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Thread: Motor setup advice

  1. #1

    Default Motor setup advice

    I am designing a turntable which will be used to spin plants to demonstrate to first year university students that they will grow towards the axis of revolution.

    To do this I need a motor. I have decided to go with a three-phase AC induction motor, and with it I am going to get a VFD (variable frequency drive). I will be using a fan-belt drive.

    My question is: am I going to have to adjust the diameter of the shaft of the turntable in my design in order to cater for the difference in speed (rpm) of the motor and the speed I want the turntable to rotate at (49 rpm), despite the VFD or will having the VFD mean I don't have to worry about that too much? That is one thing I am stuck on at the moment.

    Any help on this matter would be much appreciated. Thank you.

  2. #2
    Sprinkler Guy Wet_Boots's Avatar
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    Why not employ an old record changer as a drive? You get 45 rpm that way, and at very little expense.

  3. #3

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    I'm not convinced that would last long enough .... a record changer drive is not really designed for continuous running for hopefully (knock on wood) 10 years + .

  4. #4
    Sprinkler Guy Wet_Boots's Avatar
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    What motor is designed for a decade of continuous running? And what class is going to last for a decade? First year university courses must have changed since I was wasting time at college.

  5. #5

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    Certainly I am convinced there are motors that will last a lot longer than an old record changer.

    It's a basic physics course. It's for a New Zealand university, not an American college. Yeah courses change but this is part of the course that's in it for the long haul.

  6. #6

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    Also with plants (and thus soil) the turntable is going to be much much much heavier than the weight of a record.

  7. #7
    Sprinkler Guy Wet_Boots's Avatar
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    Well, just as a practical approach, there must be a zillion old BSR MacDonald record changers out there with nowhere to go, and they did have reliable motors, so if the weight of the plants is not too much, you have that as an option.

    Now if you enlarge the sizes and weights beyond what a phonograph could handle, then you can still manage with simple motors and belts and pulleys. Is there an equivalent to an industrial supply house like Grainger in New Zealand?

  8. #8

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    I am probably going to have to order some parts from Australia but there are a few spcialist motor stores around the joint, nothing hugely industrial.

    Yeah the motor and belts and pulleys were what I was originally going to go with but if you have any additional information I would gladly appreciate it.

  9. #9
    Sprinkler Guy Wet_Boots's Avatar
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    Grainger is an industrial supply house where I can buy motors and belts and pulleys. If you have a platform for your experiment, you can 'conect the dots' with the appropriate belts and pulleys. I can picture a platform of wood, with a groove notched into the edge, for taking the belt. It wouldn't take a fancy motor to make this work, and you can still look at how a belt-driven phonograph turntable works, as a guide to building your project.

  10. #10
    Plumber jimbo's Avatar
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    Quote Originally Posted by Wet_Boots View Post
    What motor is designed for a decade of continuous running? .
    Well, since this is a plumbing forum, I can relate about the reactor main coolant pumps on submarines. When GE or someone was designing those pumps and motors, they asked Rickover what he would spec for longevity and pump seal leak rate. He said 20 years and zero ounces leakage over the 20 years. First they fainted, but then they did produce the design. I guess the price tag was a million or so each, in 1955 dollars!

  11. #11
    Sprinkler Guy Wet_Boots's Avatar
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    Good 'ol Mil Spec!! Yee-haww!

  12. #12
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    don't forget they had to run dead quiet, too, I'll bet.

  13. #13
    Like an engineer alternety's Avatar
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    A VFD motor is real economic overkill for a fixed rate application. Actually even a variable speed application in the right circumstances.

    Get standard motor with sufficient power to turn the mass of the platform and contents. You might want to think about start up acceleration/deceleration/motor stalling when you service/add/remove the plants.

    Build the platform. Pully on support shaft. Pully on motor mounted under the platform. Belt between them. See a book for diameters.

    Protect the motor/belt/bearings from nasty stuff associated with growing plants. Use good bearings and support for the shaft/platform.

    Simple. If the motor breaks, it is a standard motor and you just replace it.

    An alternate to a simple constant speed AC motor would be a DC motor (preferably a permanent magnet version). These have nice smooth torque characteristics and can be made speed adjustable and soft start/stop with inexpensive variable voltage line connected DC controllers. I am running a 1.5 HP motor from a Baldor BC140 controller. Look at the specs on that. Other people make them also.

  14. #14
    DIY scratch-pad engineer leejosepho's Avatar
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    Quote Originally Posted by joeyar View Post
    My question is: am I going to have to adjust the diameter of the shaft of the turntable in my design in order to cater for the difference in speed (rpm) of the motor and the speed I want the turntable to rotate at (49 rpm), despite the VFD or will having the VFD mean I don't have to worry about that too much?
    A large pulley on the turntable shaft would be driven by a small pulley on the motor shaft, and the ratio would be dependent upon the rpm of the motor. A typical 1725 rpm motor and something like a 43:1 ratio would net 40 rpm at the turntable. A 2-inch pully on the motor would have a circumference of about 6 inches, and that would match to a turntable pulley of about 82 inches to end up with 40 rpm. If you do not want a turntable that large, a simple gear motor could be used to reduce the speed of the drive pulley and allow for a smaller driven pulley. Or, a small lawnmower transmission could be used ...
    Last edited by leejosepho; 01-11-2008 at 03:51 AM.

  15. #15
    Aspiring Old Fart, EE, computer & networking geek Mikey's Avatar
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    Quote Originally Posted by joeyar View Post
    Also with plants (and thus soil) the turntable is going to be much much much heavier than the weight of a record.
    The power required to keep the thing turning will be very small if the turntable is balanced and mounted on a single ball bearing, like the old high-quality record turntables were. Startup power required will depend on the total mass, of course, but you could use a separate motor (or a graduate student) to provide that impetus.

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