Revised math calcs/motor determination

Collapse
X
 
  • Time
  • Show
Clear All
new posts
  • Chenige
    Senior Member
    • Jul 2011
    • 172

    #1

    Revised math calcs/motor determination

    Thanks for all the help. As I stated before my new charger is stopping charge before the battery is fully charged. So instead of 50 seconds run time I actually got 10 minutes. Charger put ~3600 mAh back in.

    20" mono electrified, 15T speed gems 2 (4006 kv), X432 prop, 7.2V 3800mAh NiMH battery, 35 max amp Astroflight ESC

    Perfectly flat water so ran wet, not as fast as expected. However I ran later with a little chop and it scooted across the tops of the chop beautifully. Could run at full speed with no controllability issues.

    Max rpm= 4006 x 7.2 = 28,800 rpm, 20,200 RPM loaded (per OSE calcs)

    Amps=(3600 mAh/1000)/(600 sec/3600)=21.6 Amps
    Watts = (amps) x V = 21.6 x 7.2 = 155 watts
    Top speed per OSE calcs= 33mph
    I confirmed numbers on OSE speed calc link.

    At the end of run:
    ESC barely warm
    Motor very hot
    Battery was slightly warm.

    So now I want to pick a brushless motor/LiPO combo to put into 4 more boats. Must be inexpensive and hopefully a bit more boat speed.

    Turnigy 36-56-04 2600kv Inrunner on 3S 5000 mAh, $20 http://www.hobbyking.com/hobbyking/s...idproduct=6527
    Max RPM=2600x11.1=28,860 RPM (same as current boat), 23,100 RPM loaded
    Rated Watts=475 therefore max Amps=43 on 11.1V
    Run time =7 minutes
    Top speed per OSE = 38mph

    Is this a good replacement combo? Numbers seem to work but I have no idea if it is the best combo. Are the rated wattages accurate for these motors? There are plenty of other Turnigy choices.
  • siberianhusky
    Fast Electric Addict!
    • Dec 2009
    • 2187

    #2
    Just saw a 2300kv version of that motor go up in smoke last week for no visible reason, was in a SV27, temps were ok, boat had been run that way for a number of months. 3:15 into a run right as he was turning to bring it in it just quit. ESC survived but all the insulation melted off the motor leads.
    It was a well tested good running setup, up till then. You'd want at least a 90 amp esc, that motor puts out way more than the listed 475 watts. The one that smoked was running a aquastar 90 esc.
    Honestly I'd look at nitrorcx for a tacon motor, be a bit more money but cheaper in the long run. Might actually end up being cheaper as it's being shipped from California instead of China.
    If my boats upside down then who owns the one I thought I was driving the last two laps?

    Comment

    • Chenige
      Senior Member
      • Jul 2011
      • 172

      #3
      Those Tacons in 3650 size just seem a bit too much for this boat? 900 Watts! However the amps on this one is 41 so perhaps it would work? Guess I am confused on how they get amps and Watts. Should not a 900 watt motor use 81 amps on 11.1V? Or the opposite 41 Amp x 11.1V =455 watts?

      Would really like to get a motor that allows a 50 amp ESC. Gotta believe there is something out there since the boat runs great now with a 35 amp ESC.

      Price is OK at $28. 2230kv may be a bit low but loaded rpm = 19,800 which is just less than the loaded RPM I have now (20,200).

      Comment

      Working...