alternator to eddycurrent brake conversion

[jlcortex]

Hello

I am building a dynamometer for brushless based on automotive alternator. I am using small motor but some friend are asking me to fit bigger motor.

I have removed diodes and i have solder 3 phases together. then i have removed regulator and i excite coil directly, it works perfect!

seeing it works, i thought about convert it in a true eddy current brake, I made this experience some time ago (i have no pictures!), i replaced the stator for masive iron lathe part. It surprised to me than it DON'T work!,

There are any secret, iron material have to be special? any idea about why it didn't work?

Thanks

[liveforphysics]

Replacing the shorted copper stator for a chunk of iron was a mistake.

The teeth focus flux into inducing a voltage into the copper wrapped around them, and the copper has about 10x lower conductivity than iron. Overall, I agree the iron chunk has much less conductivity, but it's not designed to make use of more than the leading edge, where the iron/copper tooth design it had enables it to get the rotors flux energy into using a larger part of the conductor, and focusing it into the low resistance materials.

At least that's my guess.


Beautiful dyno! If you're willing, it would be GREAT to get to send you some various motors to test for us.

[Biff]

That is a great idea to make a nice low-power and low cost dyno. If I ever need to build a 2-5kW dyno I think that would be the way to go.

Like Luke said, the machined iron should have done the trick to create some resistance, but it is not nearly as effective as just using the copper that was in there in the first place.

You should be able to get more power out of your dyno if you hook up some external resistors rather than just shorting the coils together. By moving the resistance out of the motor you can dissipate the heat better, but you will have to increase the field strength to get higher output voltage from the alternator.

-ryan

[salty9]

Electric water heater elements in water should work well for external resistors. Could also be used to make a calorimeter.

[Biff]

Electric water heater elements in water should work well for external resistors. Could also be used to make a calorimeter.

They are pretty good. Hot tub heaters are also good. However the easiest supply of resistors for a do-it-yourselfer are heating elements from electric stoves, which are really easy to pull out of appliances at the dump, from my experiance here, there are far more electric stoves than hot water tanks at the dump in my area. You should actually be able to put stove elements in water if you want aswell. However the calirometer just measureing the temperature of the water isn't the best thing to use if you want to test output power of a motor, since there would still be significant loss in the alternator which was turned into a generator which would not be accounted for. However if you put it all in a nicely insulated box you would have a complete closed system and could determine the total power the motor was delivering with pretty good accuracy.

-ryan

[bigmoose]

I believe an aluminum sleeve about 5 to 7 mm thick would have been effective due to it's higher conductivity relative to steel. The sleeve should have a tight gap to the rotor, perhaps 0.5 mm or so. One could water cool the OD of the sleeve to remove heat.

Very nice design and fabrication!

[salty9]

This takes me back to my PChem days. To complete Biff's setup you would only need an accurate thermometer, a method for measuring either mass or volume of water, and a good time base.

Chuck

[Biff]

This takes me back to my PChem days. To complete Biff's setup you would only need an accurate thermometer, a method for measuring either mass or volume of water, and a good time base.

Chuck

The time base is the killer for me, you might have to run the motor for several minutes, maybe even 30 to get an accurate measurement. With a reaction torque measuring system you can get output power very quickly, but they aren't that easy to setup either. No matter how you look at it, determining the output power accurately is pretty difficult.

-ryan