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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.
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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 Upton-Taylor, Greased Weasel Racing.

  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.

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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.

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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

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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.

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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. ??

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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.

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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% .

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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.

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

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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)

  20. #20
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    If you double voltage you cut the ampage in half if you was making a 1000 watts at 10v

    And you want to make a 1000 watts at 20v then you cut the ampage in half.

    That is not the case if you have an electric boat and using the same motor and same prop, then decide to double the voltage.

  21. #21
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    You beat me to it Speed.
    Totally correct.
    If you reduce the kv of the motor to turn the prop at the same RPM that it was doing on the lower voltage, THEN you will reduce the amp draw with the higher voltage.
    If you leave the kv of the motor & the prop size the same, you will more than double your amp draw when you double the voltage because the prop will be spinning at twice the revs increasing the load on the motor.
    Last edited by 785boats; 09-22-2018 at 09:34 PM.

  22. #22
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    Thanks for that explanation.

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    What were the voltages? Amp draw vs RPM means nothing without a voltage alongside it to indicate the power? It obvously wasnt consistant voltage as you have more RPM with more load on the Leopard, and RPM will drop with increased load if the voltage is constant.

    You are also comparing a RTR motor with an aftermarket motor, to me your data says that a Leopard is more efficient than an Aquacraft, which is no surprise. Compare the Leopard 4092 against a Leopard 3360 if you want to draw a conclusion about the efficiency of motor sizes.
    Paul Upton-Taylor, Greased Weasel Racing.

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    Quote Originally Posted by Alfa Spirit View Post
    A brushless motor NEU 1512 have the same efficiency than a NEU 1521, the value is high, more than 92% .
    This is correct with same brand and same range motors.

    If you compare a poor Leopard and a NEU Motor, efficiency is not the same .

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    The purpose of the data log was to show that size of motor does not necessarily increase amp draw.

    And yes the leopard is holding the rpm better with the same load.

    The voltage with the k42 is 7.4 leopard and 7.4 aquacraft.

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    It is the Leopard I was talking about, it is holding RPM better than itself!

    "13389 rpm @ 24.6 amps on 2s with the k42.
    13467 rpm @ 30.4 amps on 2s with the x642."

    7.4 is the nominal voltage of the battery, I am talking about the actual voltage at the time of the amp and RPM readings, which is needed to calculate input power and compare efficiencies. It's not possible for the motor to provide the results above at the same voltage, the X642 reading was taken at a higher voltage.
    Last edited by NativePaul; 09-24-2018 at 08:32 PM.
    Paul Upton-Taylor, Greased Weasel Racing.

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    Use silicone gun oïl for the ball bearings ... LoL

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    The 7.4v is the voltage for the k42.

    The 642 was 7.7v with the leopard but I think that voltage was a little high because how can the voltage drop to 7.4v at 24.6 Amps.

    And the same pack give 7.7v at 30.4 amps without even charging the battery.

  29. #29
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    My cells have very low internal resistance.

    I did a datalogged at 100amps with a 3s. And the voltage drop is .4volts.

    That is if you start at 12v it drops to 11.6v at 100 amps.

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    Quote Originally Posted by Speed3 View Post
    The 7.4v is the voltage for the k42.

    The 642 was 7.7v with the leopard but I think that voltage was a little high because how can the voltage drop to 7.4v at 24.6 Amps.

    And the same pack give 7.7v at 30.4 amps without even charging the battery.
    Either you measured the k42 later in the discharge cycle when the battery had less juice in it, or your figures are off. What about the voltages for the Aquacraft tests?
    Paul Upton-Taylor, Greased Weasel Racing.

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