Since I have no real way to measure the power being applied to the propeller shaft while under way, the simpler reading is how much current (volts and amps) coming from the batteries, which shows up as DC.
When I look at the output of my controller (via the Clearview display)at various throttle settings, the output AC voltage and amperage is often less than the input DC current. Additionally, this output current varies faster than I can see it as the controller does its thing. This would line up with the reduced power provided by the motor at those throttle settings. But I don't think that the controller can output more watts than it consumes. Therefore, I believe that when my motor and controller are pulling 5100W DC from my batteries, the motor can be producing at the most, 5100W of effort, actually somewhat less because of losses (which show up mostly as heat from the controller, wiring, and motor).
The conversion factor of 1.73 (the sqrt of 3) is used when calculating AC load to true power consumption. If the input was 125A of 48VAC (or 50A of 120VAC), then the true power consumtion of the system drive system would be the 8.3kW that you calculated using a power factor of 0.8. But since we don't have long extension cords powering our boats, this kind of info isn't really as relevent.
We could probably use this info to roll some arcane calculations back into the the drive system performance to determine the motor's different power factors at different throttle settings but I'm not sure how valuable the information would be.
To me, the net results of a given boat speed from a given battery current (watts to knots) is a more practical piece of information. That number factors in all losses, including power factor, prop slip, wiring losses, controller and motor generated heat, hull drag and everything else. If a boat's measured "watts to knots" performance is significantly lower than similar boat, then one might want to dig into each part of the system and find out where the extra power is going. But for most of our systems in auxilary sailboats, the boat's net performance will fall into the 1kW per ton for 90%+ of hull speed in calm conditions, give or take a little.
My 10,200 lb, 30' ketch has a LWL of 24'. 1.34 x sqrt of 24 = 6.56kts. 6.56kts x 0.9 = 5.9kts. 10,200 lbs = 5.1 tons. My boat goes 5.9-6kts using 5.1kW. Weird, huh. There are some boats using inductive AC motors that have reported even better "watts to knots" figures, I guess that those motors are even more efficient than the PMAC systems that many of us are using now. The down side is an increased cost for the inductive AC drive system. But this 1kW per ton rule of thumb has worked out for a number of conversions here. The trick is to not get too bogged down in some of the theoretical details.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, Chris Hudson <clh5_98@...> wrote:
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> Eric,
> It thought you calculate three phase AC power differently than DC. Using DC I agree with the 6KW. Using the formula for 3 phase AC P=V*I*1.732*PF. Assuming a PF of ,8 I get 8.3 KW. What do you think?
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