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Thread: what type of noiseless/low noise motors are there?

  1. #11
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  2. #12
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    I searched amazon for rc motor brushless 3000rpm and found a bunch I think they will all be about the same.

  3. #13
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    Sure, I could use rc motors, they are everywhere. They do make a hell of a noise but they do run at much higher RPM usually and have a propeller or wheel attached to the shaft which adds more noise. That said I probably dont need such a high torque motor as my design is much smaller, unless that doesn't matter for the noise.

    A direct drive would work if I could find a relatively small motor with a 30mm hollow shaft because in my design that's where the beam is passing through.
    I could also use teethed pulleys on the motor and rotating element but that would introduce even more noise and wear.

    EDIT: watching the helium video now...
    Last edited by shoujin; 03-11-2018 at 16:24.

  4. #14
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    In response to helium soundproofing:
    Neat idea and while I don't think filling the whole enclosure and still preventing helium leakage is possible or viable, by using magnetic coupling with big enough magnets I can enclose the motor in an airtight enclosure. But there will still be a hole where the motor cables have to go through which can be sealed. I don't know if that one spot will make the whole idea pointless.

  5. #15
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    Have you considered an AC induction motor ??
    An unloaded 4 pole motor would run at about 1800 RPM at 60Hz.
    An unloaded 6 pole motor would run at about 1200 RPM at 60Hz.

    Jerry
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  6. #16
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    Not a fan of AC motor for this, but could work.

    I got some BLDC motors meant for quadcopters, but they have 3 cables coming out instead of 2.
    https://www.ebay.com/p/2700kv-A2212-....c100005.m1851

    In quads it appears the 3 cables are connected to a speed controllerwhich in turn is connected to a DC source.
    But if it's just a BLDC motor shouldnt you be able to ignore the 3rd cable and run directly off of a DC power source? I tried and it behaved like a stepper motor (fast and momentary rotation)

  7. #17
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    Quote Originally Posted by shoujin View Post
    I got some BLDC motors meant for quadcopters, but they have 3 cables coming out instead of 2.
    Quad-copters need variable speed motors to fly, so they have an external speed controller. That's why you need 3 wires. In a constant-speed BLDC motor you can have just 2 wires supplying the motor provided all the electronics for switching current between the stator windings is built-in to the motor itself.

    But if it's just a BLDC motor shouldnt you be able to ignore the 3rd cable and run directly off of a DC power source?
    If it's a constant-speed BLDC motor, yes. But RC quadcopter motors are not designed that way. They require an external speed controller.

    Adam

  8. #18
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    Well, then I'll need some advice on whether I should use a similar BLDC motor or an RC motor. RCs seem better as they are available off the shelf and many have nice mounting holes. But a controller instead of controlling the voltage may or may not make them usable for my project because of my constant precise speed requirements.

    Here's the requirement:
    The motor needs to rotate at constant 1200 RPM.
    With ordinary BLCD I can put an ir or magnetic sensor to measure the rotation speed and adjust it accordingly by controlling in analog the voltage.
    I don't know if the RC motor conttrollers can provide the same precision.
    Any thoughts?

  9. #19
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    The speed controllers for RC BLDC motors use a PWM input signal (from either the radio directly in an airplane application, or from the flight controller in a quad-copter application) to control speed. So, yes, you could attach an IR sensor to the rotor and use that signal to drive a circuit that would generate the PWM signal (either speed up or speed down) that you then would send to the controller. This seems like a kludge though; it adds complexity and increases the response time of the system.

    I am not aware of any RC speed controller that has any direct feedback signal from the motor regarding actual motor speed. It is simply assumed that there is no pole-slipping going on. (Since a BLDC motor is a synchronous motor, so long as you aren't slipping poles the rotor speed will match the rotational speed of the field in the stator windings.) In fact, since most RC motors never need to spin at low RPM, the motors often eliminate the hall-effect sensors entirely and simply use back-emf to detect the rotor position. This means that once RPM gets low enough that you start slipping poles, the motor just stops turning completely.

    Thus if you wanted to be clever, you could simply look at the power being sent to the motor from the RC speed controller to determine rotor speed. Again, assuming that no pole-slipping is taking place, the rotor speed should match the field speed. (Provided you are above the minimum operational speed.) So you could just look at the time it takes for the power to switch from one winding to the next and derive the rotor speed from that. From there you would still need to generate the PWM signal to tell the speed controller to go faster or slower, but that will be local to the controller and thus nearly instant feedback.

    As to how accurate this system could be? Well, the ESCs I use in my quads are designed to receive a PWM signal from the flight controller every 20 ms. If the pulse is 1 ms wide, that is zero (or off) and if the pulse is 2ms wide, that is full throttle. Anything in-between is interpreted as a % speed between 0 and 100. So in theory the controller should be able to respond in 22 ms or less. Is that fast enough for your application? Yes? Cool - then you can use off-the shelf components and save a bunch of money.

    If you need better response, then you'll want to build your own speed controller for the motor, or at least modify the input to an existing unit so it will update faster. The limiting factor in an RC speed controller is certainly not the controller itself, but rather the infrastructure in place for RC signalling between the aircraft and the pilot. Those standards were laid down in the era of glow-fuel engines which had no use for microsecond response times.

    Adam

  10. #20
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    all is clear now. thanks

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