John R.-
I absolutely love your Elektra Performance spreadsheet---it's something that everyone wanting to know how energy gets from the battery to the motor and to the prop (and water) should review and understand. I've probably said this before, but you might make the calculations more accurate by using the "No Load" Speed Constant of 45RPM/volt along with the Armature Resistance (16.75mohm) to do your calculations. The "Loaded" speed constant of 40.5RPM/volt doesn't give good estimates at lower speeds or currents. Consequently, the first line in your table shows a calculated RPM of 130 that should really be 139RPM --- a 7% difference. Similar story for the "Actual HP" column.
Use this:
ShaftRPM = (Vm – Im * Rm) * 45RPM/volt
Of course this model assumes constant "Rm" and doesn't include other resistances if your "Vm" measurement isn't actually at the motor terminals. At low currents, these effects are nil and so the above equation is more accurate. OTOH, at higher currents the inaccuracy in the above equation due to Rm not being constant or simply being wrong might increase…so there's a tradeoff.
Anyway, again, I hope everyone who wants to model their system will look at your work here and learn---great job and thanks for sharing J.
-Myles Twete, Portland, Or.
Posted by: "Myles Twete" <matwete@comcast.net>
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