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Thread: Motor peak torque RPM

  1. #1
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    Default Motor peak torque RPM

    Those who have data logged their runs, I was hoping to confirm some theory.

    I have been almost exclusively using 40mm motors (mostly TP Power 4040,4060, and 4070 models)

    I came across a graph indicating the torque curve of a brushless motor but without any numbers on it.

    Motor RPM and current.JPG

    From some data logs using Castle ESCs most of my peak speeds end up in the 34k to 38k rpm range. My assumption is that peak torque falls off somewhere around that RPM.

    Does anyone here have some knowledge on this that could confirm?

    -Liberty

  2. #2
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    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
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  3. #3
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    Cool

    The maximum torque of an electric motor occurs at zero rpm. As rpm increases, back EMF counteracts the motor’s torque until at the maximum unloaded rpm the torque and load equal each other, stopping additional rpm gains. Electric motors are not internal combustion motors. Power and torque are not the same thing. See example below.

    0167D11E-95E8-41A9-A275-A75A8F548849.jpeg
    ERROR 403 - This is not the page you are looking for


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    First of all, the 2 plots come from 2 different tests. The first plot is under load control, and the second one is under speed control. Speed control testing is the contemporary favorite, as its easier to understand. Without numbers on the graph you can't tell if it is an overly simplified, brushed DC motor plot with non-linear scale.

    As Fluid pointed out, max torque happens at low RPM. A contemporary Inverter (like in a car) can limit its current, and that is why the torque curve is initially flat up to 9K rpm in Fluids image.
    In our ESC's, there is no current limiting hardware. Hence, you put a big prop on a setup that cant handle it, the current (and heat) will rise quickly beyond the ESC ratings. Poof, you smoked the ESC.

    As the motor speeds up, so does the back EMF voltage. Once speed goes past a certain point depending on a variety of factors, the back EMF will start to limit the current, in a constant power way. That is why the red curve is flat in Fluids plot. This is where most of us operate our boats.

    Peak torque is also peak current. Its no where you want to operate for very long, because of a risk of smoking hardware.
    Cheetah, Super Rio, (Mod) Starship (Mod and sold),

  5. #5
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    Theoretically a motors max torque is at zero RPM and falls off from there, but the sensorless brushless ESCs that we typically use don't know where the rotor is positioned in the motor until it starts spinning, so will only give relatively low powered pulses to the windings at zero RPM as it knows it will likely be out of position.

    Realistically the maximum torque will be whenever the ESC first switches out of startup mode and into running mode, different ESCs, and even different firmwares on the same ESCs can have that switchover point at different RPMs, but I haven't seen one that was much over 1000 RPM.
    Paul "tug Killer" Upton-Taylor, Cat lover.
    FastElectricBoats.co.uk

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    Thanks for this info - quite informative. My question then would be just how we would apply this info/data in practice to make sound decisions when choosing motor KV, prop choice relative to loading characteristics and so on. (my basic application would be for an oval heat race boat - let's say a P-Mono.)
    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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    Default

    Good stuff!
    Vac-U-Tug Jr (13mph)

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    Quote Originally Posted by properchopper View Post
    Thanks for this info - quite informative. My question then would be just how we would apply this info/data in practice to make sound decisions when choosing motor KV, prop choice relative to loading characteristics and so on. (my basic application would be for an oval heat race boat - let's say a P-Mono.)
    Im not sure you could without some instrumentation and testing.
    Loading characteristics of a hull is the big unknown.
    You would need to know the amount of drag on each hull at various speeds, and how that drag was affected by weight.
    That would determine the amount of force necessary to get a particular hull going a certain speed.
    Another side of testing would be a dyno for our motors, where you could test a motor / esc at a given constant speed, to get a handle on the relationship between current and torque.
    Props would introduce a another round of testing.
    That way, you could figure out the most efficient way to generate the required amount of force.

