I am trying to get the shaft to run at 3600 rpm. I don't know if it will slow down much under a load. Would the AC be too unstable or would it be better to use a DC battery bank with an invertor?
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\$\begingroup\$ Welcome! Depends where you load is and the voltage of each generator. If far away and low voltage output you probably want to transform up the voltage immediately, which is easier with AC. Can you provide some numbers for voltage of each and distance? \$\endgroup\$– winnyMay 28 at 18:29
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\$\begingroup\$ Thanks, I'm working on an undershot water wheel with the grandson. the goal is 3600 rpm. and only run a hundred ft. distance with the wire. It would be for some lighting and maybe a water heater. \$\endgroup\$– TimKMay 29 at 0:05
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\$\begingroup\$ Lighting and water heater are two vastly different things these days. Where it used to be you might use 10 x 60W bulbs to light up a couple of rooms = 600W, which is a significant fraction (but still on the low end) of a small water heater, you now get that same amount of light with 10 x 9W = 90W of LED. Plus the LEDs could be run at low voltage (e.g., 12V or 24V system) where the water heater really needs to run at much higher voltage (120V or 240V AC typical). \$\endgroup\$– manassehkatz-Moving 2 CodidactMay 29 at 4:35
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\$\begingroup\$ It is also far safer to experiment with low voltage DC than with high voltage AC. For LED lighting, low voltage DC is very practical. For water heating, low voltage would be some serious (and dangerous) current levels and not practical - which means AC with the related advantages (transformers) and disadvantages (risks, especially around water). \$\endgroup\$– manassehkatz-Moving 2 CodidactMay 29 at 4:39
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\$\begingroup\$ The only reason to use a fixed AC frequency is if you have to use synchronous machines or link distribution grids. Once the frequency no longer matters you can pick what works the best. \$\endgroup\$– KalleMPMay 29 at 12:37
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2 Answers
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The #1 problem you will have is transmission. You have power at location X and you need it at location Y. And heating is a big load: you only get 3.41 BTU/hr per watt.
I hear you arm-waving it as "only 100 feet" but that's exactly the mistake everyone makes! Seen it over and over*. Let's fast-forward through the rest of that mistake chain: the person skips doing a proper voltage drop calculation because they kinda know the answer (in copper) won't be pretty, so instead they buy the copper they're willing to pay for. It's not nearly enoug, not even close; voltage drop calculates out at like 60% which makes the system not work at all.
They blew it because they were brain-locked on copper as the only viable conductor. Actually, the US AC power system and its supply chain is designed around aluminum as the principal heavy feeder. Dirt cheap commodity aluminum wire is the best answer - for low voltage DC you typically use duplex URD, or 4-wire MH feeder in 2 parallels. Same with SER indoors. With this very affordable wire on tap, now the Load Calculation isn't so frightening, and you can size your wires properly. Doing that Voltage Drop Calculation is a good teaching moment, too, because now you know for sure. A lot of people never figure it out.
Splicing from Cu to AL wire is straightforward using lug connectors. For #4 or smaller, just cut up accessory ground bars. #2 or larger, use bare lug connectors (no need for the costly insulated Polaris connectors; you can hand-insulate 24V). Note that all these lug connecters are made of aluminum, because when aluminum is the lug, thermal expansion differences Cu vs Al work favorably. Thus Al lugs are the universal donor. Torque screws to spec.
* Pure Living for Life did the same flub with their early solar panels, using #10Cu, back when 2-2-2-4 was a buck a foot. They "got back on the horse" and tried solar again; though in the end they relented and brought in utility power. If it was easy, everyone would be off-grid :)
answered May 29 at 1:11
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1\$\begingroup\$ I know of a small hydro (2kW) that has about 1/2 mile between hydro and load. Two transformers to get to about 800V and down to 230 works well - about 30 years so far… losses minimised as square of current… \$\endgroup\$ May 29 at 6:10
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1\$\begingroup\$ SI is handy here as 1 kW of electricity produces 1 kW of heat and you can avoid the 2.93 factor BTU/hr. \$\endgroup\$– D DuckMay 29 at 12:26
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\$\begingroup\$ @DDuck That only works if the society you are speaking to is accustomed to relating to heat using the "watts" unit and not another, i.e. if that is how heaters are normally labeled to consumers. SI is an interesting system, unfortunately there is not an SI unit for "being understood" lol. \$\endgroup\$ May 29 at 20:18
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\$\begingroup\$ Yep, it's confusing. A quick survey of heaters available at a major US distributor shows oil filled space heaters rated in W and kW (75% of models), BTU/hr (20% models) and in BTU (5%) \$\endgroup\$– D DuckMay 29 at 21:07
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\$\begingroup\$ @DDuck For plug-in portables, yes. Technology Connections did a thing on that. However, when you get into big heat and start dealing with fuels, it goes straight to BTU and watts is never mentioned again. The whole Anglo world loves their BTUs, even Australia is big on it. \$\endgroup\$ May 29 at 21:21
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To my mind, the issue is the gearing and losses.
What is the rpm of the waterwheel?
Then the gearing and losses to get 3600 rpm may be significant.
Perhaps a lower rotational speed for the generator and a different generator is a better option.
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\$\begingroup\$ Thanks. I found an undershot wheel calculator that might tell meif I'm nuts. I have the shop to build parts, but that might not be enough. I will look for another generator in case. \$\endgroup\$– TimKMay 29 at 0:23
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\$\begingroup\$ So it looks like the electrical is feasible. Now we can keep working on the mechanical. I just found flotation platform that should be fine with DNR regulations. Flop or fly, this should be a great learning experience. \$\endgroup\$– TimKJun 1 at 23:21