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-   -   replacing MDR with SSR (http://talk.craftweb.com/showthread.php?t=12566)

Cecil McKenzie 03-22-2020 07:19 PM

replacing MDR with SSR
 
Since my shows have been cancelled or postponed i am planning on using a small Denver furnace to fill small orders. I have been having reliability issues with MDRs sometimes failing off and worse sometimes failing on. Considering changing my MDR with a solid state relay.

Interested in whether anyone has done this and what experience they have had. Is it a simple operation and has it worked well? A Watlow representative has suggested a din na mite relay. Thank you for any input. Hunkering down in Kansas.

Rick Wilton 03-22-2020 07:37 PM

don't buy the really cheap ssr's from Amazon. They will melt down, they are NOT designed for anything close to their stated ratings. I thought I was saving some good money buying "direct from a manufacturer" and quickly (3 months) found out they were crap and dangerous.

Cecil McKenzie 03-22-2020 07:49 PM

Thanks i was thinking about Watlow equipment.

Sky Campbell 03-22-2020 10:06 PM

Ive never seen a mdr fail in the on position. Knowing how they work you would understand how thatís impossible unless you lay it on its side or hold it upside down. I donít think you can find anyone that would consider a ssr to be more reliable then a mdr. Maybe someone will prove me wrong but I highly doubt it.

That said I would use mdr relays for my positive cutoff and a scr to control my power. The scr if tuned correctly (reducing the gain, using soft start and only delivering as much power needed) can possibly double the life of wire elements.

Pete VanderLaan 03-23-2020 07:17 AM

I've never seen an MDR fail at "On" either but I have seen them fail in that they shut down in the on position and close, never achieving the temperature needed for the program. It's the solenoid that gave the trouble, not the mercury portion.

Mechanical relays indeed will fail at on.

Safety issues on an MDR would not keep me up at night.

Brian Graham 03-23-2020 08:35 AM

Din-a-mites are great- there are always a ton of them on ebay - You need to have the correct dc milivolt signal available from your controller though.

Sky Campbell 03-23-2020 10:44 AM

Quote:

Originally Posted by Brian Graham (Post 147413)
Din-a-mites are great- there are always a ton of them on ebay - You need to have the correct dc milivolt signal available from your controller though.

I think your confusing things or maybe Iíam. I know din-a-mite as producers of scr controllers not ssr controllers. A scr will require a variable miliamp input and not sure if anything uses a milivolt input.

Sky Campbell 03-23-2020 10:49 AM

I guess I should google before talking. There are some controllers that need a milivolt signal but I personally have never seen or heard of one.

Cecil McKenzie 03-23-2020 12:36 PM

In my experience mercury relays have always failed on. I have melted paperweights and multiple vases melted together as evidence. When i started using my small Denver unit I had a couple failures with the MDRs failing in the off position which surprised me . After I started my furnace this year it ran fine then failed in off position. I could tap the relay and it would work again but then fail again. I replaced it with a relay I bought 8 years ago. It was different from many I had used in that it had brass bars connecting the two sides of the coils top and bottom. The manufacturer said this was to make them synchronized.

i was pleased since it ran much quieter than the old one. About 5 days later i noticed the furnace was at 1972 F when it said it was holding at 1950F. I am pretty sure it had been higher and then become unstuck.

When I talked to Denver they said that they thought solid state relays always stick on. I talked to Brice recently because I knew he used solid state relays. He said he though his have failed sometimes in the on position and sometimes in the off. Denver suggested a fail safe of some type to keep furnace from over heating.

These considerations have prompted me to seek advice from this board.

Brian Graham 03-23-2020 12:50 PM

Sky - I may be wrong - maybe it is just switched dc. I would have to go look at them in my panels. I have always run Watlow controllers as well - old series 96 and a few EZ Zone PM's. I agree that SCR's will make wire elements last longer because they soft start.

Shawn Everette 03-23-2020 01:59 PM

I too have had MDR's fail on. An SCR too, but that was on finicky control system.

