Steam turbine generator

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RET
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Location: Toronto, Canada

Re: Steam turbine generator

Postby RET » Wed Apr 19, 2017 11:38 pm

Hi,

A properly designed steam nozzle will give a steam velocity which is close to or in some cases above the speed of sound. If you do the math, even at 40,000 rpm. for a 1/16 full size turbine, the steam velocity is at least 4 to 6 times the tip velocity of the blades. For any kind of efficiency, the blades need to turn the jet through at least 90 degrees and the closer you can get to 180 degrees without disturbing the jet flow, the better. The greater the curvature, the greater the efficiency of the turbine wheel. The blades in the picture are not curved nearly enough. Yes, they will work after a fashion, but the turbine will be "steam hungry." If you look at the Tesla design, there is way too much "slippage" built into it for any kind of efficiency.

About 4 years ago now Bill Huxhold of the Toronto Society of Model Engineers built a number of turbogenerators in all 3 scales 1/16th, 1/12th and 1/8th full size. All of them worked but the 1/8th full size ones worked very well. I purchased several of each size and I intend to use one of them on "Big Boy". In his design, Bill used a 2 pole rotor with a 3 coil stator. This setup avoids "cogging." Remember, the stator laminations must be insulated from one another. If they are not, they short out producing severe eddy currents. In that situation, you will get no output current but the stator laminations will overheat rapidly.

If you look back a few years, you will see a long thread on this subject.

Hope this helps a bit.

Richard Trounce.

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apm
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Re: Steam turbine generator

Postby apm » Fri Apr 21, 2017 6:55 am

The thing I see with most of those turbines that others have posted is that I doubt any of them are capturing the expansive energy of the steam. Real turbines have a complex airfoil and combinations of stationary blades that allow them to maximize the expansive effects of the steam. In not attempting to use the expansion of the steam I suspect most if not all of the power comes from the transfer of momentum which is only a mere fraction of what is available. Interestingly enough though I looked at some of the older drawings in the locomotive cyclopedia of the original pyle nationals and I am not sure if they ever

I think of this as the equivalent of say spraying a fast moving jet of steam on a piston located outside of the cylinder and blasting it forward. It will move but you won't get much power out of it. On the other hand take the same piston and stick in in a cylinder where you are getting the full benefit of the expansion and you get a whole lot more power. This is why I would suggest trying to configure a rotary type vane motor
Image
as eccentric nature of the rotary vanes allows you to capture expansion inside of the motor.

If a commercially available air motor won't work as I don't know if the vanes are good for steam here's a youtube video of one that should do it quite well;
https://www.youtube.com/watch?v=u0fLhtWeKjQ
I guess the only problem though on that one is the noise isn't prototypical.

One other thought I went to 3D content central and there are quite a few guys who have posted 3D models of little scale working jet engines. So I downloaded one and sent the wheel over to shapeways and the cost wasn't too high for stainless steel. I am still not sure however if it would work as a turbogenerator really isn't set up for axial admission of steam like a jet engine.

trivettj
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Re: Steam turbine generator

Postby trivettj » Fri Apr 21, 2017 8:00 am

Harbor freight has a 1/4 inch die grinder that comes apart really easy and has a separate air motor inside the tool I took one apart and played around with using it to spin a electric motor for a generator and it worked so you might want to look at that idea the die grinder is the 11.99 one so it's not much to loose that is why I tried it.

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juiceclone
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Re: Steam turbine generator

Postby juiceclone » Fri Apr 21, 2017 11:44 am

get hold of a turbocharger (junkyard?) and redesign. The exhaust half is designed to extract power from expanding gases so theoretically, should be a good starting point and size seems about right. :>)

Wolfgang
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Re: Steam turbine generator

Postby Wolfgang » Sun Apr 23, 2017 11:37 pm

I've designed and built 3/4" scale steam turbo-generators successfully. One is mounted on my CPR Hudson #2888. Pictures thereof in operation are floating around the internet somewhere...

