Sciencemadness Discussion Board - High current low voltage heating element

High current low voltage heating element

wg48 - 11-4-2016 at 22:17

I wanted to heat a 25mm dia SS reaction beaker or tube to at least 1000C. I considered using a few turns of steel strip or fence wire as a heating element either external or in the beaker. Possibly a resistance heated boat.

So some experimentation was required. I used an mot with the secondary and shunts removed and rewound with two turns of 25mm2 wire. The oc voltage was 1.78Vrms with a sc current of about 550Arms. The transformer is shown in the pics below. Sorry the pic showing the two turn secondary was lost.

The second pic shows a 100mm length of 3mm dia fence wire heated by about 300Arms.

The third pic shows the failed wire just after it stopped behaving like a sparkler. The differential heating was caused by part of wire being low resistance due to a coating of copper used to estimate the temperature. I missed the chance to snap the ignition of the zinc coating as I was too busy avoiding the zinc oxide fumes.

The last pic shows a 3mm dia tungsten rod glowing white hot perhaps at 1500C (the rod distorted) The blur is in part caused by the cloud of volatile tungsten oxide. The current was about 350Arms.

I am going to need a larger dia wire secondary for practical use with more turns to drive a longer heating wire. Perhaps a three mot pack threaded with 15mm diameter copper pipe packed with copper wire and filled with aluminium. I may be able to heat a ss tube directly with that.

[Edited on 12-4-2016 by wg48]

[Edited on 12-4-2016 by wg48]

[Edited on 12-4-2016 by wg48]

violet sin - 11-4-2016 at 22:45

Ok, care to elaborate?

Sorry, i was reading last night and this thread was a title with a single line of text same as the title. I was hoping there was more to come, and now there is pics and text.

[Edited on 12-4-2016 by violet sin]

the missing pics

wg48 - 12-4-2016 at 00:40

Here are the last two pics I did not manage to attach to my original post.

wg48 - 12-4-2016 at 01:05

Quote: Originally posted by violet sin

Ok, care to elaborate?

Sorry you are probably referring to my original screwed up post which has now been edited. If not would you care to elaborate on the elaboration you require. LOL

[Edited on 12-4-2016 by wg48]

phlogiston - 12-4-2016 at 01:57

Alloys like nichrome and kanthal have the benefit of not acting like a sparkler

violet sin - 12-4-2016 at 11:56

With nichrom etc. You have to pay attention to wire length and voltage. Too much and it will get red/orange but not hot enough. Too little and it glows like the friggin sun and burns out. Not that hard to calculate, there are plenty of online resoirces for just that.

As noted above, i was interested in the opening statement, but there was no content added yet. Sorry for the seemingly useless post. Was right before bed time, and I was pleased to see all the info this morning. Just had to bolt off to work so only had time to edit a little blurb onto my first post.

Artemus Gordon - 12-4-2016 at 15:49

Quote: Originally posted by wg48

I wanted to heat a 25mm dia SS reaction beaker or tube to at least 1000C. I considered using a few turns of steel strip or fence wire as a heating element either external or in the beaker. Possibly a resistance heated boat.

So some experimentation was required. I used an mot with the secondary and shunts removed and rewound with two turns of 25mm2 wire. The oc voltage was 1.78Vrms with a sc current of about 550Arms. The transformer is shown in the pics below. Sorry the pic showing the two turn secondary was lost.

The second pic shows a 100mm length of 3mm dia fence wire heated by about 300Arms.

The third pic shows the failed wire just after it stopped behaving like a sparkler. The differential heating was caused by part of wire being low resistance due to a coating of copper used to estimate the temperature. I missed the chance to snap the ignition of the zinc coating as I was too busy avoiding the zinc oxide fumes.

The last pic shows a 3mm dia tungsten rod glowing white hot perhaps at 1500C (the rod distorted) The blur is in part caused by the cloud of volatile tungsten oxide. The current was about 350Arms.

I am going to need a larger dia wire secondary for practical use with more turns to drive a longer heating wire. Perhaps a three mot pack threaded with 15mm diameter copper pipe packed with copper wire and filled with aluminium. I may be able to heat a ss tube directly with that.