SCR is probably going to give you the longest element life, but may be harder to source, and probably more expensive as you up the amps.

SSR is probably going to be the cheapest option of those offered.

Personally I'm still on mechanical, the metronome click lets me know it's working without looking at it, and I can usually hear when it's on it's way out. I've also got 24 of them laying around, so it's the free option.

Sky Campbell 03-23-2020 02:41 PM

1 Attachment(s)
Quote:

Originally Posted by Cecil McKenzie (Post 147422)
In my experience mercury relays have always failed on. I have melted paperweights and multiple vases melted together as evidence. When i started using my small Denver unit I had a couple failures with the MDRs failing in the off position which surprised me . After I started my furnace this year it ran fine then failed in off position. I could tap the relay and it would work again but then fail again. I replaced it with a relay I bought 8 years ago. It was different from many I had used in that it had brass bars connecting the two sides of the coils top and bottom. The manufacturer said this was to make them synchronized.

i was pleased since it ran much quieter than the old one. About 5 days later i noticed the furnace was at 1972 F when it said it was holding at 1950F. I am pretty sure it had been higher and then become unstuck.

When I talked to Denver they said that they thought solid state relays always stick on. I talked to Brice recently because I knew he used solid state relays. He said he though his have failed sometimes in the on position and sometimes in the off. Denver suggested a fail safe of some type to keep furnace from over heating.

These considerations have prompted me to seek advice from this board.


The only possible explanation for a mdr to fail on would be if you used a normally closed relay. That is the power flows through the relay until it is energized then the connection is cut.
Denver uses normally open relays as we all should for heating applications.
Understand how a mdr works. It is a tube with mercury inside. There are contacts at the top and bottom when it is energized a plunger is forced into the liquid displaced to make the connection. There is no arching and no contacts to wear out. It is in theory impossible to stick in the on position. They are rated in the millions of cycles and are the most reliable type relay you can use.

Solid state relays have a tenancy to fail and short on the output causing the heater to stay on as the circuit is welded together.

Sky Campbell 03-23-2020 02:52 PM

The Idea a normally open, properly mounted MDR will fail and melt down your work isn’t happening. Your controller on the other hand can fail and continue to deliver power to the mdr even when it’s not called for.

I’m not trying to be a smart ass. Please don’t think anything I’m saying as being disrespectful or dismissing anyone’s experience. I know freak accidents happen but usually with closer examination there will be an answer.

Cecil McKenzie 03-23-2020 04:10 PM

Sky no worries. i know the literature says the MDRs fail open rather than closed, i use MDRS that are normally open. On several occasions I have had melt downs in my anneallers. I changed the MDR and everything was fine. I have also experienced touching an element when the controller was off and received a 120 volt shock. I took this to mean that one side of the relay was closed allowing me to get shocked. The elements were not hot as per temperature but were energized by one side of the relay. Now you say that is not possible but in my case it seemed very possible.

Be that as may I am trying to find something that will work better for me.

I have been reading some about SSRs on line. One page said
mdrs are not allowed in many countries now and wonder what some other countries are using instead of MDRs,
https://www.process-heating.com/arti...mercury-relays

Still trying to figure this out. Appreciate all input.

Shawn Everette 03-23-2020 04:53 PM

I don't doubt the theory behind MDR's not melt down, but I've experienced the same kiln, with 2 different MDR's go thermo nuke. Relay was non functional after that, nothing odd coming from the controller, simple swap fixed all issues. Seen enough shorts instaweld metal to believe that one could fuse the plunger open in non ideal circumstances. Whether or not that was the fault of the relay could be debated.

Steve Beckwith 03-23-2020 06:30 PM

I ran a Denver furnace and on at least two occasions I had an MDR fail in the on position. It would be discovered when I made contact with the metal frame with the pipe while gathering. Not a full voltage dose but enough to give a nice tickle. I was running them at the upper range of their amp rating. If I recall, I had problems with a 40 amp relay. I switched to a 60 amp and had no further problems.

