Dynamo for testing steam and gas model engines

[JorgensenSteam]

I am considering using a DC motor as a dynamo for testing model steam engines.

 

[Dan Rowe]

Pat,
I think that a DC motor would be simpler than an alternator to make a dynamo. I picked up a surplus one a while back but never did anything with it.

The other clasic method to test engines is a Prony brake. We had one in the engineering lab at Kings Point and it we used on a full size single cylinder steram engine.

For photos of the setup Dave made see:
http://www.homemodelenginemachinist.com/index.php?topic=9960.0

Dan

 

[Bluechip]

Pat

I did some fooling around in the early '70's with a Miller 6V motorcycle dynamo coupled to a steam engine.

While you would be able to determine the electrical power output with some accuracy, you would have little idea of the efficiency of the generator.

I was able to get a fairly accurate figure for the efficiency of the dynamo at the local Tech. College.

About 80%, it would be considerably less on a smaller generator, I would think.

Dave BC

 

[Dan Rowe]

It is simple to calculate the efficiency of any electric motor. With a regulated power source build a Prony brake and use it on the electric motor. Then you have a fairly accurate test dynamo.

Edit, the is the advantage of a DC motor it can be used as a DC generator.

Dan

 

[Bluechip]

Quite so Dan ..

Or cut out the bit in the middle, and put the brake on the steam engine ??

Dave BC

 

[johnthomp]

im lost here are you testing for rpm or power output on the gas/steam engine tested
regards john

 

[Dan Rowe]

John,
The test is for power. See:
http://en.wikipedia.org/wiki/De_Prony_brake

Dan

 

[johnthomp]

ahhh got it the same thing you can get off an exercise bike for adjusting the dificulty of the pedals via a belt round a flywheel if you find a good digital unit at the scrappy like i did itll tell you the rpm aswell just a suggestion fellas
regards john

 

[Dan Rowe]

As far as the pony brake, I considered that, but I would like to test a variety of models, both large and small, and need to be able to dial in both very large and very small amounts of load.

I thought about using a DC motor, but when I priced one, they are extremely expensive in the size I would want.

I guess I could pull the generator off of my 1965 Beetle, but I am not sure that it and its regulator would still work, nor am I sure I could get the wiring correct.

The auto alternators are plentyful and cheap.
Just need a wiring diagram.
I will see if I can make a diagram an wire up some Radio Shack resistors in the correct series for 12 volts DC.

For small models, I guess I could use my RC airplane starter motor.
It is 12V DC, and I think they are reasonable in cost.
I need something larger for the larger engines though.

Pat, The problem with car alternators is the efficiency is poor. The DC motor I got off ebay was a treadmill take out. The simple way to approximately size a motor is to match the shaft size of the test engine and the motor. There is sometimes a lot of surplus motor buyers it took me about a year to find a suitable DC motor for the price I was willing to pay.

The tread mill motors are used for wind mills and other types of DIY projects here is a link to a wind power site that gives the pros and cons of motor types for wind power: http://www.otherpower.com/otherpower_wind_alternators.html

Dan

 

[mu38&Bg#]

If you can't characterize your generator well enough to determine shaft input power you can just measure torque directly. It could be as simple as a moment arm on the generator and a digital scale to read force. A little bit of math and an RPM reading and you have power output. The variable load for the generator could be as simple as resistors described above, or an electronic current load. I'm working on a dyno to test model airplane engines. I have pretty much everything sorted, but the data acquisition. I'll be using a large brushless motor for the generator and an electronic load. I hope to have it all controlled by an electronic governor to operate at a steady state RPM. I'm designing to 20,000 RPM, 2kW, and 2Nm as maximums, but torque could really be anything with a simple change in setup. I hope to begin cutting metal soon.

