DT) relay has three possible electrical connections for each line - one terminal where the incoming power connects, one normally open (NO) terminal that does not carry power unless the relay coil is energized, and one normally closed (NC) terminal that that carries power when the relay coil is NOT energized, but cuts power when the coil IS energized. So a DPDT relay has a total of 6 power terminals, plus two more for the coil power. A DPST relay has only 4 power terminals, plus the two for the coil.

Because DPDT and DPST are both double pole, they can both control two electrical lines, such as the two 120Vac lines on a 240Vac circuit. The difference between them is that a DPST-NO relay has connections that will complete the power circuits ONLY when the relay coil is energized. A DPDT relay has two outgoing power connections, one of which carries power when the relay coil is NOT energized and one that carries power when it IS energized. For safety, a normally closed relay (or the NC terminals on a double throw relay) should never be used to control power to kiln elements. For this reason it's fairly pointless to use a double throw relay for kiln control since two of the contacts will never be used, and double throw relays are usually a bit more expensive than single throw, due to the extra terminals and contacts. (Although I guess if the relay is one that is large enough to have replaceable contacts, it may be possible to swap the NC contacts with the NO ones when those eventually wear out. Maybe...)

Image

Image

Essentially, a double pole relay is just the electrical equivalent of a pair of single pole relays that share the same coil.

Image

TOB
Bert Weiss
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Re: relays for kilns

Post by Bert Weiss »

Thanks Brad. I misnamed the right relay. The one I used last, is actually called a contractor. I did have the right mental picture of what is supposed to happen.

That kiln has 3 zones, so I used two 3 pole normally open contactors. I believe you directed me to this model.


Zoro Item G2315381 Definite Purp.Contactors, 120VAC Contactor,120VAC,40A,3P,Open $34.93 2 $64.86

Due to bizarre electrical inspection issues, we have not yet powered it up. I believe that will happen soon after the new year, once there is an occupancy permit in place at the facility.
Bert

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tob
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Re: relays for kilns

Post by tob »

Yes, as I said, I figured it was just a typo. But I thought I would point it out so anyone else who might be unfamiliar with relay/contactor terminology and reading the thread wouldn't get more confused than they probably already are. Then, while pointing it out I got a little carried away with the details. ;)

Speaking of which, relays and contactors are the same thing and generally work the same way. Both of them are electrically activated switches. It's just that when they get above roughly 20 or 30 amps of current handling capacity that they are called contactors, rather than relays. Don't ask me why. (Because I don't know.) So a contactor is really just a more robust relay. That terminology also tends to occassionally carry over to solid state relays (SSRs), with larger, more robust ones that include heatsinks sometimes called solid state contactors (SSCs). But don't let that confuse anyone, SSRs and SSCs are still essentially the same thing.

And in response to Mark's questions:

"If relays fail in the closed position, which they apparently do, but only one side of the relay fails in this closed position:
1- I probably would not know it, as the kiln would continue to operate normally."


That is correct, although it would be very unusual for just one side of a relay/contactor to fail closed.

"2- The heating coil connected to this bad relay would be electrically “hot” even when the kiln is off, because the income line that is connected to the failing side of the relay is “hot” and therefore the coil is “hot”. (For 240 VAC both incoming lines are hot. This is not true for 120 VAC.)"

Correct again.

"3- If I touched the heating element with the kiln off and the lid open I would be shocked, if I was grounded! On my kilns the “lid open” switch only breaks the 12 VDC signal line, not the incoming voltage."

Correct once more. But as mentioned above, the failure situation you describe would be very unusual. The only situation in which I can envision it realistically happening would be if one were using a separate relay for each pole of the circuit. It's not impossible otherwise, but highly unlikely.

"This seems to say to me that until I incorporate the above information with SSR’s and switches I should never touch the coils in my kilns unless they are unplugged. Maybe it’s a good practice all the time. I am definitely guilty of adjusting them with the kiln plugged in."

That would be wise. Messing with the elements of a kiln that is still plugged in is not a good idea.

The Other Brad
tob
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Re: relays for kilns

Post by tob »

Ironically, I had essentially this same conversation yesterday with someone who just bought a new SCR from me and wondered why, after receiving it, it didn't have connections for both lines of a 240Vac circuit. Since I knew this had been discussed at length several times before, I hunted down one of those threads in the archives of my discussion board to point him to. Afterwards, I figured a link to it might be useful here too. The same principals apply to wiring both SSRs and SCRs. Here's the thread - http://handmade-glass.com/cgi-bin/conf.pl?read=30838
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