Dynamometer Results & Comparisons - How to build you own dyno..

View Full Version : How to build you own dyno..

03-18-2004, 10:43 AM


03-18-2004, 10:49 AM
It's funny you posted that from the eng-tips site, I came across that yesterday. There's a really good post about cam design on that site too.

Has anyone on here built a "home made" dyno?

03-18-2004, 01:00 PM
Original thread...


Hmmmm Make a eddy current dyno capable of holding 1100+lb ft of Tq

A very easy way to build an eddy current brake is to just use any ac induction motor, and feed dc current through the windings. It only works for low power engines though, because the solid steel (squirrel cage) rotor does not dissipate heat well.

But if you are experimenting at home with very small engines, it is a very practical ultra low cost way to do it.

A three phase motor will have three terminals, just feed a dc current into any two terminals, and ignore the third. The rating plate will have a current rating for the motor, which should not be exceeded.

Only a low dc voltage is required because the resistance of the windings will be quite low. Also the braking torque will be extremely high, even at currents far below the maximum on the nameplate.

With a given braking current, the required torque to turn the motor increases very rapidly with speed. A given motor will brake full rated torque at its slip rate, which might be only 150 RPM.

Just to explain this, if the mains frequency is 50Hz, the magnetic field inside the motor rotates at 50 x 60 = 3000 RPM. The motor speed might be rated at 2850 RPM, so the slip will be 150 RPM.

If you try to spin it faster than 150 RPM with full field current, it will readily sink far more torque, so a small electric motor can easily hold almost internal combustion engine. But the big problem is heat buildup in the rotor. It will get very hot very fast, so that is what limits the power absorption, not torque sinking ability.

So, a single or three phase induction motor would make a nice home chassis dyno for a cart or small bike, provided you were careful not to cook it. A large external blower or submerging the whole thing in oil might also work.

It is all a bit Mickey Mouse, but some of us are not that wealthy. Short power runs are all you really need with long cool down periods for home use. Also, excessive RPM and heat might cause it to fly apart, so a bit of caution is required.

Try it yourself, get a quarter horse or larger single phase motor, and while turning the shaft by hand, apply 12v dc and see what happens.

The resistance in each coil is 0.945 ohms.
There are 8 coils either side of the stator = 16 in total.

Normally they are operated in 4 stages, 2 coils either side for stage 1 & 4 coils either side for stage 2 & so on.

For the dyno i will wire all the coils together & apply power to all coils at once. Would a rotary motor controller,a high voltage potentiometer or a Rheostat do.
Do they even make potentiometers that big or are these for AC power ?????

The retarder can be wired to suit 12 or 24 volt DC, i have wiring diagrams-spec sheet.

To keep it on 12 or 24 volt DC, i was going to use truck batteries unless there is a better way.

Max current draw for 12 volt is 196 amps & for 24 volt is 98 amps.

Braking torque is 1450 Nm or 1066ft/lbs.

Would AC power make the retarder vibrate or chatter because of cycles per min?.

As i take it DC is smooth & AC comes in cycles, correct me if i'm wrong.
I mentioned before we have a 250 amp AC/DC converter that i use for tig welding can this be used instead of batteries??

I know a little about eddy currents about 8 yrs ago i built a retarder ,wound the coils & used cast iron brake disc rotors that i machined to suit. It was an experimental project,it works good for small engines but it has to much resistance per coil, wire to small for better engines. I controlled this--playing-- through a 10 amp AC rotary controller that i use for honing into a stick welder wound up full & then through the AC/ DC converter. Basically all i proved is, that it's easy to MAGNETIZE an entire workshop full of tools in one fowl swoop not to mention still having to comb my hair down every 10 mins.

The Telma retarders I have seen are used on heavy trucks, and are rated to sink 1,000 Ft/lb torque, and some up to 4,000 Ft/lb torque, according to their website. They do this at truck type tailshaft speeds, so in a truck, I don't know but maybe 1,500 RPM or something might be typical in a mid gear going down a hill?

Now with an eddy current brake, the faster you try to turn it, the more torque it takes. What I am trying to say is you might not need to sink 4,000 Ft/lb of torque continuously at 1,500 RPM in your application.

You might not actually require anywhere near the 196 Amps at twelve volts. It might be more like 19.6 amps at 400 Ft lb at 1,500 RPM, and even less current at higher RPM to hold the same torque.

The way I would do it, would be to find myself an old style heavy duty industrial battery charger. It could be 12v, 24v, or 48v. something with plenty of grunt. It will come in an earthed metal box with an ammeter, and would be ideal. These things are used for recharging electric forklifts, and telecom applications an so on. You might get lucky at a machinery junkyard or in the Trading Post.

The sixteen coils can be connected in a variety of ways to suit any suitable operating voltage. I have no idea what voltage your TIG runs at, or how happy it would be running at a continuously high dc current, but it might well be o/k.

Once you have a suitable transformer and rectifier, you will need to control the output. The easiest way is with a VARIAC. These are a continuously adjustable output transformer where you can adjust the output voltage from zero up to full mains voltage by turning a large knob through about 270 degrees of rotation.

You just connect the variac up to 240 volts, and run your battery charger from the output. In this way the dc output of the battery charger can be adjusted from zero up to full rated output at the turn of a knob.

Variacs are rated by current and come in many sizes, so you can get for example, a 2.5 amp variac, or a 20 amp variac and so on. How big it needs to be, really be depends on how much power your battery charger draws. The rating plate on the back will tell you. So get the battery charger first.

I am in Melbourne by the way, and my e-mail is:


03-18-2004, 01:30 PM
J-Rod: You find the neatest things ;)

Gomer: I found tortion bar design info there the other day, but drew a blank on cam design search. Will you share the URL?

03-18-2004, 01:46 PM
Gomer: I found tortion bar design info there the other day, but drew a blank on cam design search. Will you share the URL?

here you go..


03-18-2004, 09:40 PM
here you go..
Thank you very much, sir! Great stuff!!