calculating needed potentiometer

Discussion in 'General Electronics Chat' started by IdahoMole, Sep 15, 2010.

  1. IdahoMole

    Thread Starter New Member

    Sep 15, 2010
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    I am making a mixer for a mash tun for all grain beer brewing how ever i need to slow the motor down so as to not make a mess all over the kitchen (again). I am using the electronic controler that came with the treadmill but even the lowest setting is too fast. The motor is a 90vdc 15 amp motor. What size potentiometer do I need and how it that calculated? I would like to be able to control the motor from as slow as a few revolutions a minute and up to full speed (although i will never have to drive it that fast)> thanks for your help. If you need anymore info please ask. (I am an electronic noob).
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    You wouldn't be talking about a potentiometer, with that size motor you'd be talking industrial rheostat.

    You've going to need some way slow it much furhter down and I'll bet you $1 it's already being PWM controlled so the circuit will need to be altered. While you can run two PWM circuits in series the resultant occasional crossing timings can raise heck with all of the electronics involved.

    In all honesty I'd think of a mechanical way to reduce the speed.
     
  3. Jaguarjoe

    Active Member

    Apr 7, 2010
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    If you're looking for a potentiometer to control the motor directly, you'll never find one and it won't too well work anyway.

    If you're looking for a potentiometer to replace the one that's in the treadmill controller, you'll need the schematic diagram of it first however, I don't think this will do you any good.

    There's a possibility that there is a minimum speed adjustment or resistor that could be tweaked in the controller to slow it down but, once again, you'll need the schematic first.

    You can build a pulse width modulated (PWM) circuit that can give you 0 to 100% speed control but you'll need a 1350 watt driver for the motor which by no means is a simple task.

    Can't you just change the pitch of the mixer blades so they won't create as much turbulence?
     
    Last edited: Sep 15, 2010
  4. IdahoMole

    Thread Starter New Member

    Sep 15, 2010
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    Marshall
    when u say "raise heck with the electronic" what do you mean? cook them?
    i did a quick search of rheostats and there are hundrends of different ratings and types, none of which i know a darn thing about. what would the specs be for the proper rheostat?
     
  5. marshallf3

    Well-Known Member

    Jul 26, 2010
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    What I was talking about was adding yet another PWM circuit in series with the existing one the treadmill already has driving the motor. You could end up smoking anything.

    Putting a huge rheostat (terminology for a very high wattage wirewound variable resistor) probably wouldn't help either as the feedback loop will sense the motor turning slower and the internal PWM (speed control) circuit will just adjust it back up.

    The only safe way to do this is to get a schematic showing the circuitry that currently controls the motor speed and seeing if that can be modified, alternately you can incorporate some sort of gear, belt or roller setup as a speed reduction device.
     
  6. IdahoMole

    Thread Starter New Member

    Sep 15, 2010
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    ok, i will work on getting the schematics for the speed control.
    i have considered adding another pully for gear reduction but the fabrication would be a big PITA but not impossible.
    i had another thought today. why not chuck everything except the motor and just find/build another speed controller? any thoughts or links to posts on that would be appreciated.
     
  7. Jaguarjoe

    Active Member

    Apr 7, 2010
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    Building a 15 amp speed controller is not a trivial task.
    Modifying your existing controller could be surprizingly easy. Cost would be about $1 or so.

    Or, buy a 1500 watt Variac, a 20 amp bridge rectifier, and a big filter capacitor. For between 100 and 200 dollars you'll be all set.
     
  8. IdahoMole

    Thread Starter New Member

    Sep 15, 2010
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    well a little more insite on that $1 fix would be great!
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    We'd need the schematics first. They could prove very difficult to get.
     
  10. marshallf3

    Well-Known Member

    Jul 26, 2010
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    Depending on the brand I'd say he stands a fair chance of finding a schematic for the treadmill, whether it's more than a block diagram is going to be important.

    Aside from that you'd want to figure out how and where it currently monitors the speed. You might be able to fool it by making the controller think it's running faster than it actually is and, if need be, figure out a way to alter the duty cycle it currently outputs if it won't go low enough.

    I'll have to admit I've made some beer before but never had to agitate anything, seems to me it would defeat the purpose. That was one big mess to produce something that wasn't anything stellar and ended up costing a lot more money than expected.
     
  11. IdahoMole

    Thread Starter New Member

    Sep 15, 2010
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    YAY! I got it sorted. There are 4 little adjusting "pods" on the board marked "max", "min", "torque", and dont remember the fourth. I turned min and max to the lowest setting and torque to the highest. messed with the fourth one a little bit but it made the motor start and stop in a jerky fashion so just left it alone. works pretty good now. Thanks for the input from everyone. If it wasnt for y'all I never would have examined the board that closely.
     
  12. IdahoMole

    Thread Starter New Member

    Sep 15, 2010
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    Oh. When all grain brewing it is nice to keep the grain moving so it doesnt scorch on the bottom of the pot....but all of that is for another forum...:)
     
  13. Jaguarjoe

    Active Member

    Apr 7, 2010
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    90
    Hey, and it didn't even cost you a dollar!
     
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