Author Topic: How many amps before burnout? (Read 5812 times)

imsmooth · « **on:** December 11, 2009, 07:14:27 AM »

I took one of my test coils of 17g wire and 105 turns and subjected it to 10A of current. I had the coil resting on an old stator pancake disc. Using an IR thermomemter the temperature got just under 200C. As the coil heated and the resistance increased I rasied the input voltage in order to maintain 10A.

Does anyone have an idea how many continuous amps is too much for various gauges? I know there are a lot of factors, including ambient temperature and convection cooling.

DanB · « **Reply #1 on:** December 11, 2009, 07:42:02 AM »

This is a good question and I was thinking about posting nearly the same thing about 2 hours ago! Some simple guidelines on this.

Maximum sustained output will depend some on outside temperature I think and a little bit on the materials in the casting (But that doesn't have great effect). In my experience polyester and epoxy and other resins Ive used will warp before the wire burns up, I've never had that happen with vinyl ester/ATH.

My experience so far would say not to push 17g wire over ab out 12 amps sustained.

14 g wire... 20 - 25 amps

15 g, 18-20

so when folks build say our 10' machine with 17g wire (which I now suggest they use 16g because it will fit), I tell them to watch it - if they see anything over 600W sustained it needs to furl earlier.

Using the larger disk magnets in our design and winding with 15g gives some safety factor, to date I've not seen one of those burn out.

As flux has pointed out - a lot of folks worry a great deal about better ways to cool the machine. I dont believe that having holes in the coils... mixing in Aluminum powder, leaving the coils somewhat exposed to the air etc etc will have much impact -as flux has pointed out, I^2 (heat is related to current squared) will catch up to you fast no matter what you do). Fact is - this basic design will usually hold together fine down to about 60% efficiency (and I expect most alternators won't tolerate much worse than that no matter what the design). So long as we keep them above that at maximum rated power and furl before, or at that point things should be fine.

Most burnouts I see are documented to have been producing a good bit more power than I would feel comfortable with considering the wire in the stator.

Flux · « **Reply #2 on:** December 11, 2009, 11:24:56 AM »

Checking with an IR thermometer will only give you the surface temperature. The inner turns will be significantly hotter.

Without a thermocouple or ETD you will not know the maximum temperature. If you measure temperature by resistance rise you will get a mean value and knowing your surface temp you will have a good idea. Over the range you need copper increases in resistance by about 4% per deg C.

Bear in mind that the rms phase current is roughly 76% of the dc with a 3 phase rectifier into a battery. I would have thought any dc output over 10A would be pushing your luck.

As Dan said the resin will probably be as much a limiting factor as the wire. I suspect polyester is not much good above 140C for any length of time. Normal epoxy has no strength above 100C. Vinyl ester does seem to be the best stator material of the three.

Issues such as cooling in wind conditions are very much unknown, Personally I don't think there is as much wind cooling as often assumed so I would be reluctant to run things hotter than they will stand on a bench test.

The other unknown is the power duty cycle under wind conditions and that may be very site dependent. Turbulent sites may have an rms that is way below the peak current but in a high wind on a good clean site the thing could be producing rated current on a substantially continuous basis.

I have no real experience of pushing these things to the limit, I would be quite bothered if any of my windings exceeded 100C but obviously many are working above 200C at times.

Flux

letERblow · « **Reply #3 on:** December 11, 2009, 08:54:21 PM »

I ran across this High Temp epoxy potting compound. (275 C, 527 F)

http://www.mgchemicals.com/products/832ht.html?PHPSESSID=0a687fb3a7eaec13dbfcef3a8a5a9367

Kinda pricey and maybe to viscous to soak into the windings.

I used some of the regular stuff (140 C ,284 deg F)on a stator, soaked in well, very strong, looks good , but it hasn't had a chance to fly yet.

birdhouse · « **Reply #4 on:** December 11, 2009, 09:03:58 PM »

flux- quick question. do you consider west systems epoxy to be in the "vinyl ester" class?

Flux · « **Reply #5 on:** December 12, 2009, 01:00:29 AM »

I am not familiar with West system but I believe it is an eopxy 2 part system. Certainly the Structural Polymers stuff I used is a true 2 part epoxy and I think it was claimed to be very similar.

I am not a chemist but it seems to me that vinyl ester is basically one of the epoxy components cured by catalytic means like polyester. Epoxy is a 2 part mix and the mix ratio is fixed. The catalyst systems are not critical on mix, it just affects the cure time.

I think there are others here more qualified to answer this question. My guess it is epoxy but the temperature is most likely too low for stators. For magnets then it is probably excellent.

Flux

Dave B · « **Reply #6 on:** December 12, 2009, 09:49:39 AM »

Just a comment on Wests System Epoxy. I have not used anything else for both the stator and adhering the magnets (I do not cast the magnets in a big chunk, just properly prepare and glue them on.) The bond is incredibly strong and if you accept what the max. sustained output of these machines is then heat is not an issue with this epoxy either.

Take a look at the progression the Dan's have been through with the rotors and stators. Epoxy now to secure the magnets and trying to squeak out a bit more power with slightly higher heat rating of the vinyl ester.

Bottom line is acceptance not a band aid. If you want 1000 watts continuous the popular 10' design is not the one to build, you will burn it up, build a higher rated and (yes, likely more expensive) machine. Dave B.

taylorp035 · « **Reply #7 on:** December 12, 2009, 10:16:02 AM »

My experience with 16 gauge say about 15 amps continuous. I have run a motor at 15 amps for 20 min with 16g, and the temp was not outrageous. Once you get to 20, the rubber insulation will melt. With my homemade electric motor, 40 amps melts the insulation in about 15 sec with power being switched between two wires (brushes on the motor).

