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Posted by Sherman in Idaho [72.47.9.37] on Friday, October 05, 2012 at 22:40:27 :
In Reply to: Re: Thanks posted by gmharris [71.105.35.169] on Friday, October 05, 2012 at 14:38:39 :
I'm doing a direct drive because belt drives waste way too much power, when there's so little available anyway. This is just a battery-charger system running off my existing domestic water line (140 psi static head), so I work backwards from that to figure out the max hydraulic power and speed of water coming out of the jet. The inefficiency of automotive alternators is in the need for about 2 amps of field current for max output, so that costs me about 10% right there. The 5 HP figure is pretty much impossible even if the alternator was 100% efficient and all the power was going to the load. The power capacity of any motor or generator is pretty much a function of the weight of copper and iron in it, regardless of its speed, voltage, or whether it's running as a generator or a motor. Sure there are higher-efficiency units, but as a rule of thumb, power is still proportional to weight. A car alternator has about as much copper and iron as a 1/2 hp squirrel-cage AC motor, which draws about 500 watts, and that's about what the alternator puts out. I have some 200 amp 24 volt military alternators, and not surprisingly they're about the size and weight of a 5 hp AC motor.
V-belts with small-diameter pulleys are pretty bad at low power. In one application I had, I had to go from a 1/4 hp motor to a 1/2 hp motor just because the 1/4 hp motor couldn't even make the machine turn with no load on it. It had 2 v-belts in series with step pulleys.
Yes, I have a small mountain stream for power. All the water goes right back in so nobody downstream knows that I took some energy out of it.