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Thread: RC motors, 3 phase..? Tell me about them.

  1. #21
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    You'll need gearing to get that much power. Slowing down the RPM has the same effect as throttling back. The power drops.

    For example if the gears in a hand drill would take the beating you could connect a 750 or more watt outrunner to the gearing and get a reasonable RPM on the output. And in fact brushless motor hand tools are coming on line rapidly. But not 750 watt tools! At least not homeowner options.

    More likely you'd need to look at a more robust gearing setup. Or adapt the gearing from a old large Hilti or similar to withstand the power.

    Oddly enough as RB211 says the motor, ESC and battery is the least of your worries and would not be that expensive. You'll likely want to run your 750 watt motor on a higher cell count than a 3S pack That's because to run at 750 watts from a 3S pack which is nominally around 12 volts you'd need to give 'er at a hefty 62.5 amps. And even those motors would not take that for more than a few seconds. Instead go up in voltage to lower the current and the motors will be happy to run with a little cooling air over them almost continuously.

    Figuring an average voltage for the cells at 4 volts (makes figuring easier too ) a 5S pack would give you 20 volts nominal and 750/20= 37.5 amps... .still fairly high so let's go with 6S providing 24 volts. That drops the current down to 31.25 amps. A little better. And easily done by any reasonable pack.

    Let's further say you want full power for 6 minutes so it's a little better than what you asked for. That makes it 1/10 of an hour and again eases the math. So technically you'd need a 3200mah pack to supply your motor. And since you're only pulling the power out at a 10C rate the pack will be loafing along easily.

    Go for a 6S 5000mah pack and you would get more like 10 minutes of use. Run it off a couple of deep cycle lead acid batteries that are rated for higher current use such as golf cart batteries to get your 24 volts and you could get a lot more run time from it. But you'd then be limited by the recharge time from the charger you would have connected the rest of the time.

    Here is a 1000W Max motor that only costs $50 just as one example. And from the specs of 1000w max and the max current of 55A for 60s it's clear that you could run it at 750 watts (1HP) and push the 31'ish amps through it all day long provided you have some cooling air flowing through the motor.

    The bigger deal is coming up with the gearing to get the RPM down to where you can use it. If you NEED the 1HP then you can't run it at a lower throttle setting to lower the RPM. It's not a constant power source like that. And due to running at 24 volts to get the current down where it's under control you give up any sort of lower RPM from the motor itself. With the Kv value of 530 RPM/volt it's going to TRY to run at 24 x 530= 12,700rpm. The load will cut that by some amount and likely you'll end up in the lower 11,xxx range. But that's still a lot faster than you are after.

    Not bad for a 10.5oz motor that's only about 2" diameter by 2.5" long, eh?

  2. #22
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    Thank you very much for the excellent information on motor, controller and battery.This forum is wonderful. Edwin Dirnbeck

  3. #23
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    Quote Originally Posted by BCRider View Post
    You'll need gearing to get that much power. Slowing down the RPM has the same effect as throttling back. The power drops.

    ?
    With the motor that I linked it should work without gearing. 195kv takes about 18 volts for desired rpm, and aroundish 50 amps. So nicely still below max rated current of 80 amps.

  4. #24
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    I see a lot of the brushless motors rated at 60,000 rpm max. Some of the larger ones top out somewhat below that. And I thought that 28000 rpm coming out of a tee-dee .020 was fast.

    I agree with BCRider on the gearing point- the motors power comes from rpm, and seldom can you use the shaft power directly. With some of these motors being capable of over 2 hp, you'll need substantial gearing to bring that down to a useable rpm. In model use you are expected to be wearing out parts and replacing them, but in any serious use you'd want durable parts that can last.