DanB · « **Reply #8 on:** December 12, 2009, 05:53:53 PM »

It's really not that we're trying to 'squeak out a bit more power' with the higher temp rating of the vinyl ester Dave... so much that its just a superior resin in every regard to polyester, and I do believe that in the application of the stator, it's better than epoxy (and a lot less expensive). Vinyl ester resin comes out of the mold feeling very much like a rock - not plastic, it's more rigid. So far as adhering the magnets goes, it's lots better than polyester - not as good as the epoxy though. I use it for both - it's a bit of a compromise. All that said - polyester works fine for stators, vinyl ester is just preferred... my 20' machine and lots of others we've built with polyester work just fine with no problems.

The 10' machine built as we build it, with 1" x 2" x 1/2" magnets really should be considered a 600W machine (sustained output above that is dangerous). Using larger magnets and significantly heavier wire in the stator ~ it can sustain much closer to 1000 Watts.

Dave B · « **Reply #9 on:** December 12, 2009, 08:50:16 PM »

Dan,

Point being, the design has evolved and material selection has improved. Changes have been made over time for certain reasons.

As many machines as you have built and have running nobody knows better from the experiences, good, bad or otherwise why we see changes here and there as time moves on. I am sure we will continue to see more improvements implimented from others ideas and suggestions too as we have seen in the past.

Maybe as time moves on there will be a second edition of your book ? Maybe an e-version of revisions. I see a start of this with your "active project tags" and the "we don't do it this way anymore" notes. Things have come a long way since the wood A-X. Thanks for all the help along the way. Dave B.

ghurd · « **Reply #10 on:** December 13, 2009, 02:00:06 AM »

Anyone capable of doing the calculations to get to this point should already comprehend the wire's limitations.

Quote "Sustained output above about 700 Watts may overheat this stator so you should design your machine to furl before that point."

Key sections being

"may overheat this stator"

and

"design your machine to furl before that point"

If someone builds a DanB Machine, they should have seen that text.

This discussion seems the equivalent of placing larger fuses in an inverter to make it a bigger inverter.

Or putting a 5000W sticker over where 700W is printed.

G-

jaskiainen · « **Reply #11 on:** December 13, 2009, 10:03:26 AM »

But it seems to me that the original question was needed.

I'm about to start my next genny and don't know wich AWG wire to use in it.

Ofcourse the heavier i can squeeze in, but still i don't have a finest idea

of what it will tolerate continously in real life.

Someone smarter than me should try to give somekind of info on that.

There's two different values in AWG-table of using the wire.

Example below:

AWG gauge 14

Conductor Diameter Inches 0.0641

Conductor Diameter mm 1.62814

Ohms per 1000 ft. 2.525

Ohms per km 8.282

Maximum amps for chassis wiring 32

Maximum amps for power transmission 5.9

Wich value i should use? 32A or 5.9A if i use that wire in my stator?

If 32A, does it mean that i shouldn't try to get over 32 amps out of my genny?

In that case the max. power out should be 13.2*32 = 422 watts.

imsmooth · « **Reply #12 on:** December 14, 2009, 09:23:28 AM »

If you see this table

http://www.powerstream.com/Wire_Size.htm

it says that AMPS for chassis wiring refers to wire that is not in a bundle and open to air. You need to use the lower amp value. As I said when this started I took a coil of wire (bundled) and connected it to a 10A source, giving a temperature of 200C after 10-15minutes. This was the external temperature; the internal temperature would be higher.

bracken · « **Reply #13 on:** December 14, 2009, 09:24:41 AM »

Spots on the coil will get hotter if there are tight bends or kinks in the wire. You need to be very careful when winding the coil for that reason, however, it is almost impossible to get everyone coil and every turn perfect. The hot spot will burn thru its insulation and then the surrounding wire's insulation shorting the coil out, dropping the resistance, and thus creating more heat.

I also assume that you put 10A of continuous current thru the coil during your testing. In operation of course the current will be intermittent and reducing your heat significantly.

Thru my experience designing motor I would guess 17 awg wire (HAPT or MIL spec/ 200-240C) to be acceptable for 10A in relatively open air. If you are going to embedded the coils in solid epoxy I would recommend a larger gauge.

dlenox · « **Reply #14 on:** December 15, 2009, 06:55:07 AM »

Dave,

I talked with West Systems Epoxy technical support the other week and their response to my question about what is the highest working temperature - and their response was 200 degrees Farenheit...

Dan Lenox

Beaufort · « **Reply #15 on:** December 15, 2009, 07:36:04 AM »

What about switching to parallel-star at a certain windspeed? Bring the ends of the coils out, increase the size of the hookup wire, and it's half the amps through the coils. This also has a big benefit on electrical efficiency over pushing series-star to higher amps. Given some of the excellent comments from everyone, it seems like there are big cost limits to changing to something other than vinyl ester for heat reasons and the wire bundle can only take so much. So for those who want more out of the receipe designs, is this a better way to go? (if so, there have been many discussions on coil switching before)

SparWeb · « **Reply #16 on:** December 15, 2009, 12:41:57 PM »

Yes, this has been discussed before, though usually the discussion is about Star/Delta switching. Both are possible, but for Series/Parallel switching of a Star stator you need a lot more wires, and more relays. Switching from Star to "Jerry" is also possible, where the phases aren't interconnected but rectified individually.

If you're switching for the sake of heat, then definitely DON'T switch to Delta because the current circulation and the harmonics will do you in just as fast. If you're switching for improved performance, then going to either Delta, Jerry, or Parallel-Star could either be a coin toss or dependent on many other factors.

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