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I need help - motor speed control

  1. Feb 19, 2015 #1
    Hi,
    I`m going to design turning table (diameter of 60-80cm) made from aluminium, it will spin by the mean of an electric motor. I want the spinning speed to be variable, so I can control it from 0 up to 300 rpm (gradually or by steps where each step is 50 rpm) by them mean of remote control, switch or whatever. The power will be used is 220-240V 50Hz.
    My questions are:
    1- What is better to use and easier to implement? AC or DC motor?
    2- Whatever has been chosen AC or DC, what is the best and easier way to control the speed? How will i control the speed?

    Thanks.
     
  2. jcsd
  3. Feb 19, 2015 #2

    davenn

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    hi there
    welcome to PF :)

    before we get to type of motor ( I prefer DC) ....
    how heavy is the aluminium table ?
    how heavy will be any loads on the table ?

    for a start, those 2 things will determine motor power requirements

    PWM ( pulse width modulation) using a H-Bridge is the preferred method for speed control
    but for the accuracy in steps you require and remote controlling, you are going to need a microprocessor
    to control the PWM section., do you know how to program micro's ?
    You need to decide what sort of remote control you want to use .... Bluetooth ?, Infrared?, other radio type ?

    This is going to be a pretty complex system .... what is your electronics experience ?

    Dave
     
  4. Feb 19, 2015 #3

    wirenut

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    Another possibility for an AC motor is a variable frequency drive (VFD) with either a potentiometer speed control (continuous variable speed) or a selector switch with a trimmer pot (for each step in frequency).
    I have installed these on many different types of machine, and you can set the maximum speed in the programming. Some other pluses are they are reversible, can be programmed to ramp up from a stop slowly or quickly, and can ramp down slowly or quickly.
    They are available in single or 3 phase, any voltage you can imagine, and for any size motor.
    You can quickly change the program during initial setup.
    They are easy to program.
    NOW.... The down side is you need to understand what each parameter in the program is and does. Don't worry most are self explanatory, and those that aren't can be figured out by CAREFULLY reading the programming guide.
    Also a con is that these units can be pricey, but ease of use and flexibility can offset the initial investment with increased productivity.
    Just my two cents. I am bias toward AC because I work with it everyday.
     
  5. Feb 19, 2015 #4

    davenn

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    hi there :)

    OK on the weights involved, that's quite significant :smile: .. then you need to find a suitable motor for the purpose, I cannot offer any suggestions offhand, others may be able to.
    Yes a wired control will make it easier

    a dimmer is usually a variable control so you would not be able to set your speeds accurately
    I still would prefer a micro controlled PWM system

    Am not sure about what @wirenut is suggesting, maybe he can provide some links so information can be viewed and assessed for its suitability
    to your project :)

    You didn't answer my last question in my first post

    cheers
    Dave
     
  6. Feb 19, 2015 #5

    davenn

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    why did you delete and repost, now my answer is out of time sync with your post ?
     
  7. Feb 19, 2015 #6


    Thanks Dave :)
    Well, i`m have my degree in marine engineering and doing now more studies in mechanical engineering. So i think i have the basics of electronics :)
     
  8. Feb 19, 2015 #7
    I thought you won`t be able to see my reply unless i click on (+quote) bottom, se i deleted it and posted one with quote, then i saw that you have already answered me :) sorry for the misunderstanding :)
     
  9. Feb 19, 2015 #8
    Thanks for the reply.
    So let`s say with a selector switch with a trimmer pot (for each step in frequency) will i be able to set for each step what is the speed correspondent? and how will i know what in the speed at each step? is it by using something like optical sensor (optical tachometer)?
     
  10. Feb 19, 2015 #9
    Then again, you can go cave man on this. Toss a micromotor's 24V brushed motor with integral gearhead in. Use any number of cheap 24V industrial power supplies, and any number of cheap prefabricated speed controllers from Ebay. I would take the precaution of putting the speed controller in an over-sized metal box because there's little to no chance that they will have safety approvals and metal enclosures are a mitigation for shock and fire.

    If the speed isn't critical, you may have 1/2 the solution in your shop - 120Hz flicker from fluorescent work lamps combined with stripes on your wheel gives an old school means of checking the speed. 300RPM -> 5RPS. That's 24 blinks of the overhead light per revolution, so 24 evenly spaced stripes will blur, slow, and then stop rotating as you come to speed.
     
  11. Feb 20, 2015 #10

    wirenut

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    Sorry for the delay, I posted just before bed,
    Yes you can set as many steps as you want. You usually put a pot across the com (0 volt) terminal, and the +10 volt terminal, then the wiper part will go to the 0 to 10 volt terminal. If you use a 2 pole multiposition switch ( all one side of the trim pots go to 0 volt, each wiper goes to a different pole on the switch and each other side of the trim pots go to corresponding poles on the switch. The commons of each pole then go to +10v and the 0 to 10v inputs of the vfd.)
    The speed is adjusted for each switch position with a trimmer pot, and the VFD readout displays the speed.
    If I can I'll try to post the connection page from on of the guides I have.
    Lenze and Eaton are two brands of reliable VFDs. If you google them you might be able to download an installation guide with the connections and programming options.
     
  12. Feb 20, 2015 #11

    wirenut

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    I think he will need to use 20 stripes, If I am not mistaken he was using 50 hz.
     
  13. Feb 20, 2015 #12

    wirenut

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    Here I found a quick start guide. On the bottom of page 7 terminals 5 , 6 , 7 are +10v , 0 to 10 v , and 0v respectively.
     

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  14. Feb 20, 2015 #13
    That`s lovely, and thank a lot for your help :)
     
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