Steam turbine generator

[Carrdo]

Hi all,

Just to inject a couple of sobering thoughts on all of these miniature turbogenerator designs.

There are still two main problems which have only been partially resolved with any and all of the designs produced to date including the Mosley.

If any these turbo generators are operated with either saturated or superheated steam and as they are located on a boiler, they are going to heat up and cool down through many cycles. On any turbogenerator, you have the "hot" (turbine end) and the "cold" (generator end). Sooner or later, no matter how well you try to isolate the hot turbine end from the cold generator end, condensation from the hot turbine end is going to make its way into the generator casing which will result in the rusting of the generator lamination steel and short out the generator. You can take all kinds of measures to slow the process down but you can't stop it.

The second real problem is with the generator rotor magnets themselves all of which have a Curie temperature; that is, the elevated temperature at which ferromagnetism disappears. And at somewhat lower temperatures, magnetism decreases significantly. Traditionally, Alnico was the material of choice as it has a very high Curie temperature and is quite corrosion resistant but when producing very small turbogenerators as we use in our models, you want to have the strongest magnetic field possible which led to the use of rare earth permanent magnets. Probably, one of the best rare earth permanent magnets available today is Samarium Cobalt. However, Samarium Cobalt has a significantly lower Curie temperature than Alnico but it has the highest Curie temperature of all of the rare earth magnets. Oxidation of Samarium Cobalt is also a concern so we are back to the old condensation problem again.

[Carrdo]

Hey Wolfgang,

Tell them about your 3/4" scale Duplex steam operated feedwater pump also!

I was one of the first to buy his set of plans for this piece of model locomotive jewellery...?

Sixteen pages of the most detailed and exquisite CAD drawings ever produced but a "tour de force" to machine.

Just an incredible thing to see working.

I haven't had the courage to take this on yet.

[Wolfgang]

Aw shucks Carrdo. yer making me blush. It's easy don't cher know. Ferst take a big lump of brazzz and machine everything off that doesn't function or look like a duplex pump. Thanks for the compliment.

May I suggest that we start a new thread if we talk about those intriguing devices. w

[Wolfgang]

Hi all,

Just to inject a couple of sobering thoughts on all of these miniature turbogenerator designs.

There are still two main problems which have only been partially resolved with any and all of the designs produced to date including the Mosley.

If any these turbo generators are operated with either saturated or superheated steam and as they are located on a boiler, they are going to heat up and cool down through many cycles. On any turbogenerator, you have the "hot" (turbine end) and the "cold" (generator end). Sooner or later, no matter how well you try to isolate the hot turbine end from the cold generator end, condensation from the hot turbine end is going to make its way into the generator casing which will result in the rusting of the generator lamination steel and short out the generator. You can take all kinds of measures to slow the process down but you can't stop it.



The second real problem is with the generator rotor magnets themselves all of which have a Curie temperature; that is, the elevated temperature at which ferromagnetism disappears. And at somewhat lower temperatures, magnetism decreases significantly. Traditionally, Alnico was the material of choice as it has a very high Curie temperature and is quite corrosion resistant but when producing very small turbogenerators as we use in our models, you want to have the strongest magnetic field possible which led to the use of rare earth permanent magnets. Probably, one of the best rare earth permanent magnets available today is Samarium Cobalt. However, Samarium Cobalt has a significantly lower Curie temperature than Alnico but it has the highest Curie temperature of all of the rare earth magnets. Oxidation of Samarium Cobalt is also a concern so we are back to the old condensation problem again.

Glad to hear that I wasn't the only one with this corrosion problem. In fact the rust particles would wind up in the bearings too, and as we know iron oxide is a very fine lapping compound that plays hell on the bearings running at up to 60 000 RPM... The bearings are shielded, not sealed as this would provide too much friction, thus very fine material will work its way into the race.

This problem was solved by cleaning the the rusting parts meticulously and applying a very thin coat of epoxy. So far so good. I suppose one could dunk the parts into some lacquer when new, saving some time I think.

That leaves the problem of dynamic balancing. Anyone have any ideas? I do mount the T-G assembly on a resilient mount which helps a little.

As to mounting on the loco, I did not mount it onto the boiler shell because my boiler is not lagged.

