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Looking for a small, low rpm, higher torque motor. Help please?

Discussion in 'Sensors and Actuators' started by Madsalts, Oct 8, 2014.

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  1. Madsalts

    Madsalts

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    Sep 25, 2014
    I'm looking for a motor to put in an array of four other motors to be run for a long time, and off of a computer power supply. I'm looking for one that can, for all intents and purposes, go "infinitely" slow, by limiting current with a potentiometer (does such a thing even exist?)

    At most, I'd want it to be about 30 rpm. I saw some for sale that rate 30 rpm at 12 volts. If I were to use such a motor (I'll provide the dealer info if asked, I don't know if that's permitted) with a computer power supply (most are in the neighborhood of 19 V), I'm guessing that I could slow it to 30 RPM w/ the potentiometer. I wonder if I could run it at 19 V, or if this would burn it out. Is amperage even a concern if I'm going to use a computer power supply? If so, what parameters would I look at? (I'm an electronics novice). I'd want the motors to be of moderate to high torque, as they'd be used for a grinding application.

    Speaking of burning out, the motor must be able to run for hours without a fan, or have an integral fan.

    I'm hoping that the motors will be simple (not having many extraneous wires to which I'd have to hook capacitors, etc. to get them to work) and be about $ 25 or less. Thanks.
     
  2. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Hi and welcome to Electronics Point :)

    Sounds like you need a stepper motor with a gearbox.

    Controlling motor speed with a potentiometer is inaccurate, especially if the load varies. A stepper motor can be controlled precisely for any speed you want, though you'll need a suitable stepper motor driver circuit. These are available for 12V DC supply.

    I don't have any specific suggestions, but I'm sure Google does, now that you know what to look for.
     
  3. donkey

    donkey

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    Feb 26, 2011
    A) it does exist, but potentiometer is extremely silly. the ways a pot works is its a resistor limiting (it could be both but am guessing) voltage..... could be current... anyway whichever it is the way it works is a voltage(or current) is fed in and then the pot (which is a resistor) downgrades it. the excess is given off as heat. this leads to 2 big issues, first is energy wasted. the second is with the wrong size pot they go black, a nice flame can be seen and then resistance goes to infinity and motor no worky.
    I would recommend a PWM. what this does is keeps the current and voltage constant, but it turns it on and off ultra fast. so theoretically you turn it of 80% of the time (so evey 80 milliseconds its off 20 its on) then theoretically it goes at 20% speed. this happens at such a fast rate that to the average human its just running slower not stopping and starting.
    you could get a genius to design you one but you can also get them of Ebay for cheap. most are rated at about 2amps but you can find higher rated ones if you know where to look.

    the power supply is another issue again. you state a computer power supply, is this a laptop transformer or a desktop one? the reason I ask is laptops are general 19volts(some higher some lower) but desktop power supplies have a 12volt rail. the issue is you have to dummy load the 3.3v rail to give it a full 12volts.
     
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Oh yeah, you can use PWM if you don't need high accuracy or if there's speed feedback from the motor to control a servo driver. I think you would want a gearbox though.

    I missed your comment about 19V. As donkey says, that sounds like a laptop power supply. You could drop it down to a fixed 12V with an adjustable buck switching regulator. They're available for a few bucks on eBay.
     
  5. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    PWM is no different to a pot. You need my Motor Speed controller if you want to reduce the RPM of a motor at a varying load.
    But the best solution is a gearbox and 5 pole motor, or a stepper motor. The 30rpm motor you saw has an inline gearbox (cyclic / sun/planet) or compound gearbox.
     
  6. Gryd3

    Gryd3

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    Jun 25, 2014
    There are differences between PWM and a pot.
    Although slightly more complicated, PWM can drive larger loads than using a pot. As mentioned above, using a pot to slow down the motor requires that the pot dissipates the excess energy as heat.
    If too much energy needs to be dissipated, you end up with smoke, or fire.
    Everyone agrees on a gearbox or stepper though.
    How much torque do you need?
     
  7. Madsalts

    Madsalts

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    Sep 25, 2014
    Thanks, I appreciate it. I just contacted someone regarding some stepper motors. Hopefully, it goes well.
     
  8. Madsalts

    Madsalts

    73
    2
    Sep 25, 2014
    Well, I did some reading and watched a couple videos on stepper motors. I have two concerns at this point: one is wiring the driver/controller, the second is a question of speed.

    I get the basic gist of how a stepper motor works. I don't get how to wire it to a driver/controller. This isn't really an issue yet, as I don't know if I'll even proceed with the stepper. It seems that the speed of the motor could be controlled via: computer program, digital dial, jumper, or adding/removing a component to the driver/controller. Looking at the site of a firm that makes stepper motors, I see that they sell optical encoders. So, this leads me to believe that the "digital dial" method of controlling speed is among the possibilities.

    This brings me to my second problem:
    The optical encoders at the site have specs of: tracks from 0 to 100,000 cycles/second, 32 to 1250 cycles/revolution, 128 to 5000 pulses/revolution. If the motors run above about 250 Hz, some the apparatus could be damaged. I don't like the idea of the only thing standing between the motor running at below 250 Hz and far above 250 Hz to be an accidental bump of a controller. Is there some way to govern a maximum speed (without using a laptop, ideally) of the stepper motors? One solution is to simply put the controller under something so that it can't accidentally be bumped. This negates the possibility of adjusting the speed on the fly, however. I wonder if there's a better solution... Thanks.
     