    This is just mt scientist / engineering brain trying to take a problem apart into smaller, more manageable chunks.
    I'm SURE its not the only way.
    Aside, This is why the science and engineering of racing is really expensive.
    Cheetah, Super Rio, (Mod) Starship (Mod and sold),

  9. #9
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    I suppose I needed to re-phrase my question. (thankful for the input thus far, always great knowledge shared on this forum!)

    If you were going for peak speed and have changed props up and down several times. You have finally reached a peak speed for that motor kv, esc, battery, and hull design...
    Have you found that in the data logs your peak rpm was around 34-38k rpms?

    From what I have gathered that seems to be where the motor cannot push the RC any further. Obviously hull design, drag and other factors all play a part, but just looking to see if my data matches anyone else.

    If I know that my setup's always peak at X RPM... This can help me get closer to the best setup initially with less trial and error. (although I always expect some)

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    Quote Originally Posted by larryrose11 View Post
    Im not sure you could without some instrumentation and testing.
    Loading characteristics of a hull is the big unknown.
    You would need to know the amount of drag on each hull at various speeds, and how that drag was affected by weight.
    That would determine the amount of force necessary to get a particular hull going a certain speed.
    Another side of testing would be a dyno for our motors, where you could test a motor / esc at a given constant speed, to get a handle on the relationship between current and torque.
    Props would introduce a another round of testing.
    That way, you could figure out the most efficient way to generate the required amount of force.

    This is just mt scientist / engineering brain trying to take a problem apart into smaller, more manageable chunks.
    I'm SURE its not the only way.
    Aside, This is why the science and engineering of racing is really expensive.
    Up until now, and this goes back a-ways with lots of trial & error and considerable fun and successes/failures, I've been using pretty much what can best be described as "seat-of-the-pants intuition." Without the ability (or truthfully, motivation) to get too wildly technical I guess I'll continue to do the 'ole test/tune day at the lake. It's what I enjoy (and can afford). I also have had the good fortune to have been able to look over the shoulder of some rather highly accomplished racers as well
    Last edited by properchopper; 09-06-2019 at 02:39 PM.
    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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    Interestingly when looking at a data logged graph. At low rpm the amps is also low, little throttle in put. When you give full throttle the amp peaks and the rpm goes to it max.

    If max torque happens at max current. Then it only seems to me that the max torque is almost at the max loaded rpm in our rc boats.

    If you use a bigger prop the max rpm will drop and the max amps will increase.

    But the same will happen max torque will be at almost max rpm for a give prop.

    I state almost max rpm because a spike load happens when giving full throttle then the amps falls a little lower and rpm increases a little.

    In relation to peak power at 34 to 38k rpm. That depends on cell count, kV and prop used. I am assuming that 34 to 38k rpm is with 6s to 8s and 1800kv or higher.

    And amp draw is over 200 amps.

  12. #12
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    Quote Originally Posted by Speed3 View Post
    Interestingly when looking at a data logged graph. At low rpm the amps is also low, little throttle in put. When you give full throttle the amp peaks and the rpm goes to it max.

    If max torque happens at max current. Then it only seems to me that the max torque is almost at the max loaded rpm in our rc boats.

    If you use a bigger prop the max rpm will drop and the max amps will increase.

    But the same will happen max torque will be at almost max rpm for a give prop.

    I state almost max rpm because a spike load happens when giving full throttle then the amps falls a little lower and rpm increases a little.

    In relation to peak power at 34 to 38k rpm. That depends on cell count, kV and prop used. I am assuming that 34 to 38k rpm is with 6s to 8s and 1800kv or higher.

    And amp draw is over 200 amps.
    I agree, and yes my example was 6s and 8s use.
    Peak RPMs could be in the 28k-42k range depending on the prop used as well as the drag created by the hull design.

    I personally have seen most of my data logs peak around 38k, however it was a very heavy 8s setup.

    Thanks all for your comments!