The whole thing is 7/8" dia. and perhaps 1 1/2" to 2" long. The output is 3 watts at 2 volts or so. Controlling the electrical output so as not to burn out the bulbs I found problematic without using an electronic voltage regulator, and this I did not want to do.

I tried steam pressure regulators with various brass and bronze diaphragms, but these would crack after some time. I finally used a 3 Watt LED as a headlight (using the entire power supply at 120PSI steam pressure) and this proved very successful, so much so that during the night runs at the MLS track outside Montreal the by-standers complained that the head light was way too bright! True dat!

The turbine wheel is 5/8" diameter and 3/16" thick. The 12 turbine buckets are milled into the periphery of the wheel in the shape of a tangential U, 5/32" wide. The steam is admitted through one .032" dia. nozzle which is at a distance of .003" from the wheel perimeter. Most importantly the steam jet impinges the bucket along one of the edges of the U, with the jet being deflected 180 deg. by the curvature at the bottom of the U.

This all works very well and the steam consumption is barely noticeable when running. However, if anyone is anticipating any efficiency here they are in the wrong hobby! Suffice it to say that it does its job very well and leave it at that.

The permanent magnet rotor is two-pole and contains one strong button magnet 1/4" dia. and 1/8" or so thick.

The field laminations are a stack 3/8" thick and 13/16" OD. These hold 4 coils of wire with 13 turns each, thus not too difficult to wind in situ.

The rotor turns on two stainless steel ball bearings 1/8" ID and 1/4" OD, purchased from an outfit in California. The supplier even furnished them with a light high temperature grease.

The supplier estimated that the life expectancy was about 100 running hours at 60 000 rpm. I have not achieved this because the dynamic balancing of the rotor assembly leaves much to be desired, putting un due stresses on the bearings. This balancing could definitely stand much improvement.

I do not have a complete set of dimensioned drawings, but have a few CAD sketches with the important dimensions. The rotor shaft sketch is completely dimensioned IIRC.

This is a fascinating item that is not too difficult to build, and adds a very nice finishing touch to any 3/4 scale locomotive without having to worry about batteries and stuff. Just open the steam valve and voila, instant light. BTW, the water glass and pressure gauge are also illuminated by a pair of small current white LED.

John Hasler
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Re: Steam turbine generator

Postby John Hasler » Mon Apr 24, 2017 9:46 am

For authenticity you should use a wound-rotor generator and an electromechanical regulator. If you are going to accept the anachronism of a PM generator anyway you might as well just use a small DC motor.

Wolfgang
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Re: Steam turbine generator

Postby Wolfgang » Mon Apr 24, 2017 5:47 pm

John Hasler wrote:For authenticity you should use a wound-rotor generator and an electromechanical regulator. If you are going to accept the anachronism of a PM generator anyway you might as well just use a small DC motor.


It's been tried, not by myself though.

Small motors often cog at the beginning, consequently the low torque of the turbine is insufficient to start the rotation of the generator. Also, how long would the brushes last at 50 to 60 000 rpm?

Anyway, the issue is neither here nor there. I chose to design and build an assembly that suited my psyche; I trust others will do the same.

I stated that this small turbine puts out 3 Watts, this is with a single 1/32" dia. nozzle jet. With a full admission nozzle ring I would opine that the power could very well be 10 to 20 Watts. But the scale size alternator would be insufficient to absorb the available shaft power.

jscarmozza
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Re: Steam turbine generator

Postby jscarmozza » Mon Apr 24, 2017 6:10 pm

Wolfgang, I'm impressed, a 3/4" scale turbo generator, I was wondering if it was possible to make a 1" unit! What tool did you cut the wheel buckets with? I've experimented with a number of set ups and tools without success. Is there a chance I could get a copy of the drawings? John

John Hasler
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Re: Steam turbine generator

Postby John Hasler » Mon Apr 24, 2017 6:26 pm

Just an offhand comment: no criticism intneded. I took your remark about not wanting to use an electronic voltage regulator as expressing a wish to avoid anachronism.

Good miniature motors (e.g., Escap) don't cog. I don't have any on hand that I'd run at 60,000 rpm, though.