Pete VanderLaan 03-23-2020 06:35 PM

well, everything works well until it doesn't.

Cecil McKenzie 03-23-2020 07:11 PM

Steve Beckwith
"I ran a Denver furnace and on at least two occasions I had an MDR fail in the on position. It would be discovered when I made contact with the metal frame with the pipe while gathering. Not a full voltage dose but enough to give a nice tickle. I was running them at the upper range of their amp rating. If I recall, I had problems with a 40 amp relay. I switched to a 60 amp and had no further problems."
Steve i can recall noticing that the red light that indicated the elements were on started to come on while the door was open. The furnace had been running normally as far as heating but I figured that the side of the relay that runs the clock and the warning light had failed in the on or closed position. I remember thinkng that when the furnace was in that condition you might get shocked by 120 ac even though I never did. Maybe I should get a higher rated MDR. The salesman for Watlow thought SSR s are appropriate for heating applications. Thanks for your input.

Shawn Everette 03-23-2020 07:52 PM

I know the tickle of a element, but I don't get what would have caused it in your case. Even if a relay fails on it "should" be isolated enough from the frame to prevent transfer. Don't know all denver's, but the one I serviced seemed like the coil was a safe enough distance away from the glass and frame.

Then there's holding an a frayed end of an cord in the middle of rewiring while your "assistant" plugs it in. That's a good jolt

Steve Beckwith 03-24-2020 05:09 AM

I always assumed that because molten glass will conduct electricity and the inside of the furnace was coated in glass from the dust from the batch while charging that is how I was able to get a poke. A couple times I used "percussive maintenance " on the MDR and it would function normally for a while.

Pete VanderLaan 03-24-2020 07:00 AM

Electricity is always drawn to the best ground. If you stand in a puddle of water with a crackerbox welder and use it, it's quite likely to nail you despite your ground cable.

True of a furnace and its wiring as well. Years back, I had a moly element droop and contact the crucible and I did not see it. I went in with a pipe and there was no current draw. The double insulated transformer was doing its job. If I'd been in water, a different story.
I di shut down and repair the problem as soon as I saw it was there.

Greg Vriethoff 03-24-2020 08:37 AM

Quote:

Originally Posted by Pete VanderLaan (Post 147438)
Electricity is always drawn to the best ground.

Path of least resistance.

I worked with a guy at my first metal fab job in Seattle that majored in sculpture at UW. He told me of the time he was working on a large piece outdoors in the yard where students could work on large projects. It was 20-25 in length. He hooked his buzz box at one end and tacked the corner. He moved to the other joint without moving the ground lead. He moved further away, and was then standing on damp grass. Suddenly, pow! Said he saw stars. When he came to he realized he'd wet his pants.

Cecil McKenzie 03-24-2020 11:49 AM

Talked directly with the Watlow representative. He said there was no real advantage to change to a SSR and the fact that the MDR I had installed was 8 years old may have contributed to its failure. He recommended putting a new MDR on and go from there.

He also said there is no truth to the belief that MDRs always fail open or SSRs always failing closed. So I am going to get a new MDR and trust the force. Thanks for your responses.

















ssr

Pete VanderLaan 03-24-2020 12:02 PM

Dudley does sell MDR's

Pete VanderLaan 03-24-2020 12:03 PM

Quote:

Originally Posted by Greg Vriethoff (Post 147439)
Path of least resistance.

I worked with a guy at my first metal fab job in Seattle that majored in sculpture at UW. He told me of the time he was working on a large piece outdoors in the yard where students could work on large projects. It was 20-25 in length. He hooked his buzz box at one end and tacked the corner. He moved to the other joint without moving the ground lead. He moved further away, and was then standing on damp grass. Suddenly, pow! Said he saw stars. When he came to he realized he'd wet his pants.

*****
Thank you.
The correct term was alluding me at the moment. I too have had your friend's experience.


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