I have spend several decades working in Electronics, though I admit my experience with transformers is limited. I am almost completely lost trying to read this post. Here's my comments and best guesses:

"I used an mot with the secondary and shunts removed" - secondary implies some sort of transformer, but Wikipedia has no entry for a "mo" transformer.

" rewound [the secondary windings, I'm guessing] with two turns of 25mm2 wire." - Twenty-five square millimeter wire???

"The oc voltage" - Open-circuit voltage. That makes sense.

" 1.78Vrms" - Volts, Root Mean Square. Should have a space between the V and the rms.

"a sc current of about 550Arms." - "sc" = secondary, I assume. Needs space between A and rms.

m1tanker78 - 12-4-2016 at 17:56

I used to burn off the galvanized coating as well as straighten fence wire (for decorative welding) in one shot with a rewound MOT. I made a large coil and embedded it part way in castable insulating refractory. The furnace got plenty hot but would melt the refractory near the wire. It eventually sagged really bad or got pulled apart by the force of the cooling refractory glass. Graphite rods will get so hot that you need brazing goggles to look directly at them! Making electrical connections to them isn't so straightforward. Neither is finding a suitable refractory for that matter.

If you plan on running the MOT for any length of time you'll need some moderately aggressive cooling.

wg48 - 12-4-2016 at 19:11

Quote: Originally posted by violet sin

With nichrom etc. You have to pay attention to wire length and voltage. Too much and it will get red/orange but not hot enough. Too little and it glows like the friggin sun and burns out. Not that hard to calculate, there are plenty of online resoirces for just that.

As noted above, i was interested in the opening statement, but there was no content added yet. Sorry for the seemingly useless post. Was right before bed time, and I was pleased to see all the info this morning. Just had to bolt off to work so only had time to edit a little blurb onto my first post.

Yes matching the wire to the power supply is important. I have previously melted six inch nails with the current from a few turns of heavy copper wire wound on a large transformer. What I did not have was the currents, voltages and approximate temperatures. A small furnace will be much simpler to construct if the heating element is say half a meter long and low voltage reduces high temperature electrical insulation problems.

violet sin - 12-4-2016 at 21:44

The MOT(microwave oven transformer) is an accessable component to many people to experiment with. That said, the reading i have done here and there seems to say a few things about them. The cores are too big for the turns count on the primary, leading to core saturation straight off. Some people had calculated that the 90 or so turns primary is too few, and said it needed more like 120-180 total primary turns, and there isnt enough room, nor can it be easily ordered to maximize the room that is there by hand.

One person had said a source( factory worker friend) claimed the cores were intentionally saturated to force it into all or nothing behavior. Because the load size inside being cooked, would affect a the power draw. They didnt want to have it do that, so instead the thing runs full tilt wether there is nothing in there or a whole frozen chicken. Regardless it made for a 10A load with open secondary, heating rapidly. Adding even a few turns would diminish the open draw though.

Removing the shunts allowed more amperage to be drawn, but withdrew some of its saftey(you and appliances are subject to more power should any thing short).

I was wondering if it would be possible to slap one of those bad boys with a grinding wheel, to diminish its core mass to an acceptable size. Ya know, round some of those big square corners down a bit. Perhaps if you were carefull not to heat it up enough( with all that friction) to strip enamel off the windings while doing so, it would behave more in the area of open secondary draw and saturation. Cant be a fun project, and i assume symetry would be fairly important. But could be a relatively easy way to make them more useful.

Personally i wanted a multi-tap x-former from one(MOT) that could be run quite a while, and not use a pile of electricity while idle or low use. Dont get me wrong, the 1.5 turn secondary i put on one works great as a spot welder( titanium wire to MMO coated titanium mesh 1-1.5 sec bursts = great strong tacks). But even just tacking a few things together, it got noticibly warm fast. So the limitation was apparent. But a hefty, cheap/free and long running version would be sweet for say a powersupply

As far as heating elements, i have seen some hexalloy in various shapes on ebay. Sillicon carbide that is. Rods, disks, tubes, etc., etc. Maybe take a peak at those, the cost was low enough to afford. Heres a link to the guy i follow (havent purchased or tried them yet, so no feedback there

-works on as low as 27V
http://www.ebay.com/itm/Silicon-Carbide-Igniter-with-FASTON-...

