Eddy current dyno DIY. help with absorbers and sensors

[bob1111]

We are going to build another dyno, but an eddy current this time around. We are looking at dedicated brakes right now but have read a few threads about using an induction AC motor and running DC to it as a brake. I was curious if this would have any chance of working for 200HP?

Regarding the power to the brake, is the common method of controlling power to them with PWM controls?

I have heard that eddy brakes fall off fast when heated up but I have never found any data to compare temperature and braking power. Can anyone help?


Also, one the biggest factors is determining sensor inputs. WE would like to build one with IR thermal sensors if possible for fast setups. Also looking for a simply induction tach for the engine as well as tach sensors for the roller and test vehicle axle. Where would a guy find economical solutions for these?

ONe other, because of vehicles will have have EGT sensing, we thought we may get at least an idea of EGTs by IR sensing the header pipe. Is this advisable at all or will this certainly lead to inconsistent data?

 

[Lou Scannon]

It's likely that most of the engine professionals here rely (as paying customers) on dyno and instrumentation suppliers to provide the solutions you are seeking. You'll probably get more traction a sites dedicated to dynometer and instrumentation design, if you can find any.
Your IR idea for EGTs may be suitable, it just depends on what you need the data for and how accurate it needs to be.
Speed sensing is pretty straightforward, once you determine what you are going to use to generate a signal. It's pretty simple with spark ignited engines, since they all have a spark triggering frequency proportional to rpm. For a chassis dyno you don't necessarily need the axle or roller since you can get engine rpm so easily from the ignition; considering that the axle in a manual trans application is proportional to the engine (and not an accurate indicator of engine rpm in a auto trans application assuming not in lockup) and the roller is rendered inaccurate for rpm due to tire slip. If you want the axle or roller signals anyway you probably want a magnetic pickup and will need suitable target on the shaft, such as a toothed wheel. You will also need a circuit to process the frequency signal received from the pickup, which can be built around an LM2907 integrated circuit, for instance.

 

[IceStationZebra]

You can find IR thermal sensors that will simulate thermocouples. Omega is one source.

One BIG warning --- IR sensors will not read accurately on most "shiny" objects (like chrome, unpainted aluminum, polished steel, cadmium plating, etc.) unless the emissivity value can be adjusted, and then you would have to determine the new emissivity value anyway. The easy solution is to only point at things that are painted, as most non-metalic paints have an emissivity of 0.9-0.95, which also happens to be the default value for most IR sensors.

Also be aware that the farther away from a surface the sensor is, the larger the area it "sees" and is averaging. Do not expect to accurately measure an exhaust pipe from 4ft away.

And obviously they will not measure transitory temperature changes in the exhaust or coolant water very well compared to an immersion probe due to the mass of the item they are measuring.

ISZ

 

[Bribyk]

I think using a motor will be very expensive. Since you are looking to absorb 200 bhp as heat you will need a motor that is designed to cool itself that much. Assuming a 85-90% motor efficiency you'd need a 1300-2000 hp electric motor to have the required inherent cooling capacity with air. The eddy current dynos I've seen run water through them to cool. For the cost of the motor you could probably buy a water brake dyno as you'll need to have water cooling anyway.