    And just to put motor size vs horsepower in perspective, there are brushless motors smaller than a beer can that are rated at 15 hp.
    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

  5. #25
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    https://hobbyking.com/en_us/turnigy-...ner-motor.html

    Around 6-7hp without gearing at 3400rpm

  6. #26
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    Quote Originally Posted by MattiJ View Post
    https://hobbyking.com/en_us/turnigy-...ner-motor.html

    Around 6-7hp without gearing at 3400rpm
    Ya know... this is where the watts and horsepower comparison start to fall apart. If we look at the 9800 watts that motor says it is rated for and are willing to accept the modest climb rate of current ultralight airplanes that motor could fly a plane and pilot that totaled a little over 220 lbs based on the climb performance of 45 watts per lb. Granted it would need to be a skinny teenager to give us enough head room to allow for the craft itself and the batteries but still... Or a twin motor?

  7. #27
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    Quote Originally Posted by BCRider View Post
    Ya know... this is where the watts and horsepower comparison start to fall apart. If we look at the 9800 watts that motor says it is rated for and are willing to accept the modest climb rate of current ultralight airplanes that motor could fly a plane and pilot that totaled a little over 220 lbs based on the climb performance of 45 watts per lb. Granted it would need to be a skinny teenager to give us enough head room to allow for the craft itself and the batteries but still... Or a twin motor?
    You can find video from youtube where they lift 55kg of beer with four of hobbyking motors.
    And its quad-rotor heli, not a fixed wing!
    HK beer lifting contest or something like that.

  8. #28
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    Quote Originally Posted by BCRider View Post
    Ya know... this is where the watts and horsepower comparison start to fall apart. If we look at the 9800 watts that motor says it is rated for and are willing to accept the modest climb rate of current ultralight airplanes that motor could fly a plane and pilot that totaled a little over 220 lbs based on the climb performance of 45 watts per lb. Granted it would need to be a skinny teenager to give us enough head room to allow for the craft itself and the batteries but still... Or a twin motor?
    9.9kW is going to be input power and while 85% isn't bad efficiency for quite a small motor, you've got to take the controller efficiency into account too. The Agni (now part of Agility Saietta) brushed motor gets into the 90% range and it's possible to use a combination of a PWM controller and series/parallel battery switching to achieve very high controller efficiency.

  9. #29
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    Quote Originally Posted by MattiJ View Post
    With the motor that I linked it should work without gearing. 195kv takes about 18 volts for desired rpm, and aroundish 50 amps. So nicely still below max rated current of 80 amps.
    It might work at that. It's rated for 1500 watts so tuning the voltage down to get the RPM would still be within the current range AND below the maximum wattage.

    There's another issue with this idea though. And not with just this motor in particular but with all these model airplane motors generally. These motors are built very lightly and are actually somewhat fragile. We model flyers destroy them fairly regularly during "interesting arrivals". They won't stand the side loads of a pulley and belt power connection. In fact I'm not sure that they won't distort sideways even from the side loads of a regular spur gears when loaded down to the 1HP torque level. So they would require being coupled whatever machine is contemplated in a way that reduces any side loads to some very minimal amount. They are fine for torque. That's what they are intended for. But weak in the other directions.

    That would be where a planetary reduction setup similar to but more robust than those found on a hand drill would be ideal. The planetary setup introduces no side loads and would allow the motor to spin faster with a higher voltage for less current draw. AND it would allow for a nice sturdy support on the more standard size of output shaft.

    I also think I'd want to run with a smarter power supply. In model use the prop sets the load on the motor and thus the maximum current level. The simple aircraft or boat speed controls only seldom incorporate a built in current limit. And these motors will easily keep asking for current and burn themselves out if we load them down past what we planned. And in a shop situation and depending on what the use is that could be a problem.

    For example I'm sure that the controllers found in the newer brushless motor hand tools are including such a safety feature. We LOVE to lean on and try to stall our hand drills after all. And it's not like such a feature would be hard to incorporate. And in fact there may well be some of the model related speed controls that do have that feature. I know that one of my older Castle Creations ESC's has an over current shutdown to save the ESC from destruction if I try to suck more than 25 amps through it.

  10. #30
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    Quote Originally Posted by BCRider View Post

    There's another issue with this idea though. And not with just this motor in particular but with all these model airplane motors generally. These motors are built very lightly and are actually somewhat fragile.
    Agreed. The bearings are truly tiny and might last only tens of hours in model use.
    I have actually machined/redesigned some small bldc motors for lot bigger bearings when re-using them for something entirely different.

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