Dynamometers

Tim Wescott fired this volley in news:k7adnTdGp6TObS snipped-for-privacy@giganews.com:

Why? Will they only measure at high speeds? Torque is torque, yes?

Lloyd

Torque is torque, yes, but if you need to weigh an envelope to see how many stamps to put on it you don't head out to the nearest truck scale.

And yes, to my knowledge many of the cost-effective ways to make engine and chassis dynamometers are speed-sensitive.

I suspect that the instrument they want to calibrate wouldn't even be able to overcome the friction on an engine dynamometer, much less be in a range where the measurement will be accurate.

Tim Wescott fired this volley in news:k7adnTZGp6QxYS snipped-for-privacy@giganews.com:

And likely calibrated over the entire range of rpms...

Even a top fuel dragster _starts_ at zero rpm, and the torque at the low end (as transferred to the axle) is as important as high-end torque.

Lloyd

AC induction and shaded-pole motors make nice adjustable brakes if you run some DC into the windings. You could measure the torque on the motor case pretty easily, an arm connected to a scale of some sort.

For smaller cases like this, a DC permanent-magnet brush motor can make a VERY good dyno. A resistor bank and an ammeter are the other things you need. A shunt-wound motor is even better, as you can vary the load with a small rheostat or an adjustable DC supply.

Jon

Build one, they are not complicated.

But the torque at anything under about 3000 RPM is not measured because the engine can't run that slow..

What the OP needs is a "prony brake" which is relatively easy to build and calibrate, or an "eddy current dyno cell" made out of an electric motor or generator and a load resistor. Both require either apressure transducer or a scale of some sort to measure the force on the reaction arm.

I have seen a small dynamometer (like for use with a Briggs and Stratton lawnmower engine or similar) in the mechanical engineering department of a university. The 80 RPM might be a problem, though.

I would think that a more clueful electric motor shop might have one - again, the 80 RPM is tricky.

You can get devices that are designed to calibrate torque wrenches, but these aren't designed for full rotation. I don't know if you could hack one to do what you want. Sears has them in the catalog but not at the store; somebody like Snap-on might also have one.

A power tool company like DeWalt might have one, but maybe only at People's Shining Electric Screwdriver Factories #1-#7.

Somebody like Bosch, Delphia, Visteon, or Eaton that makes small electric accessory motors for cars probably has the right equipment, but I don't know if they rent time on it.

Matt Roberds

That's pretty much it. I need to poke at it a bit, to find out just how much traceability they want -- they wanted a lab that would do it, but I think that maybe they just need me to whomp something up.

so, power upto about 1 horsepower but at slow rotational rates? water wheel? stationary bicycle?

that sort of precision is should be fairly easy with a de Prony Brake and a half-way decent force-meter (eg: electronic scale)

Years ago there was a guy posting here that made a Don Foreman inspired eddy current dyno for testing electric motors. I think his name was Gerry. I had some pictures of it.at one time. An eddy current dyno (also called an absorber) can be air or water cooled, has no motoring capability, and doesn't use a "load resistor". Take apart an elliptical exercise machine and you may find a version that has a band of magnets around a steel rotor.

With the proper drive ratio you could also use a gear pump (like a Brown and Sharpe 3S) as a loading device. I'd use an in line torque cell but that's because it's available to me.

Interesting request, good luck!

Probably cast iron...you could try experimenting with a brake rotor, those are designed to dissipate heat....

How about a regenerative DC motor controller and a DC motor? DC motor mechanically coupled to your source. Power is put back on the mains.

Dc onto the mains. Now THAT is a trick (unless you forgot the inverter)

The _regenerative_ motor controller does the trick.

Like this:

And how does that work?

Jamie

"Maynard A. Philbrook Jr." fired this volley in news: snipped-for-privacy@news.eternal-september.org:

"Electro-Majic DC". It couples directly to AC lines with no problems. No need for syncronous inverters, no voltage control, no nothin'. Just hook it up and pedal your bike for an instant reduction - and even payback - on your power bill!

Lloyd

I think you're going to be disappointed with your understanding of it.

Jamie