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Thread: Motor length vs amp draw

  1. #1
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    Default Motor length vs amp draw

    Does the length of the motor have any effect on amp draw? For example if I take two motors with the same KV running on the same voltage but one can is 60mm long and the other is 100mm will I see about the same amp draw from both? I generally like to run the longest can possible in order to dissipate heat as much as I can but often wonder if that causes my amp draw to go up.
    Team Liquid Dash

  2. #2

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    Quote Originally Posted by Raydee View Post
    Does the length of the motor have any effect on amp draw? For example if I take two motors with the same KV running on the same voltage but one can is 60mm long and the other is 100mm will I see about the same amp draw from both? I generally like to run the longest can possible in order to dissipate heat as much as I can but often wonder if that causes my amp draw to go up.
    The longer the can the higher the amp draw.


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  3. #3

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    Assuming that the magnets are of a similar material and the air gaps are similar, the Io (no load Current) of the larger motor will be proportionally higher than that of the smaller motor. The smaller rotor will also have less mass and will take less power to accelerate up to speed.

    As well as those efficiency losses to the bigger motor, the bigger motor will have more torque and thus will be closer to its unloaded KV with a given load, doing this extra work takes power and that comes at the price of amp draw.

    The boat will also be a little heavier, and therefore take a little more power to run at a given speed.

    Having said that, motors do have an efficiency curve (take a look at the Lehner website for the best overview I have seen of this, whether you are interested in Lehners or not the principals apply to all motors) and you can overwork a small motor so much that its efficency is terrible, and running a bigger motor at its peak efficiency can result in using less current for a given speed, as well as cooler equipment.
    Paul "tug Killer" Upton-Taylor, Cat lover.
    FastElectricBoats.co.uk

  4. #4

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    The amp draw would be around the same, I run different size motors. 4074, 4092, 3656, I have also run 2860, 3660 and 3674. In my experience the bigger motor is by far superior. The speed will increase by a lot versus the small motors. At least with a prop of 45mm and bigger.

    Imagine I run my impulse on 2s with a 2858 3200kv and a octura x642. And I also run on 2s with a leopard 4092 1730kv with a m545.

    And the leopard 4092 is more than 3mph faster. 2s vs 2s.

    The big motor is the way to go.

  5. #5

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    any electric motor with a larger size rotor assembly is going to draw more amps than a smaller motor of the same kv size. how on earth would a 4092 size be equal in amp draw to a 3656 motor ? what have i missed ??

  6. #6

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    Quote Originally Posted by rol243 View Post
    any electric motor with a larger size rotor assembly is going to draw more amps than a smaller motor of the same kv size. how on earth would a 4092 size be equal in amp draw to a 3656 motor ? what have i missed ??
    Same Kv, same voltage applied, same prop, same boat, the 4092 is a lot more efficient than the 3656.

  7. #7

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    Well if not the same very close. In my experience.

    How does 84 amps sound for a 4s setup with a 4092. That has 1820kv with an m545.

  8. #8

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    I did some data logging on the aquacraft 3656 1800 vs a leopard 4092 1730kv.

    The data log shows the leopard to have 1820kv and not 1730kv.

    It shows the aquacraft to have 1789kv and not 1800kv.

    I did some data logging with 2 different props on each motor on the same cell count.

    The log have them to be almost the same even though the leopard kV is slightly higher.

    So this confirms that length does not change ampdraw.

    The leopard has a 60mm rotor vs a 30mm rotor in the aquacraft.

    The leopard also has much lower internal resistance.

    But where the leopard separate it self is rpm underload. And my test was with a octura x642 and a graupner k42. With these small prop the leopard is reving significantly more.

    With bigger props the gap will be even further.

  9. #9

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    But what was the amp draw of each motor, with the same prop under load, with the boat travelling in a straight line? And were the temperatures different?

