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Choosing the correct potentiometer

  1. Feb 19, 2010 #1
    hello to all. I'm new here and hoping that I'm in the correct forum. my question is:
    I'm desiging a hoist to use as a strength tester at my job. I am using a winch that uses DC current. The motor is 1.3 hp using a 12v battery with line speeds of around 10.8 ft/min to 8.2 ft/min. I need to slow that down hopefully using a pot. I would like to get the line speed down to about 1-2 ft/min. Is this possible and what size pot do I need? i'm thinking 500k but at what wattage 2-watt 4-watt?
    any help is greatly appreciated.
     
    Last edited: Feb 19, 2010
  2. jcsd
  3. Feb 19, 2010 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF. I've broken your post off as a new thread, rather than leaving it on the end of the other thread.

    You do not usually make a speed control for a motor by lowering the input voltage with a potentiometer. That reduces the torque as well as the speed, and the motor doesn't generally function well on reduced voltage.

    Instead, you generally control the speed with some form of pulse-width modulation (PWM) of the input voltage. That is, you turn the input 12V on and off at a fast rate, with the ON/OFF duty cycle ratio determining the speed of the motor. Are you familiar with PWM in speed control of DC motors?
     
  4. Feb 19, 2010 #3
    thanks for your reply. I am familiar with PWM but have not used in quite a few years. Iwas reading in another forum that I may possibly use a rheostat. Particularly a 150w/12ohms rheo. I need to have this operational early next week for my testing. thank you for your advice.
     
  5. Feb 19, 2010 #4
    For a DC motor, the speed is approximately proportional to the voltage and the torque is approximately proportional to the current. The main reason these proportions are approximate is the internal resistance of the motor. The current through that resistance creates a voltage that must be subtracted from the applied voltage to make the voltage-speed relationship more linear. Many DC drive circuits monitor the motor current and adjust the applied voltage to provide the same speed under varying loads.

    Obtaining 1.3 HP with a 12 V supply means your motor is drawing about 80 Amps. The simplest solution would be to switch from 12 V batteries to 6V batteries. Depending on the voltage drop due to the internal resistance of the motor, you may be in the ball park of the speed you want.

    The best solution would be to find an adjustable supply that can supply 80+ amps at only a few volts. That will not be a cheap solution.
     
  6. Feb 19, 2010 #5
    In the good ol' days, before electronic controls, the speed of a DC motor was often controlled by varying the field winding current. A motor has a "back EMF" voltage where a motor acts like a generator. When you put 12 volts on the armature winding, the motor spins up until the back EMF reaches a bit under 12 volts and the motor does not spin faster. If you increase the field winding current (and voltage), the motor will generate higher back EMF.

    If the DC motor has a separate field winding, you could increase the voltage/current which will reduce the speed. Double the field winding current , halve the speed. Halve the voltage and double the speed. Set the field winding current to zero and burn out your motor. The motor probably is not designed for higher winding current and will likely overheat. Also, the field magnetic laminates might saturate and you will not get a slower speed.

    For direct control of a 12 volts at 80 amps DC motor, you would want a 0.5 to 1.0 ohm, 1 or 2 kilowatt http://en.wikipedia.org/wiki/Rheostat#Rheostat". A rheostat is a two terminal potentiometer.

    Might find something from an old golf cart.
     
    Last edited by a moderator: Apr 24, 2017
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