40$ rod 12" long.
http://www.ebay.com/itm/12-LONG-HEXOLOY-SILICON-CARBIDE-CERA...

They also had 18", 24" & 30" rods, the last of which is 94$, take a look at the auction, the details there in are pretty darn impressive.
-VS

3DTOPO - 14-4-2016 at 12:31

You can't just heat up metal to 1000c in atmosphere and expect it will last. Catastrophic oxidization is a serious problem for practically all metals above 1000C, nickel based super alloy Iconel 601 is rated to 1175C, but that is about the best you can do with metal in atmosphere (excluding platinum).

If you bought one of these little SiC heaters they sell on eBay, you can run it off a 12v power supply and is good to 1500C:

http://www.ebay.com/itm/FF003-SIC-Hot-Surface-Ignitor-for-Rh...

And it will last - these things usually are good for 10,000 hours of operation.

[Edited on 14-4-2016 by 3DTOPO]

yobbo II - 14-4-2016 at 14:59

http://www.ebay.co.uk/itm/Mellen-Company-Silicon-Carbide-Hea...

What would the wattage of this element be?

Microwave transformers can be wired with two primaries in series to give a much more sensible load to the mains when not drawing current.
Put lots of taps etc on output that you put on. Put the output winding on using very flexible cable. It can be purchased and is like a great big noodle.

3DTOPO - 14-4-2016 at 20:34

I'm not sure - not many. I think its probably around 50 watts - email them and ask.

But - SiC heater rods are available practically any wattage you want (I have some that are 500 watts a piece). You can also have a reasonable amount of resistance, so you can run them direct of AC with a $20 SCR.

yobbo II - 15-4-2016 at 14:43

How much do they cost and where do you purchase.

If the home constuctor was making a very temp. furnace/oven would they be better trying to obtain SiC or Molybdenum Disilicide elements for the job.

3DTOPO - 15-4-2016 at 21:47

You can get them on eBay and or Alibaba. I would recommend Sic over MolyD - unless you need to go above 1500C.

MolyD has much less resistance, so you would need a high amp low volt power supply.

[Edited on 16-4-2016 by 3DTOPO]

wg48 - 17-4-2016 at 00:21

Quote: Originally posted by Artemus Gordon

Snip

I have spend several decades working in Electronics, though I admit my experience with transformers is limited. I am almost completely lost trying to read this post. Here's my comments and best guesses:

"I used an mot with the secondary and shunts removed" - secondary implies some sort of transformer, but Wikipedia has no entry for a "mo" transformer.

" rewound [the secondary windings, I'm guessing] with two turns of 25mm2 wire." - Twenty-five square millimeter wire???

"The oc voltage" - Open-circuit voltage. That makes sense.

" 1.78Vrms" - Volts, Root Mean Square. Should have a space between the V and the rms.

"a sc current of about 550Arms." - "sc" = secondary, I assume. Needs space between A and rms.

Sorry my post almost completely lost you. In my defence of my not introducing my abbreviations and scruffy punctuation I am on a small phone and tend to minimise my key strokes. My abbreviations are also common. My meaning of mot come up as the first result of a search with mot and transformer. Its not a good idea to truncate an abbreviation when searching for its meaning.

Mot - microwave oven transformer, oc - open circuit, sc - short circuit (I surprised you missed that after oc)

Of causes practices and conventions of punctuation vary but I would say I missed out a space between the numbers and the units and between the units and rms. I don't thing either omission was likely to cause confusion.

wg48 - 17-4-2016 at 02:04

Quote: Originally posted by violet sin

The MOT(microwave oven transformer) is an accessable component to many people to experiment with. That said, the reading i have done here and there seems to say a few things about them. The cores are too big for the turns count on the primary, leading to core saturation straight off. Some people had calculated that the 90 or so turns primary is too few, and said it needed more like 120-180 total primary turns, and there isnt enough room, nor can it be easily ordered to maximize the room that is there by hand.

One person had said a source( factory worker friend) claimed the cores were intentionally saturated to force it into all or nothing behavior. Because the load size inside being cooked, would affect a the power draw. They didnt want to have it do that, so instead the thing runs full tilt wether there is nothing in there or a whole frozen chicken. Regardless it made for a 10A load with open secondary, heating rapidly. Adding even a few turns would diminish the open draw though.

