www.TK560.com

[sulla]

I'll try not to fill up the forum with piles of newb posts, but when it comes to vacuum forming I am a total newb.

I am trying to understand the voltage calculator. And am trying to work out some preliminary designs.

1. What reason would I need to change the value of wattage from 2400?

2. I think I understand 'Segments' for a single coil oven, but how would I use the calculator for an oven design like Doug's Proto form machine with segmented heating elements? I am thinking that is a design I'd like to try.

Perhaps an oven of 24" x 32" or so with 5 elements.
_________________
-Andy

[crashmann]

There's a fair amount of math involved with oven design. Or you could just wing it until the circuit breaker starts tripping, then back off a bit...

The limitations:
Your AC power supply is 120 volts (at best) Stick a meter in there and see what you are actually getting. No, I'm not kidding. Set your volt meter for AC volts, stick the red in one side, and the black in the other. What do you get? That's one of three values in Ohm's Law. ( http://en.wikipedia.org/wiki/Ohm%27s_law )

I (current) = V (volts) / R (resistance)

Next, your circuit breaker is limited to either 15 amps (standard household issue), or 20 amps. How do you find out which one it is? Plug a lamp into the outlet for your oven. Go to your circuit breaker panel and turn off breakers until the lamp goes out. What's on the breaker, a 15 or a 20? There's the second value that you need for the Ohm's Law equation.

Multiply these two numbers together, and that will give you your total oven heat output, in watts.

Power (watts) = V (volts) * I (current aka amps)

For example, 115VAC * 15 Amps = 1725 Watts

Plug that into the Oven Calculator page instead of 2400

All of the nichrome ovens on this board are broken into segments. Whether it's a single spiraling coil like Thurston James, or separate planks like Doug Walsh. Each nichrome coil between the hot and neutral power terminals equals a segment.

A segment is a resistor, like a lightbulb. Put them in series, and they will all illuminate, but dimly, because each lightbulb adds resistance to the circuit, one right after the other. However, put the lightbulbs in parallel (like all of the oven designs on this board), and they will all glow VERY brightly. Putting resistors in parallel is like adding lanes to a highway - more traffic (current aka amps) can flow through faster.

So, let's engineer our oven:

You've got 115 VAC from the outlet, and a 15 amp breaker

115VAC / 15 amps = 7.667 ohms of total resistance for the whole oven

You want 5 segments.

5 * 7.7 ohms = 38.33 ohms per segment (12.778" of tightly coiled nichrome wire)

Wire up your oven in this manner:

[sulla]

Wow Charlie, thanks for that, and thanks for taking the time to explain all that. It's a lot of food for thought. I should have been thinking of the nichrome wire as resistors all along, I see... I usually use ohm's law for calculating resistors in LED circuits in small-time prop projects. I am no electrician and have a lot to learn.

I will 'plug in' my voltmeter tonight and I will double check my relevant breaker (I am pretty sure I have a 20 amp breaker going to the outlet I want to use in the garage.)
_________________
-Andy

[sulla]

OK, I took my measurements. I have 122.6 V AC at the outlet (WOW!), a 15 amp breaker, so that's 1839 watts ( V x AMP = watts)

It's 8.173 ohm total oven resistance, so with 5 segments it's about 41 ohms per segment (13.622" of tightly coiled nichrome wire.)

That sound right?
_________________
-Andy

[crashmann]

Excellent, you actually read the whole thing! Very impressive!

Your numbers look exactly right. Just remember, this is a starting point, and you'll probably need to trim the nichrome back a bit to maximize the output. The trick is tuning it without causing the breaker to trip all of the time.

Good luck, you are definitely on the right path!

Charlie

[spektr]

remember that there are 3 variables to use when heating plastic. Watt density, which is what you are messing with by "optimising the heater", distance and time. You might find (as I have) that the heat distribution pattern is as important as squeezing out the last watt of heat. Most plastics have a thermal "speed limit" and if you heat them faster than that, the surface facing the heated pays the price. A balanced heater without cold corners is a lot easier to use.....

If I had a 5 segment nichrome heater to tune, I might be making the connection at 1 end of the wire with an alligator clip so I can vary the
resistance by changing the connection location instead of clipping the wire..... It works on my foam cutters and such........

Scott.

[sulla]

Thanks for the tip Scott, but are you talking about a spiral nicrome wire oven?

What I want to build is an oven like Doug's Proto-form machine with 5 separate panels rather than one long coil. So trimming the lengths of the coils in the segments to change the resistance is how I understand Charlies advice to work.



An even heating surface with an optimized heating element sounds like a wining combo to me. I know different plastics heat at different temps and burning is a hazard. Perhaps an infinite switch like in one of the vacuum forming books I have is a way to go? This is just conjecture as I have not seriously looked into using anything other than a simple SPST switch for the oven.
_________________
-Andy

[Fredo]

The Proto-forms optimized panels each draw about 1200watts@ 220volts x 8 panels = about 44 amps. You would need a 220volt 50 amp breaker to run that 2x4 oven. A 24x32 with 5 of the same type panels would still draw 6000 watts, or about 27 amps at 220 volts...........Fredo

[sulla]

Thanks Fredo. I appreciate the advice.