 

[FIXIT]

There you go as asked

Steve

 

[Jasonb]

Here's a bit about running a model traction engine on a Dyno, all you need is a dynamo, bucket of salt water and a multimeter.

http://www.bailey-steam.com/hogdyno.htm

Jason

 

[steamer]

For the smaller size engines, you could go eddy current with little more than some good Neo magnets , some bearing pedestals, and an aluminum disk....it would be simple enough.....it would take a little work to size it, but the magnets are relatively cheap....I'll work on a sketch

Dave

 

[Kermit]

http://www.google.com/search?q="how+to+build+dynamo"&hl=en&tbs=bks:1,bkv:f,bkt:b&sa=2

Some reading material for you to peruse if you haven't seen these before.

Kermit

 

[Lakc]

What got me into this hobby was a meeting with Bob Washburn at NAMES, and buying my first issue of Strictly IC, which proudly displayed on its cover a model engine dynomometer. As I was heavily into dynomometers and drag racing at the time, I considered it the best way to go about qualitatively measuring the changes I wanted to make in certain engines.

This is the listing for the issues from the SIC index pages.

KIRK MINIATURE DYNAMOMETER,
by Bud Kirk & Bob Washburn.
A Hydraulic (Mechanical Water Brake) Dynamometer.
Issues # 62, 63, 64, 65, 66, 67, 69 & 70.....$8.00ea.....TOTAL: $64.00

 

[jpeter]

Regarding power, horsepower, you need to know torque and RPM. Back in the day when I was teaching Physics classes I came up with a prony brake for calculatiing torque. I would loop a cord around the output shaft of the running engine or motor, with a spring scale attached to each end of the cord. I'd then pull the cord tight reading the force on each scale. I'd subtract one reading from the other to find the force required to drag the motor down to the desired RPM. A strobe light was used for finding RPM but however you find it you need it to calculate power. I'd then multipy the force by the radius of the motor shaft to find torque. The loop of string was acting like the prony brake. After knowing RPM the most understandable way to find work is to multipy the distance traveled per minute, circumference of shaft times rpm, times the force, the differential reading of the two scales. One HP is equal to around 33,000 foot-lbs of work per minute. It was a pretty simple and understandable way to fashion up a prony break and I felt my reading were quite accurate. All the tools I needed was a length of cord appropriate for the task, two scales and a way to read the rpm. I've often thought a known weight would replace one of the spring scales. Try it, you'll find it easy.

 

[mu38&Bg#]

Selection of generator motor requires that you take into account the RPM of the engine. Gearing would allow changing the speed ratios and certainly increase the operating torque range of the dyno. I'm using a Neu Motors 2215/3Y. This motor is rated up to 40,000RPM and 5kW continuous. If I have a need for low power I have salvaged hard drive platters to make a hysteresis brake, for maybe 100-200W at 10-20kRPM.

There are many options for a torque brake: propellers, generators, water brake, friction brake, eddy (hysteresis) brake etc. Each has it's benefits and drawbacks.

 

[mu38&Bg#]

Carefully look at those torque figures. Some are quoted as stall torques. The motor can't handle that kind of torque for more than a few seconds. Maybe something like a treadmill motor would be more practical.

HP is dependent on RPM. The torque output of the engine will also vary with RPM. The voltage output of the generator varies with input RPM. There are many things to consider.

 

[IronHorse]

Take a look at my thread on a engine dyno.

http://www.homemodelenginemachinist.com/index.php?topic=4543.0

This is a very simple dyno that I made last year. I did a few tests with it and although it will give you some data, it was hard to get a consistent reading on the tests.

 

[Dan Rowe]

Pat,
I have tested a few DC motors with a variable speed lathe and larger ones with a larger lathe. If you have something to indicate RPM also the tests will generate more usefull data.

The guy in the wind mill link stated that if the rotor of a DC motor is hard to turn when the leads are shorted together than that motor will make a good choice for a wind generator.

The tread mill motor I have is difficult to turn when the leads are shorted and it had good voltage results with the lathe test. It will be nice to have a small 12 VDC system that is portable or simi-portable.

Dan