# Dynamometer

Discussion in 'Metal Boat Building' started by SheetWise, Sep 6, 2010.

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### SheetWiseAll Beach -- No Water.

I recall having read articles over the years (many years) on some innovative ways to build a dynamometer. Now that I'm looking I can't find anything -- even with the help of Google. I need to build and calibrate a small dyno to do some testing on a scale prototype (< 10 hp). Has anybody saved any plans? I think it was Popular Mechanics about 30 years ago, but I can't find what I'm looking for in their archives. I may be wrong -- but I know I've seen several creative solutions. At the very least I need readings that are relatively accurate, but I'd like it to be calibrated. Ideas?

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### MikeJohnsSenior Member

Sheetw
For small motor testing the simplest method is to drive a brake drum of some sort with a variable friction ring clamp on the drum and an arm on the clamp connected to a scale.

Then HP by US units torque, in ft-lbs, looking this up .... HP is RPM * ft-lbs/5252

Sophisticated electrical methods abound and all involve driving an alternator or generator into a big resistive load and measuring the amps and Volts.

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### baeckmoHydrodynamics

For that kind of power (and considerably more....), use a disc brake with its caliper from a car front wheel. It is better than a drum, because of easier balancing, and they come ready and cheap from any car scrapper. The caliper to be attached to a lever; its inner end resting on the shaft in ball bearings, the outer end pushing your scale. Make its length (from shaft cl to scale contact point) one unit of the preferred dimensional system, ie one meter if you use Nm or kpm, and one foot if you are going lbft, so you have less scale factors to keep in mind. Note that for precision, the lever must be counterbalanced, otherwise it will show a torque while at rest.

Those brakes can handle a lot of torque and heat, but for safety, build a sturdy protection cage around it.

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### LandlubberSenior Member

....I had an electronic one years ago, it was French. It measured the acceleration rate in seconds (parts thereof) to reach a set cut out rev limit, then calculated the hp from the time taken. You had to know capacity sizes etc, but that side is easy. No drums, no engine load. It was very accurate when compared to a standard drum rioller type, so I assumed it worked well.

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### SheetWiseAll Beach -- No Water.

Thanks for the replies. I like the disc brake design, will do more research down that path.

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### SheetWiseAll Beach -- No Water.

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### gonzoSenior Member

Get a test wheel for small outboards. You only will need a water tank to make it work

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### SheetWiseAll Beach -- No Water.

It's not an outboard -- I'm really just looking for relative efficiency in some various configurations right now, but will need more details later. What's a test wheel?

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### LandlubberSenior Member

...basically a propeller with no pitch...just a balanced ring of the same size and weight as a prop...no wash of course.

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### SheetWiseAll Beach -- No Water.

How do you rate the load?

11. Joined: Aug 2007
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### CDKretired engineer

What's against using a real dynamometer?
Alternators from small gen sets are cheap, the load can be a bucket of water, some carbon piles and a teaspoon of salt. A real marine solution.

The alternator output gives you rpm, torque and power.

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### gonzoSenior Member

The test wheels have several settings. They are rated for a HP at calibrated RPMs

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### baeckmoHydrodynamics

...and according to the affinity laws for turbomachinery, power is proportional to rpm^3.

Anyway, for serious development or fine tuning, you have to resort to methods where the engine can be kept at a constant operating point for a period long enough to let the temperatures throughout the engine to stabilize. This rules out the inertia methods.

The outboard test wheels are ok for comparative tests, like production acceptance, but not for much else. Measuring the electrical output from a generator requires a precise knowledge of the generator's efficiency map at all combinations of rpm and torque.....forget it, use a brake (mechanical, electric or fluid)!

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### SheetWiseAll Beach -- No Water.

Thanks CDK. I really do appreciate your posts, your knowledge, and your approach to problems ... I really do. More than you'll ever know. But ... if by "real" dynamometer you mean an off-the-shelf purchase, it's out of my range for this hobby application. For me to hobble together some components (as you are so capable of), it's outside my experience. I'm looking for some plans. If I have to build something bigger than I want, or with more features than I want, I'm ok with that -- it may be useful another day. But I've got to keep this under 3-4k US, and I haven't seen anything commercial that fits that budget. On the other hand -- I've got 30+ years experience programming and interfacing processors with machinery, so I don't have any problem with the software side (but I was not the guy doing the mechanical side

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### SheetWiseAll Beach -- No Water.

Thanks Baeckmo, Gonzo, and Landlubber --

Apparently I've got some more research to do on how I intend to interpret the relational data.

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