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Eddy Brake

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jason_gze

Wed Jul 09 2008, 07:17AM

Registered Member #1176 Joined: Sat Dec 15 2007, 12:29AM
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Posts: 5

Hey all. New here. Ive always been into power electronics (build a SSTC couple years ago).

But at the moment im trying to build a eddy current brake for a home made engine dyno. It's currently in research stage. Instead of trying to wind my own magnets, i decided to use the MOT's I had laying around.

What I have done is cut open the cores removed the secondary and doubled up the primaries on one of the cores. Im currently running them in series to help with over heating issue and get a bit more voltage drop :P

For the disk itself im using an old car disk brake. Its not ideal and to say the least its tricky to keep the magnet off it. But ive managed to get quite significant loading on the disk at about 50-70v (dont have a volt meter on my varaic) @ about 20amps. This is only at hand turning speeds.

This brake is going to have to put up with about 200-250kW peak power.

Do you guys have any suggestions on how to up the braking power? I plan to run two electromagnets per disk and have two disks. Or have 4 magnets on one disk in pairs opposing each other.

I have no real world experience with eddy current brakes so i don't know how powerfully they are at low RPM (hand turning) and how much more resistance it will provide at higher speeds aproching 8000RPM? Can any one provide some incite.

Cheers J.

Steve Conner

Wed Jul 09 2008, 09:41AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The braking torque should be the same at any speed. So you can measure it at low speed with some kind of Rube Goldberg mess of string and weights, and since power is equal to torque times speed, you can calculate how much power it'll absorb at the RPM your engine will go at.

I'd expect you'd need a LOT more stuff to absorb 250kW.

Aluminium discs would probably work better than iron for eddy current braking. Overall, I think car brake discs are best used as mechanical brakes with the calipers and pads that came with them. You'll get far more braking torque than you could by using magnets on them. If they can stop your car, that more or less proves they can absorb the power of its engine.

An old induction motor might make a better brake. You can get a powerful eddy current braking effect by feeding DC through the stator windings, and maybe you can pick up a big 100hp motor at a scrap dealer. Then "all" you need to do is get rid of 250kW of heat from the rotor...

jason_gze

Wed Jul 09 2008, 10:00AM

Registered Member #1176 Joined: Sat Dec 15 2007, 12:29AM
Location:
Posts: 5

Hmmm the thing is commercial units normally run about 200v @ 20 amps (for a 400HP ~ 300kW) dyno load cell.

Sorry about that but i kept on getting an error when using the link.

Steve Conner

Wed Jul 09 2008, 11:51AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Well like I said, if you used a rotor made out of aluminium you'd get much more braking torque for a given amount of power to your magnets, into the ballpark of the units you linked there. Iron discs conduct electricity poorly, so they don't develop much in the way of eddy currents.

The Dynojet unit you linked has 16 poles and two rotor discs, from looking at the picture, and you can bet the rotors will be cast aluminium or even copper. They may be bigger than car brake discs, as the picture gives no idea of scale. You also don't know whether it's continuous rated. I doubt it.

The lab I used to work in had a Heenan & Froude 150hp eddy current brake with water cooling. It was about 15" diameter by 10" long.

Weston

Wed Jul 09 2008, 03:40PM

Registered Member #1316 Joined: Thu Feb 14 2008, 03:35AM
Location: Cambridge, MA
Posts: 365

I think that the motors you are talking about are 4kw or 4000 watts. (200*20= 4000)

Fraggle

Wed Jul 09 2008, 10:32PM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

I thought the braking torque was proportional to the angular velocity !?

Steve Conner

Thu Jul 10 2008, 09:48AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Sorry, I goofed, the braking torque is indeed proportional to the RPM. If it weren't, electricity meters, car speedometers, and that trick with the neo magnet and the heatsink all wouldn't work.

Fraggle

Thu Jul 10 2008, 09:25PM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

Thank God! You had me panicking and reaching for the books ;o)

jason_gze

Fri Jul 11 2008, 01:36AM

Registered Member #1176 Joined: Sat Dec 15 2007, 12:29AM
Location:
Posts: 5

Yeah thanks for that :) You had me flicking through the texts book going wait???

I maybe getting myself a nice big disk of aluminum. But ill have to wait until i get my test engine before i settle on anything

Cheers J.

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