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crashmann · Guru Joined: 27 Sep 2005 Posts: 501

Which link is broken? The oven calculator is here:
http://www.imperial-armor.com/ovencalculations.html

According to Cod, each segment (A to B, B to C, etc...) should be made from 12.8" of tightly coiled nichrome wire. You can double that length if you plan to stretch it from A to B to B 1/2. Just mark where the middle is, and mount that at B (or D if doing the inner coils).

Charlie
TI-386

tubachris85x · Posted: Fri May 04, 2007 11:35 pm Post subject:

Thanks crashman, the link wasnt working for me earlier for some reason.

I entered in the "volts," 120, cause thats what my readings consistantly said. Im still not sure what exactly I need to putt under watts though, I noticed that Cod put in, 1500, so Im assuming thats the max number of watts the outlet will produce. Though, I dont know how to account for having two circuits on the same oven. However, I am wondering if I can then can try this and see if the breaker wont blow if I switch it back to one switch, thats if Im reading and entering the numbers right of course. if thats the case, then would use 12.8 inches per segment, 51.2 inches total, where the outlet is producing 120 volts, and I can only use (assuming thats what it means) 1500 watts. Now when it says "coiled length," it means unstretched assuming. Just want to double check before I try it. I think im almost close to solving this, thanks again guys!

-tubachris
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cod · Posted: Sat May 05, 2007 3:12 am Post subject:

Stomper · Posted: Sat May 05, 2007 4:38 am Post subject:

One more thing... electrical circuits & overload protection are rated at a CONTINUOUS LOAD of 80%, so if you exceed the rated MAXIMUM amount the breaker/fuse will trip/blow (I won't get into "time delay" overloads), if you pull more than 80% of the circuit's rating for a CONTINUOUS amount of time your overloads will kick in as well. Electrical circuits are designed to handle the "inrush" when you flip a switch to the "on" position (this is also referred to as "locked rotor" and can have a 300% factor), then even out on a "full load ampreage" (when everything is up running at full spead ahead) which should not exceed 80% of the given circuit size.

so a:

15-amp circuit will handle 12-amps CONTINUOUS DRAW

20-amp circuit will handle 16-amps CONTINUOUS DRAW

Whatever sized circuit you attempt to use, multiply its value by .8 to get the proper load range you should be shooting for on that circuit.

Hope that helps, and doesn't muddle it up worse for you! (it's 3:30am and I'm bleary eyed ATM, LOL)
_________________
Tony Krewson
FEAR... is not my god!!

drcrash · Posted: Sat May 05, 2007 8:40 am Post subject:

That's VERY interesting, Tony.

Sounds like it means that a 20 amp oven should systematically blow a 20 amp circuit, after being on continuously for a while. Uh-oh.

It also sounds like me dividing a 26-amp oven into 2 13-amp halves on different circuits gives me enough headroom to run on two 20-amp circuits, but not two 15-amp circuits, as I thought it would. (Luckily, I do actually have mostly 20-amp circuits, but ideally I'd like to be able to use the oven anywhere.)

This is important info. It sounds to me like the standard advice for a TJ oven on 110/120V should be to break it into two two-segment circuits.

BTW, about inrush currents... my impression is that resistive heating elements shouldn't have a big inrush like motors and lightbulbs. The resistance only goes up something like 10 percent in heating a nichrome element up to 1400 degrees, IIRC. (For a lightbulb, the resistance is very low and goes way up in the first fraction of a second, because the filament burns way hotter.)

If I've got that about right, then a 20.8 amp oven per the oven calculator should draw less than 20 amps after the first few seconds---the resistance of the coils will go up a bit above their cold measured resistances---but it will still draw more than 80 percent of 20 amps.
_________________
Paul (a.k.a. Dr. Crash)

Tired of buying cheap plastic crap? Now you can make your own! www.VacuumFormerPlans.com

Stomper · Posted: Sat May 05, 2007 11:25 am Post subject:

Yeah, I threw that inrush part in because I deal with electrically driven motors and refrigeration compressors all the time, but yes resistance heating/lighting will not TYPICALLY have a huge inrush like a motor because the motor has starting torque to overcome from a dead stop to 1170-rpm (and much higher) in a fraction of a second which is the "locked rotor" issue I mentioned. The 80% CONTINUOUS LOAD still apllies in either case.
_________________
Tony Krewson
FEAR... is not my god!!

cod · Posted: Sat May 05, 2007 12:11 pm Post subject:

cod · Posted: Sat May 05, 2007 12:23 pm Post subject:

faithblinded · Posted: Sat May 05, 2007 2:13 pm Post subject:

One factor missing from the oven calculator, is compensating for the change in resistance nichrome experiences with change in temperature.