Removing the shunts allowed more amperage to be drawn, but withdrew some of its saftey(you and appliances are subject to more power should any thing short).

I was wondering if it would be possible to slap one of those bad boys with a grinding wheel, to diminish its core mass to an acceptable size. Ya know, round some of those big square corners down a bit. Perhaps if you were carefull not to heat it up enough( with all that friction) to strip enamel off the windings while doing so, it would behave more in the area of open secondary draw and saturation. Cant be a fun project, and i assume symetry would be fairly important. But could be a relatively easy way to make them more useful.

Personally i wanted a multi-tap x-former from one(MOT) that could be run quite a while, and not use a pile of electricity while idle or low use. Dont get me wrong, the 1.5 turn secondary i put on one works great as a spot welder( titanium wire to MMO coated titanium mesh 1-1.5 sec bursts = great strong tacks). But even just tacking a few things together, it got noticibly warm fast. So the limitation was apparent. But a hefty, cheap/free and long running version would be sweet for say a powersupply

If anything mot cores are too small so trying to make them even smaller is likely not going to improve them.

I would be very surprised if mots run at half the number of turns of a more conventional transformer. I took some measurements of some transformers I have under no load conditions.
#1. audio transformer probably rated at 150W.
#2. step down autotransformer rated at 500W.
#3. a cuboid looking mot.
#4. an other cuboid looking mot.
#5. a larger more conventional shaped mot.
#6. an other lager mot.
#7. The rewound mot, shunts removed and probably a small air gap caused by poor reassemble.

The input voltage, input current and power are tabulated in that order.

#1. 246. 0.045 4.65
#2 246 0.089 8.36
#3 244 3.98 108
#4 244 2.97 61.5
#5 244 1.19 49.2
#6 243. 1.56 46.8
#7 242 5.02 160

Wow all the mots are apparently high no load currents compared to the auto transformer particularly #3.

#7 very high dissipation is perhaps understandable give the removed shunts and air gap.

Perhaps your correct about the turns.

[Edited on 17-4-2016 by wg48]

[Edited on 17-4-2016 by wg48]

yobbo II - 17-4-2016 at 06:10

MOT transformers always have a load connected (unless the oven/magnetron is broken).
The manufacturer knows this and therefor is not worried about rather large no load currents as they never occur.
Two MOT in series (primaries) work great. You now have twice as big a transformer with no worries about high no load currents.
You will put the secondaries (which you have wound yourself) in series too.
If you want to put the secondaries in parallel you would need to wind things very exactly or the load current will not split the way you hope it should. Not recommended putting secondarys in paralllel.

3DTOPO - 17-4-2016 at 20:33

Quote: Originally posted by violet sin

As far as heating elements, i have seen some hexalloy in various shapes on ebay. Sillicon carbide that is. Rods, disks, tubes, etc., etc. Maybe take a peak at those, the cost was low enough to afford. Heres a link to the guy i follow (havent purchased or tried them yet, so no feedback there

40$ rod 12" long.
http://www.ebay.com/itm/12-LONG-HEXOLOY-SILICON-CARBIDE-CERA...

They also had 18", 24" & 30" rods, the last of which is 94$, take a look at the auction, the details there in are pretty darn impressive.
-VS

I am nearly certain the Hexoloy you link to is a non-conductive form of SiC (conductive and non-conductive are both available, with non-conductive being far more prominent).

That said, they will definitely work as a susceptor in a microwave, I think that is what that vendor primarily sells them for.

3DTOPO - 17-4-2016 at 21:28

Quote: Originally posted by violet sin

As far as heating elements, i have seen some hexalloy in various shapes on ebay. Sillicon carbide that is. Rods, disks, tubes, etc., etc. Maybe take a peak at those, the cost was low enough to afford. Heres a link to the guy i follow (havent purchased or tried them yet, so no feedback there

40$ rod 12" long.
http://www.ebay.com/itm/12-LONG-HEXOLOY-SILICON-CARBIDE-CERA...

They also had 18", 24" & 30" rods, the last of which is 94$, take a look at the auction, the details there in are pretty darn impressive.
-VS