    Last edited: Oct 11, 2014
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
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    Nov 28, 2011
    You need a stepper motor controller that can run the motor continuously at a steady speed, with some way of adjusting that speed. This would either be a stepper motor driver that includes that capability, or a stepper motor signal generator that can generate an appropriate continuous stream of pulses at a settable frequency (this can be done with a small microcontroller or a fairly simple circuit), feeding a stepper motor driver that will translate those pulses into a suitable voltage and current for the actual motor.

    A rotary encoder could be used to generate the signals needed to drive a stepper motor, but that would make the stepper motor follow the rotary encoder. In other words, you would have to turn the rotary encoder constantly, and the stepper would turn at some speed related to the speed at which you're turning the rotary encoder. In other words, I don't think you would adjust the position of a rotary encoder to adjust the speed of the stepper motor. But I could be wrong here.

    Have you found any stepper motors or stepper motor drivers that you think might be suitable? If so, post URLs of them and we can see what they can do, and what else you would need.
     
  10. Fish4Fun

    Fish4Fun So long, and Thanks for all the Fish!

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    Aug 27, 2013
    Hey Madsalts, Welcome to EP!

    I am in 100% agreement with everyone else on 1) Stepper Motor 2) Geared DC motor or 3) a Geared Stepper Motor....Unless you have serious positional accuracy issues I cannot see you requiring an optical encoder....In my CNC hobby I have little trouble holding +/- 0.002" positional accuracy with open-loop steppers at linear speeds up to ~600ipm (inches per minute) Since you mentioned 30rpm MAX I cannot imagine you will exceed the positional accuracy or RPM accuracy available in a simple open-loop stepper motor....The biggest questions you need to answer are 1) How much torque do you need? 2) How accurate do you want your speed control to be? 3) How much do you want to spend to achieve 1 & 2?

    On the low budget/low torque side of the matrix is something like this: http://www.ebay.com/itm/DC-5V-Stepp...409?pt=LH_DefaultDomain_0&hash=item2ed7e06a99 You can get the same thing from the US in a couple of days for ~$5ea. This will NOT solve all of your problems, you will need a uController or a PC to complete the system. Only slightly up the cost matrix, something like these are available: http://www.ebay.com/itm/RepRap-Step...802?pt=LH_DefaultDomain_0&hash=item4177c19222 which will drive stepper motors like: http://www.ebay.com/itm/1-PC-NEMA-1...883?pt=LH_DefaultDomain_0&hash=item1c2f3784ab . It would be easy enough to design a simple signal source for this type of driver/stepper w/o involving a uController or PC and the increase in torque would be HUGE.

    As far as steppers go....much like the name suggests, they operate by moving in incremental steps...while stepper motors are produced in a wide range of steps per revolution, the most common number of steps per revolution is 200 (1.8 degrees per step).....to increase the "resolution" and improve low RPM performance, typical drivers will break "full steps" into "micro-steps". Common drive configurations are 1/2 step, 1/8 step, 1/10 step and 1/16 step. There are certainly other configurations.....assuming 1/16th step, this implies 3200 "uSteps" per revolution.....so to rotate the motor @ 1 RPM would require 3200 step pulses to a 1.8 degree stepper being driven by a driver in 1/16 step mode. to achieve 2 RPM would require 6400 steps per second ...etc, etc. So by varying the number of steps sent to the driver you have VERY tight control of RPM.....there is no "wiggle room" here, the motor will turn exactly 1/3200 revolutions per input step pulse...so to vary the speed you simply need to change the pulse rate....so, continuing with the 1.8 degree stepper in 1/16 step mode, to turn 30rpm would require a pulse train of 1.6khz. (3200ppr * 30rpm/60m/s = 1600pps = 1.6kHz). @1.2khz the motor would turn @ 1200pps*60s/3200ppr = 22.5rpm....etc, etc. So....in this example you would have a speed range from 0rpm to 30rpm in 0.01875rpm increments (30rpm/1.6kHz = 0.01875rpm/Hz)....If you need higher accuracy than this, then gearing the stepper will increase the rotational speed accuracy by the gear ratio multiplied by 0.01875rpm/Hz within the rpm limitation of the stepper motor.....for instance if the stepper motor had a maximum rpm @ the required torque of 3000rpm and it had a 100:1 gear reduction (still assuming a 1.8 degree stepper and a 1/16 step driver) then you would need a pulse train of 100 * 1.6kHz = 160kHz, and you would have 0.01875/100 = 0.000875rpm/Hz rotational speed accuracy...for the record, 3000rpm is a bit faster than typical stepper motors are capable of with any type of load.....

    Anyway, unless you need extreme rotational speed control or a great deal of torque, the "easy button" is likely a NEMA17 1.8 degree stepper with one of the Pololu drivers....I haven't bothered searching, but I believe a similar driver complete with a pulse-train generator and a "speed control adjustment knob" is available from ebay for testing small steppers for << $20.

    Fish
     
    Gryd3 likes this.
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