But I am not making the exact proto form machine in Doug's plans. I am making a custom sized oven for my own less-grand-than-Doug's-former.

So if I use each panel as a 'segment' and restrict the nichrome wire in each segment to 13.622" (give or take some tweeking) with a total oven bed sized for 5 segments/panels of appropriate nichrome wire distribution (what ever that may end up being), won't that end up still fitting into the oven calculator findings of 122.6 V AC at the outlet per voltmeter, a 15 amp breaker, with 1839 watts ( V x AMP = watts)?

It's 8.173 ohm total oven resistance, so with 5 segments it's about 41 ohms per segment or 13.622" of tightly coiled nichrome wire per segment wired in series, right?
_________________
-Andy

[crashmann]

Wow, I had no idea the ProtoForm oven put out that much heat. That's some hot stuff baby! You'll never get that much output with a regular household outlet. A clothes dryer outlet is usually 30 amps at 220 VAC = 6600 watts. A kitchen stove is usually 50 amps at 220 VAC = 11,000 watts, plus you'll need some 8 gauge wire to run that much current without melting the jacket.

Your question should be "is 1839 watts enough to heat a 32" x 24" sheet of plastic?"

I suspect your heat output on the 15 amp circuit may be a little low. It will take longer to heat the plastic to forming temperature, but that's not a completely bad thing either. As Scott mentioned, heating the plastic too quickly can result in orange peel finish. I can't imagine where you would run into trouble heating it too slowly.

One really cool feature of the over under design is that as the plastic heats and starts to sag, the middle droops further from the heat source. This will help avoid burning the plastic in the middle of the sheet before the edges are soft enough to form. Are you building an over under style vac table?

My oven heats .125 or 1/8" HIPS from underneath in about 4 minutes while putting out 2400 watts. I suspect your oven would take at least 6 to 8 minutes. You may become impatient and want to hold the plastic closer to the oven coils, but then you ruin the distribution pattern - the infra-red energy needs to bounce around and diffuse, otherwise you'll see the pattern of the coils in your sheet

Do you have a 20 amp outlet (to create 2452 watts) near your workshop that you could plug into?

Charlie

[sulla]

Thanks Charlie. Yes I do plan on building an over-under former. All the breakers in the box are clearly labeled and I tested them. there is only one breaker for the garage and it is 15amp. I don;t want to hire an electrician just for this so I will work with what I have.

Perhaps if I change the size of the oven bed and build it in two 3-segment sections wires in series... Each plugging into a 15amp 122.6V wall socket?



122.6V AC
15 amp
1839 watts
3 segments (per half of oven)

total resistance: 8.173 ohm
segment resistance: 24.52 ohm
segment length: 8.173" (tightly coiled)

24" x 24" total bed size (includes both sets of 3 panels/segments)
_________________
-Andy

[trapperdale]

Sulla,

You said you tested the breaker...is it dedicated?
did you turn it off and look around the house to see if any other appliances/ outlets are using that same breaker?
here in Ca. it is common to run the bath room lights and out door lights off that same breaker. if the house was built before 1980 it may power some plugs also.
if it shares you do not have the full 15 amps !!!
just my 2 cents.
good luck with your project.

[sulla]

The breaker is only for the garage outlets, garage lights, and outdoor outlets and outdoor lights, like the porch light and a light on a pole in the yard. The garage has a few things plugged in that are in regular use - some shop lights, and a water softener, but nothing else, except at night when the yard light comes on.

The house is less than 10 years old.

Could I just lower the amperage in the equation to play it safe and still get reasonable results?
_________________
-Andy

[crashmann]

Splitting the oven in half and running them to separate circuit breakers would work perfectly. That's what another member did to solve the "circuit breaker tripping all the time" problem. Now you need to find two outlets on separate circuits from the garage. Remember, you'll need the garage outlet for your vacuum pump and other various implements of construction.

Backing off by a couple of amps on each side should work ok. That means a little more resistance per segment. I like the alligator clip suggestion for tweaking, just be sure there's no way the clip will fall off. And in the event it might fall off, make sure it doesn't land on anything conductive. A big plyboard might be good to have underneath. A piece of aluminum would be bad!

Charlie

[trapperdale]

whao! hoss!!!

just an other breaker will not due!!!!
THE SECOND BREAKER MUST BE ATTACHED TO THE OTHER PHASE WIRE IN THE PANEL BOX!!!!!!!!!!!!!!
two breakers on the same phase wires will just melt the nichrome heating wire/ or burn something...

advice comes with responsibility !!!!!!!!!!