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Thread: Does the motor size and length make a difference?

  1. #1
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    Default Does the motor size and length make a difference?

    What i want to understand better is what does the difference in motor size(the can..36/40/56) do for performance? I get the kv thing with higher vs lower kv, but i am still not sure on the can size and how that effects power? speed? amp draw(something else i don't quite fully understand either). Oh and motor length too.
    Thanks!!!!

  2. #2
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    Can size and rotor length (magnet size) determine power in Watts. Pay attention to watt rating though not an industry standard they are fairly accurate. In general large means more power and as rotors get bigger in diameter centrifugal force determines rpm limits.
    I like TP's numbering where 4070 means a 40mm can and 70mm rotor length. This is a 4092 motor which seems to be standard sizing.
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    Mic Halbrehder
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  3. #3
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    For a given diameter spec, longer cans mean more wire and more thermal capacity, that comes with a slight weight penalty which can sometimes matter depending on the hull.
    Vac-U-Tug Jr (13mph)

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    Lenght increase the power and diameter increase the torque.

    Outrunner motors have a high torque

  5. #5
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    Kv is related to amp draw, so the lower kv the lower amp draw.
    Length and diameter drive the power.
    Last edited by Jesse J; 12-25-2020 at 12:47 PM. Reason: Correction to info
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  6. #6
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    Ummm, not really. 10 motors all from the same manufacturer and of the same size, but with different Kv, will all have the exact same torque.

  7. #7
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    Quote Originally Posted by donhuff View Post
    Ummm, not really. 10 motors all from the same manufacturer and of the same size, but with different Kv, will all have the exact same torque.
    Please post the data, I would like to be corrected with this information.

    I’m going by this equation:
    Torque (lb.in) = 63,025 x Power (HP) / Speed (RPM)
    If rpm goes down and power remains constant (can geometry), then torque goes up..
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  8. #8
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    Boy that was quick!

    It took me a while to get this through my head, and I'm not saying that I know it all on this subject. And I do not follow a lot of the calculations because I suck at math! But I'll try to explain what I understand.

    I'm not trying to say that your wrong in general, because this is how the majority of people "describe" engine/motor power. They see it as, Low rpm equals torque, and high rpm equals power.

    In internal combustion engines, this is more of a problem because the torque peak will usually occur somewhere in the middle of the engines rpm range. And that peak can be moved through the rpm range, depending on how the engine is tuned.

    In the electrics, peak torque is always "theoretically" at 0 rpm. I know we can't do that with a brushless motor because the speed controller won't do that. But with a brushed motor you could. Or with an induction motor like in a window fan for instance. Hold the blade and turn it on, and you will feel how much stall torque that motor makes. And when you let go of the blade and the fan starts turning, the torque will start declining. BUT the power will increase as rpm builds.

    But back to our motors. Say we have those 10 motors and they are all Leopard 4082s and they all have different kv. BUT they all have the exact same amount of copper fill in the stators, regardless of the kv. If you could put a torque wrench on the output shaft, and give it full power, they would all measure the exact same amount! Because they are all basically exactly the same except for the number of times that the wire goes around a stator tooth. And the length of the wire (number of times it goes around the tooth, ie-turns) has direct affect on the length of TIME that the stator tooth's magnetism will rise and collapse. Setting dead still like with our torque wrench test, time has no relevance. But when we add time to the mix, now we are talking about HORSEPOWER, (the ability to accomplish work.) And remember that our motors at max unloaded rpm make NO torque, and of course then, NO horsepower.

    So it's really just a word thing that most of us get wrong when we say "that motor is real torque monster". What we really mean is that the motor's power curve is more useful in the lower rpm range. And a high kv/rpm motor will have it's power peak occur at a much higher rpm.

    The guy in this video explains it pretty good if you understand all the electric motor terms, (I don't) https://www.youtube.com/watch?v=xi7jxIkX2vY

  9. #9
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    Cool video, and good discussion.
    Good food for thought.

    I recon the more accurate description should center around amp draw for any given motor, since this is more relative to the KV than torque.
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  10. #10
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    That guy has several good videos that explain most of our motor questions.

    Kv is just simply, RPM per volt, period!

    It's kind of like a transmission. Say we that we have several Leopard 4082 motors in different kvs, and all we have is a 10 volt battery. We're gonna try a few different motors and the first version of the motor is 500kv so it's like stuck in first gear at only 5000 rpm. In a car, this setup would be able to go straight up a mountain with no strain and no heat, but on flat ground, it's going to be very slow, and we aint never going to get there (where ever There is).

    So let's up that kv (shift gears) to 1000kv. Still got that 10v batt but now our top rpm is 10,000. We are in second gear now and beginning to get a little speed. But the motor is struggling some to get to the top of that mountain, and we're beginning to overheat a little. On the flat ground now, and making decent speed. this motor might work, "for my application" but I want to try more!!!!!!!!!

    2000 kv, yeah baby, we're on our way back now, and in third gear and flying across the flat at 20,000 rpm, but uh oh. We're getting a little hot just running on the flat. Now we arrive at the mountain, and home is on the other side. We better hit the bottom of that hill with some speed and maybe we'll make it. This is where "burst amps" comes into play. When you start up the grade, the rpm will drop and the amps will rise as will the temperature, and not in a linear fashion either. All of them will go up sharply and will continue to increase because the heat makes the magnets get a little weaker, and that makes the kv increase, that makes the load increase, that makes the heat increase----and repeat, repeat, repeat. FINALLY the Genie comes out of the motor with a big puff of smoke! And I just bet you that your wish will be that "I wish I didn't let the Genie out" And that's the one wish he aint gonna grant you!

    Sooooooo, what changed in the above motors? Torque? no, they all have the same torque. POWER? no, they all have the same amount of power. Remember they all have the same Watts rating, which is basically the electric way of saying horsepower. RPM is all that changed, and mostly the change was that the rpm to match the point that makes sufficient power, TO ACCOMPLISH THE WORK REQUIRED and that's really all that you should consider when selecting a kv. What rpm range do I want to operate in.

    If building a tug boat, go with low kv so you can turn a BIG prop at low rpm to get massive thrust without a lot of speed or heat.

    If going for straight away runs, then this is about the only place where those high kv motors will be of any benefit. Because this is the one place where the "burst amps" can pay dividends. High rpm on a smallish prop with huge amp draw and high speeds, for just a few seconds. Then get off the gas and let it cool down before making the return trip.

    And then there's second gear, or rather the middle of the road kv that allows you to push it a little when you need to and cool down when your on the straights/flats. And that's why OSE recommends you stay between 25-35 thousand rpm for a play or race motor.

    I keep hearing this "low kv low amps and high kv high amps", And TPs are amp hoggs, That's not really true or how it works, but that's for another story.

  11. #11
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    Bigger motors have the capacity to handle more power/ higher watts. Spin bigger props.

    If you have a 4074 2000kv and it has the ability to turn a x442 at 25000rpm on 4s.

    Choosing to use a 4092 2000kv will allow you to turn a x445 at 25000rpm on 4s.

    The above example with the 4074 and 4092 illustrates what running a bigger motor does for performance.

    Basically let you spin a bigger prop at the same rpm.

    At the same time the ampdraw increases.

    So you should be mindful that you need a higher rated esc and better batteries when choosing a larger motor.

    Patrick Smith
    Tortola, BVI.
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