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I think that alternator is way overkill for "topping off" those batteries. And, while it may be too big for that motor when run at full juice, the resistance of the alternator is proportional to the load on the alternator, plus losses, so it'll probably run. You might want to look at a smaller one, though.Also, if you're using SLA batteries, that will probably work well enough, as SLA aren't particularly picky about how you charge them. If your batteries are LiPo or similar, then keeping them topped off is actually a good way of killing them, and charging them with a rectified sine wave is another good way of killing them :-/
You ought to take a look at an article by Ken Gracey in Robot magazine. He used a gas motor and generator to power the electronics (including electric motors) on his robot. It seems like there are similarities between your project and Ken's.There's a pdf of the article in post #2 of this thread.http://forums.parallax.com/showthread.php/96081-Great-Article-on-the-Hybrid?p=663845&viewfull=1#post663845
With 5 Ah of battery, and at full stall current (20 Amps each) you can run for seven or eight minutes after the gas runs out. That seems like quite sufficient buffer.Also, your motors won't actually be drawing the full stall current most of the time. Unless you're planning to do something truly heinous, which would probably end up burning out the motors before you ended up wearing down your batteries.So, if stall current is 20 amps, times two, then a 50A alternator ought to be able to sustain the motors for as long as you have gas. If your run current is more like 5A (1/4 of stall; a good design target) then a 20-25A alternator ought to be quite enough. And easier for your engine to drive :-)
when the rotor is locked, currents can approach around 100 amps!
Quotewhen the rotor is locked, currents can approach around 100 amps!If that is the case, then yes, you need a beefy alternator! When you said "no load current 1.8 A, rated current 19A" I thought the "rated current" was the "stall current," but from what you say, it's actually the "holding load current." Thus, scale the math I did by a factor 5 :-)Convenient formulas:horsepower == footpounds * rpm / 5252horsepower == kilowatts / 0.75Your alternator is approximately 3 horse-power, and your motor is also approximately 3 horse-power, so there's some risk of stalling out at full load. It's unlikely your batteries will ever be depleted enough to draw the full load on the alternator. However, the voltage regulator may actually generate significant load even when the batteries are full, although that 16 Ohm resistor seems to limit it to 1A, which I'd think isn't big enough to worry about in this system.
I'm not great with car electronics. If I had an AC alternator, I'd run it through a full-wave rectifier, and if I needed to control the charge of a battery, I'd use either a linear voltage regulator as a current limiter, or a PWM regulator. That circuit you posted works differently, so I'm hoping someone with a better knowledge of that side of things can chime in!
I think the circuit above, and a 40A or better alternator, would do that just fine for you!
I'm thinking about ditching the alternator idea, since it will take about ~1 HP away from the engine