Hi John-
Thanks for the data---perhaps the first propulsion data I've seen of an actual LEMCO motor driving a boat J.
It's clear how your calculations work and how you arrived at the efficiencies---yes, I went over this in detail…
A couple observations & suggestions:
· RPM calculation assumed constant "loaded" RPM/volt of 40.5. Better to use 45 and subtract the loss in motor:
i.e. RPM = (Vm – Rm * Im) * 45
Impact: Speed estimates are low across the board and the error increases to about 6% at the low end.
· SHP calculation assumed constant loaded motor efficiency of 89%. Calculate instead: EFF = 100 * Pmotor/(Pbatt – Prelays)
Below 20amps, the efficiency drops off a ledge from 87% to 68% at your 1.6amps measurement.
All in all, using the constants is good enough, especially since you used a loaded value for the Kb and given that we don't spend much time at the unloaded or weakly loaded end anyway. J
A story: I recently helped a friend get lithium batteries set up in his boat. Next thing I know he's talking about going cruising in the Puget Sound. I gave him a spreadsheet that best modeled the battery capacity vs voltage based on specs, data and measurements. Later he texted me from the San Juan islands saying something like "well, I didn't know what to do with your spreadsheet and all those equations, but I'm getting about 4 miles per volt as the battery voltage drops---and that simple calculation works for me". He had a great trip and I'm envious at the overall efficiency he gets---cruises at something like 1kw.
This, with fantail hull, huge prop, old GE forklift motor, Curtis controller and lithium batteries...
Anyway, great job and thanks!
-Myles
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of fitloose
Sent: Saturday, July 23, 2016 2:22 AM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Re: AC vs DC motors
Hi Hannu/Miles,
My max shaft speed high torque/low rpm Lynch D135 brushed DC motor is 800 rpm. My cruise is 1,000 watts at around 4 knots on average and at my voltage/amps combination of my motor it is running at 89 to 90% efficient at that output . Peak motor is 92%. Overall system efficiency Inc prop is about 51% with direct drive, so a perecent here and there at that power output is nothing, and using efficient Lithiums helps too.
See this spreadsheet I did for confirmation of my performance.
http://john.rushworth.com/Elektra_Performance_June_2016.xlsx
John R.
Posted by: "Myles Twete" <matwete@comcast.net>
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