    -Liberty

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    Nice to give information from all of you.
    Thanks

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    Quote Originally Posted by LibertyMKiii View Post
    Has anyone done this sort of testing on RC motors?
    It would basically be a brake dyno, where the test just started at a fixed max throttle, and then increased the load to slow the motor down. the batteries used could just be a deep cycle lead acid. 12 or 24 v to approximate 3 or 6s. You could use a super capable ESC as a standard. You would need to log torque, speed, Volts, Amps.
    I would love to see something like this.
    it would be a great way to compare motors on performance results, not just construction quality.
    Cheetah, Super Rio, (Mod) Starship (Mod and sold),

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    That sort of testing is not good in my opinion. I think the best testing method is performance on water. Get a data logging ESC. Choose a common prop, choose common kV motors between different brand. Select a similar cell count.

    Then compare which motor turns the prop the fastest.

    I would say select a location which have like 300 to 500 feet of straightaway and in calm conditions. Hold full throttle for 5 to 6 seconds.

    The idea is to see how high the motor rpm is a when the motor wind out like 5 seconds at full throttle.

    During that full throttle burst you will get to see how long it takes to build up to peak rpm under load by moving the morse at the start of full throttle down to the end of full throttle.

    You will see which motor builds rpm quicker, which give the highest loaded rpm and you can look at ampdraw and so on.

    Also you can check the heat between each motor and see which is more efficient.

    I would like to say, Leopard motors are very good motors much better than tp motors. They run cooler and is a lot faster in my experience.

  16. #16
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    The entire point of doing dyno testing is to remove variables like prop difference, battery difference, and hull difference.
    Further, without being able to measure torque, you would have no idea of output power, and efficiency.
    There is a reason that these sorts of plots are industry standard.
    You could use these sorts of plots to determine the sweet spot for the different motors, and choose a prop to get to a target RPM.
    It would be really handy for racers, where you run at max throttle for most of the race, and efficiency matters.

    Leopard VS TP? I prefer TP, but even that is just a guess.
    There are some rules of thumb about motors, such a Neu and LMT, BUT without measured data, its all just conjecture and opinion.
    Cheetah, Super Rio, (Mod) Starship (Mod and sold),

  17. #17
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    So if you had a dyno pull on let's say a tp4050/4y 1550kv. 14.8v as supplied power.

    The readout was something like 500amps stall. Then at 3000 rpm 500 nmm of torque, at 14000rpm the motor is at 250nmm of torque, at 20000rpm 150nmm of torque. Max power is 2.5 hp @12000rpm and 300nmm of torque.

    With this readout you would select a prop to make the motor rev 12000rpm for max speed for a straightaway record run.

    Then for oval racing you would select a prop for 18500rpm.

    Is this what you mean?

  18. #18
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    Pretty close.
    Prop efficiency is a huge factor, as we all know. Prop performance goes way beyond diameter and pitch, such as rake, blade area, not to mention the many mods people do.

    So, if you had some data logging on your specific setup, you could take the motor RPM and reference the dyno chart, assuming the input voltage in the dyno test was close to what you had in your boat.
    From that, you could see what your target RPM is to get the most power out of the boat, heat generation from efficiency, what the current would be, and so on. you could also see how close your setup is to the margin of your rated ESC current.
    Switch the prop to get a different RPM, and reference the same chart.

    Further, it would be the only real way to compare two different motors of the same size.
    Which motor makes more power? at what RPM is that max power at?
    Or, for a given power output power and steady input voltage, which motor draws more current?
    Cheetah, Super Rio, (Mod) Starship (Mod and sold),

  19. #19
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    here is something interesting...,

    https://www.ecalc.ch/torquecalc.php?neumotors
    Cheetah, Super Rio, (Mod) Starship (Mod and sold),

  20. #20
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    Quote Originally Posted by larryrose11 View Post
    here is something interesting...,

    https://www.ecalc.ch/torquecalc.php?neumotors

    Sorry for the late reply. I had looked at this but didn't find it to provide answers in the way I had hoped.
    I am not certain what torque Nm value should be input.

    -Liberty

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