Wolfgang
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Re: Steam turbine generator

Postby Wolfgang » Mon Apr 24, 2017 9:25 pm

jscarmozza wrote:Wolfgang, I'm impressed, a 3/4" scale turbo generator, I was wondering if it was possible to make a 1" unit! What tool did you cut the wheel buckets with? I've experimented with a number of set ups and tools without success. Is there a chance I could get a copy of the drawings? John


John, I moved into my "new" digs 1 1/2 years ago and am still looking for stuff. I just got my workshop up and running; I needed 220 AC and the VFD connected for the gear hobbing machine. Thankfully a friend helped with this.

I see no problem with increasing the size to reflect 1" scale; up sizing a reasonable amount is usually not a problem although a little more attention needs to be given to the smaller details. I made no effort to make an exact copy of the Pyle National generator; I wanted a reliable piece of machinery that appeared reasonably representative.

The turbine buckets (cannot really talk of blades here) are machined peripherally/tangentially onto the outside diameter of the wheel. The buckets are U-shaped as made with a 5/32" dia. end mill -a slot drill for preference. The milling pass is tangential to the wheel and starts in thin air, progressing into the wheel perimeter until the U end of the cut falls onto the radial line vertically up from the table.

Recall that Richard above said that the steam from the nozzle needs to be turned through 180 deg. as closely as possible, and this is the best way to achieve efficiency for this type of (impulse) turbine. To fool around with the nozzle this small to make a proper expansion nozzle is a waste of time and will not add to the efficiency on this small scale because the surface friction of the steam flow is the governing factor here. The outlet of the nozzle is simply a small 60 deg. included angle cone, followed by a 1/32" dia. drill. The thing is then back drilled to some dia. (I forgot which), and then pressed into the turbine inlet pipe stub. Then the outlet end of the nozzle is machined in situ by chucking the turbine housing and boring the projecting nozzle until it is in line with the turbine wheel cover plate registering bore 5/8" dia. plus a few thou to clear the turbine wheel.

I'll dig out the drawings and post them here if I can. This may take a little while but I will do my best. w

jscarmozza
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Re: Steam turbine generator

Postby jscarmozza » Tue Apr 25, 2017 7:58 am

Thanks Wolfgang, in rereading your previous post I recognized one problem in my approach, I was trying to cut too many buckets into my wheel. I also came up with some crazy setups and 'special' tools to make the cuts, I spent more time doing that than thinking about the wheel! I'm going to do a trial run with your method and see how it turns out. Thanks again.

I'm also still thinking about apm's comments regarding steam expansion and the rotary engine. The balance and sealing issues associated with a rotary engine are things I don't want to tackle, way above my pay grade. For that matter so is the steam expansion issue; but I still can't help wondering if the laws of physics and thermodynamics begin to reorganize themselves in small scale? Are things that are insignificant in the design of the full scale prototype dominant in the small scale model, other words, is a small scale turbine more of a pinwheel than a turbine? John

RET
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Re: Steam turbine generator

Postby RET » Tue Apr 25, 2017 9:13 am

Hi,

Wolfgang, thanks for posting. Your design is a practical one that obviously works because you have been using it for years. An electric motor will work as a generator, but the winding resistance is too high for any kind of efficiency. A generator needs a larger diameter wire for a lower resistance with fewer turns. The PM rotor with a wound stator is the practical way to make something that works.

As far as the nozzle is concerned, you need the entrance curvature of a steam cone injector combined with the recovery cone shape. This will produce the high exit velocity needed. Remember, mass times velocity equals momentum and velocity also has a direction component. By changing the direction 90 degrees, you have transferred the velocity component to the blade. This is equivalent to having the jet go "SPLAT" against a flat surface. If you can turn it 180 degrees you get as much work again.

Wolfgang's description is the way Bill Huxhold built his turbogenerator units and they not only work well, but they look right. Bill is the best model maker in TSME (Toronto Society of Model Engineers). He routinely builds things that the rest of us wouldn't even think of attempting and when he describes how he does it, it sounds Sooo simple.

Richard Trounce


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