  10. #10

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    I hesitate to join in a bench racing discussion () but here's something I found years ago :[disclaimer : take away anything you want from this]

    I was planning to enter the current P-Mono that I raced (usually a fair amount under 3 minutes with a J start) at the time [DF33 / CC240/Neu 15151Y/X447/4S2P 10Kmah] in a 4 minute Offshore event at Winter Warmups - to choose an appropriate motor that could work mileage-wise with the longer length race I compared the mah used by my Neu 1515 1Y (can length 69mm) to the mah used by my Neu 1521 1.5D (can length 84mm) in a comparable timed run. I found that the longer can 1521 1.5D ate LESS battery mah's than the 1515 1Y under racewater conditions so I used that setup for P-Offshore. (Newland and Peterson both kicked my butt definitively btw).

    This is by no means a statistically valid experiment with enough trials to conclusively "prove" anything but it did get published in The Scientific Journal of Small Sample Experiments(University Press) right next to a small trial sampling experiment which conclusively discovered that All Cars are RED
    2008 NAMBA P-Mono & P-Offshore Nat'l 2-Lap Record Holder; '15 P-Cat, P-Ltd Cat 2-Lap
    2009/2010 NAMBA P-Sport Hydro Nat'l 2-Lap Record Holder, '13 SCSTA P-Ltd Cat High Points
    '11 NAMBA [P-Ltd] : Mono, Offshore, OPC, Sport Hydro; '06 LSO, '12,'13,'14 P Ltd Cat /Mono

  11. #11

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    For the test I deliberately used 2s to minimize risk of damaging equipment.

    Here is the result.

    Aquacraft 3656 1800

    12103 rpm @ 23.4 amps on 2s with a graupner k42.

    11817 rpm @ 30.9 amps on 2s with a x642.

    Motor kV 1789 according to data logged no load rpm.

    Leopard 4092 1730 rated kV. The datalog measured the leopard at 1820kv.

    13389 rpm @ 24.6 amps on 2s with the k42.

    13467 rpm @ 30.4 amps on 2s with the x642.

    The leopard having a slightly higher kV should pull a bit more amps.

    But it clear to me kV, cell count, prop used, friction in the drive. Are some of the factors that significantly change amp draw. Can size does not appear to affect amp draw accord to the results from my log.

  12. #12

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    shpuldn,t this test be done with the use of more suited battery power such as a 4s and or 6s ? testing with this low voltage 2s lipo is not really working these motors to maximum limits. 3 x T.P. motors. 4050 = 151 amps, 4060 = 248 amps, 4070 = 300 amps, smaller rotor lengths to longer rotor lengths. ??

  13. #13

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    The amp draw increases in proportion to the increase in cell count.

    4s will double the amps and 6s will triple the amps that I saw.

    The aim was to show the kV for kV should draw around the same amps regardless to size of motor.

  14. #14

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    Length of the motor change the maximum power , Pmax is higher on a 4092 than a 4060 , efficiency is the same.

  15. #15

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    The pmax is referring to how much power input a motor is rated to handle as power input.

    The bigger motors can handle more power provided the quality is the same.

    Efficiency in my opinion refers to amount of output power out of the power input.

    And if that's the case a bigger more is more efficient.

    As proven by my test.

    Despite having ruffly same input the bigger motor turned the same prop faster.

  16. #16

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    A brushless motor NEU 1512 have the same efficiency than a NEU 1521, the value is high, more than 92% .

  17. #17

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    Back in the old drag racing days, they used to say " there's no replacement for displacement" . Now days it's hard to say!
    SW26 V3 Green 35mph , Swifter Hydro 26. Delta Force 35. cars ,3 Rustlers 45-76 Mph.

  18. #18

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    Maybe I'm mistaken, but I always thought if you double the voltage, you cut the amperage in half.

  19. #19

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    Quote Originally Posted by bhorowitz View Post
    Maybe I'm mistaken, but I always thought if you double the voltage, you cut the amperage in half.
    That' s right,

    P (watts) = U (volts) x I (amperes)

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