I made a small mounting bracket that bolted to the front of the cab wall, and fastened the T-G on top of this angle bracket, keeping it away from the hot boiler shell. w

[Asteamhead]

Hey Carrdo, Wolfgang,
Look there are some solutions to the problems you mentioned.
First: you must not use superheated steam to feed the turbine! Even with the prototype engines they turned back to saturated steam after some trials. In case using saturated steam the turbine won't heat up to more than 100 °C due to the exaust steam's preasure is null above the ambient's. Thus the generator is embedded (cooled!) to this temperature, which is well below of what modern brushless motors will allow.
The windings may be heated up too much by using superheated steam, too.
And the turbo generator should be mounted on a separate bracket made of stainless steel to improve thermal insulation from the boiler anyway.
Second: balancing is the major problem in my opinion. I'm using simple teeth wheels made of aluminium to minimize that problem. The complete engine plus turbine wheel is supported in the case by two o-rings to allow a bit of movement when starting. At normal speed ( 30 000 rpm +) there should not any movement to be felt at all. Very low noise can be achieved, too if nos. of teeth x rpsecond is above the frequency you are still able to hear (depending on your age ...).

In my experience 500 hours of useful lifetime can be achieved.

In case you are modeling a Pyle Natinal style generator, please try if you can achieve a slight vaccum due to the exaust is much more directly after just 90 °. I found this by happen!

Asteamhead

[RET]

Hi,

Very interesting discussion. Thanks Carrdo & Wolfgang. Just to add a bit more, Here is a picture of Bill Huxhold's turbogenerators in all 3 sizes. From left to right, 3 1/2"gauge, 4 3/4" gauge and 7 1/4" gauge (since that's what is in our neck of the woods).

As an aside, oh how I wish Bill Fitt or one of his buddies had struck the 1/4 key on his typewriter instead of the 1/2 key. Apparently that's how all this gauge mismatch business started.

1/16 scale on the left, 1/12 scale in the middle and 1/8 scale on the right.

Anyway, according to Don, Bill used ceramic bearings with ceramic cages in his generators. Bill & I talked about the generators and exchanged ideas when he was building them and that's how I know what I do plus my engineering training and experience. All of Bill's units run very well on air but the big one is VERY impressive. They run at least 40,000 rpm. I believe Ron Melvin took video of all 3 running under load at one of the TSME exhibits at the Model Show one year. That video should be on this website. I have tried out the small one and it will easily run an LED flashlight. I plan to use the 1/12 size on Big Boy. Because Big Boy is a large model, that size should look right.

To give you an idea of size, the one on the left is 1 5/8" long while the one on the right is 2 5/8" long from the turbine housing on the left to the generator housing on the right. I bought several of each size because I wanted to own something that Bill had made.

Richard Trounce

[Wolfgang]

In case you are modeling a Pyle Natinal style generator, please try if you can achieve a slight vaccum due to the exaust is much more directly after just 90 °. I found this by happen!

Asteamhead

Hello Asteamhead,

Would you mind stating your last paragraph above in German, as it is a little difficult to understand. Thanks, Wolfgang

[BClemens]

Here's half of one:
http://www.ebay.com/itm/Mini-Alternator ... 1762732953

There is even a 'water wheel' generator offered.... (not advocating the use of the site - just info for the DIYers)

BClem

[John Hasler]

Here's half of one:
http://www.ebay.com/itm/Mini-Alternator ... 1762732953

There is even a 'water wheel' generator offered.... (not advocating the use of the site - just info for the DIYers)

BClem

What sort of bearings does it have, though?

[BClemens]

Here's half of one:
http://www.ebay.com/itm/Mini-Alternator ... 1762732953

There is even a 'water wheel' generator offered.... (not advocating the use of the site - just info for the DIYers)

BClem

What sort of bearings does it have, though?

Looks like these use plain bearings... not too good for sustained high speed. The attention to this particular alternator is that the magnets and their support housing is inside an outer shell. Most of the super small alternators have an outer magnet ring cup housing that rotates on the outside..That could be hard to modify for a turbine but this one could be coupled to the turbine shaft - and also modified to run in ball bearings.

Just a thought.....anything to slow down progress on the loco..

[jscarmozza]

I was thinking along the lines of a pump impeller operating in reverse with the steam jet entering tangentially and the exhaust leaving at the eye of the wheel. I made a jig for my rotary table but it loosened up half way through my first try at the impeller and went out of adjustment, the product was pretty bad but the method seems like it will work with some tweets to the jig. Any thoughts, opinions or advice on this style of turbine before I get into it? John

[Builder01]

Are you following something you have seen, or, are you just winging it?

David