Eric
actually some thing ins the graph seem consistent with the makers claims for the motor - the 82% is useful
Modelling that motor from 48-77 volts the efficiency range is very similar to the graph - the difference is the fall in efficiency at near peak rpm will be a little quicker
I think its safe to assume at 100% throttle the PWM controller runs at 98% efficiency and from the modelling the motor's efficiency will be very close to the 83% shown in the dyno chart at the 106A level - it really is designed to operate consistently over the 36-77 voltage range
Part throttle: Graphs published some years ago by Neutronics suggest the losses are 7-8% in motor controller efficincy for each 10% reduction in throttle.However they also showed different makers had different results. Brushed motor PWM controllers maintain performance better across throttle settings but the motors are limited by higher heat losses.
Your power /weight figures are very interesting/ I agree the torque important . An extreme example is hydroplanes ultralight, high speed planing craft - who needs torque its all horsepower right - the Hp is high but its the boats with good torque/weight and a merely competitive hp figure which run hardest. In a low speed , relatively heavy, displacement craft, pushing a high inertia fluid torque is the priority.
Usually hi torque is a synonym for low Kv, in other words its the same motor different windings, but smart designers get much better torque per amp yields from improvements to motor architecture.
Andrew
On 2/29/2012 10:59 AM, Eric wrote:
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:
>
> 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?
>
>
-- AJ Gilchrist Fastelectrics 0419 429 201
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