hi Bongz,
You still working on this project or is it done? I did this same thing for my BScEE thesis: an "active" dyno for an SAE supermileage car using an alternator.
Here's what you need to do:
STEP1:
Pick an alternator which is rated greater than your motor, or downsize your motor, or physically chain a couple alternators together. A 90A alternator putting out 13.2v = 1188watts.
1188Watts/ 746W-per-HP = 1.6HP
Because the conversion from mechanical to electrical isn't 100% efficient, to produce this 1188Watts would load an engine by, say, 2HP.
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Step2:
Find someone who knows electronics.
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Step3:
Alternators have a coil on the spinning armature, which is fed by a control circuit on powered by the 3-phase stator coils on the housing. By varying the current thru the armature, the unit's field controller keeps a constant ~13.2V coming out of the stator, no matter what the engine speed, to correctly charge a battery.
Rip out the field control circuit, to be replaced by your own, described below
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Step4:
Create a purely resistive load. You can buy 12v resistor load banks from a car battery shop (they use em to test batteries cold-cranking-amps). You may also use coat hangers, coz they're low-grade metal. Make sure you have enough wattage for your motor! A resistive load is linear, and will suck proportionally more power for the current you feed it.
If your alternator still has a diode bridge to rectify the generated AC into DC, you can hook your load on the outputs of this bridge. Sometimes the field controller and bridge are combined into one circuit. Phone an alternator shop if you need just a bridge.
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Step5:
To build an "interesting" dyno, what you want to do is precisely control the speed of your engine on a rotation-by-rotation basis. You'll need some kind of angular sensor to know your MUT's RPM.
Your control software or circuit needs to set the alternator's DC field current to 0A if you're below the desired speed. This causes no current output from the alternator, and hence no load...causing the engine to speed up.
If you're above your desired speed, have your control software or circuit set the *DC* current on the armature to the maximum rated (not over coz it will overheat!) This induces maximum load from the alternator, and causes the engine to slow...provided the alternator is rated greater than your engine!
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That's it. I found the reaction time of the alternator so fast you couldn't possibly get the engine to go any other speed, even slamming the throttle from end-to-end...unless you completely starved it of gas.
For our super-mileage car, by fixing the engine speed on the dyno, you could then tweak the amount of gas and spark angle to find the greatest output voltage from the alternator for the least amount of gas. Because the resistive load is linear, greater output voltage means greater engine output.
It then becomes a question of stepping through all possible engine RPMs and tabulating the precise fuel and spark angles.
NOTE: A rubber "spider" between the engine and the alternator might extend the life of your dyno.
hope it helps!
'dyno