The reason that an oven calculated to pull 20 amps, actually ends up pulling less once the wire heats up, is because of the change in resistance of the wire. I think. Comparing your oven calculations with the reading of an amp clamp once it heated up would answer this question.

just my two bits

Ken in Cleveland

drcrash · Posted: Sun May 06, 2007 6:28 am Post subject:

Stomper · Posted: Sun May 06, 2007 11:17 am Post subject:

Good point on the voltage differences Paul. FYI- All electrical devices (at least in the U.S.) are designed to motivate with a voltage tolerance of +/- 10% because of the slight fluctuations in the electrical supply. You can get differenet voltage readings from the same outlet at different times of the day because of the demand being placed on the grid as well. The middle of the road for "110-volts" devices is 115-volts, and 230-volts for "220-volts" devices.

Voltage should be thought of as "pressure" (i.e 30-psi of water). Amperage should be thought of as "volume" (i.e. 1/2"-dia. hose at "X'- gph) You will get far more water at 30-psi from a 2"-dia. hose than from 30-psi going through a 1/2"-dia. hose. Another thing to keep in mind, some of you may opt for a 220-240 volt setup; the higher the voltage supply, the smaller the wire size needed because the less amp-draw you will need, BUT you have to use 220-240 volt rated components.

The size of the device (aka load) determines the amperage draw. If you try to draw to much volume (apms) through wires that are too small, the wires will get hot and possibly melt.

14-ga. wire is used for 15-amp circuits.
12-ga. wire is used for 20-amp circuits.
10-ga. wire is used for 30-amp circuits.
08-ga. wire is used for 40-amp circuits.

Volts x Amps = watts (3.41 btu's per watt)

Here's a image copy of an ohm's law wheel I have:

I'm seriously contemplating using a 220-240 volt setup on my machine (when I get off my duff and build it, LOL) using a "clothes dryer" circuit. This being the type with FOUR prongs, with two of the prongs being the "Hots" @ 110-120 volts each (L-1 & L-2), one prong being the ground (required for safety), and the fourth prong going to the nuetural buss in the service panel, so I can "split" one of the "Hot" prongs and have an "onboard" 110-120 volt outlet for my vac-pump, while I run the heater elements off 220-240 volts. This will mean I need a 10-ga. 4-conductor (aka. 10/4-awg) wire to handle a 24-amps CONTINUOUS load set-up.

OK, I'm finished hi-jacking tubachris' thread now.... Embarassed

_________________
Tony Krewson
FEAR... is not my god!!

tubachris85x · Posted: Mon May 07, 2007 7:15 am Post subject:

Jeeze, im gone for a few days or so, and you guys go on an informational chat about electricity. Too bad I cant talk much about it since Im still learning. I like that wheel with the equations, Im gonna keep that so I can learn more about electriclty

Well, since I know the equations, I know that 15 amps x 120v = 1800 watts coming from the outlet. But I now since Ive realized the breakers are labled either 15/20, I just need to figure which outlet that one of the 20 amp breakers are connected to and I can try the 2200 watt oven. So that means I would need about 34 inches total with 8.7 inches per segment of nichrome. Now just need to find that particular outlet...Thanks guys, Im learning alot from your discussions

-tubachris
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tubachris85x · Posted: Tue May 15, 2007 1:37 pm Post subject:

Hey,

Ive been away for awhile, but im going to try to get this oven working next week, assuming that the numbers from the oven calculator are correct and that I dont mess it up lol

Gradutating soon, and I got finals this and next week. Almost outa school!

-tubachris
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tubachris85x · Posted: Wed May 16, 2007 9:48 am Post subject:

I have a question thats been bugging me for awhile, I forgot the rule, for it has to do with the height of the mold and the thickness of the plastic when casting. I am worried that my helmet mold is going to tear the plastic upon casting. The top lip of it it comes to a thin edge, and I am afraid that this is either going to rip through the heated plastic or leave a very thin and weak area. Here is a picture of the top of it. I have to wait to get back home to measure the height of it, but I believe that its just under a foot tall.

Thanks

-tubachris
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jegner · Posted: Wed May 16, 2007 9:53 am Post subject:

Measure from the top of the mold, to to the surface of the platen. If over 12 inches, there might be an issue using .080, but .093 should work. Also, there no real knowing how the plastic will stretch until you try it, but my guess is, you will be ok.

Jim

nice work on those molds BTW.