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Calculating the value of resistors

  1. Mar 3, 2010 #1
    if my 12volt motor running with 12v draws 10amps, am i right in thinking if i put 6 volts on the same motor then it would run at half speed? how would i work out the value of the resistor needed?

    i know i need to use ohm's law somehow but it's been a while since i've had anything to do with this and cant remember just how to do it....

  2. jcsd
  3. Mar 3, 2010 #2
    I don't think the speed of an electrical motor is necessarily proportional to the voltage. It depends heavily on the characteristics of the load you are putting on the motor. I think you need to experiment to find the correct values.

    Say that a 12V motor can barely lift a given weight, running at some non-zero RPM. On 6V it will then not be able to lift the weight, and the RPM will be zero. So the relasionship will be complicated, and I think it is even for an "unloaded" motor (i.e when the only load is the ball bearing friction and air resistance within the motor itself).

    EDIT: This seems like a good source of info:
    http://www.4p8.com/eric.brasseur/emamem.html [Broken]
    If you know the power needed to drive the load for each value of RPM, then you might be able to use those formulas to derive the approximate relationship between voltage and RPM for your motor. I didn't look thoroughly, though.

    Last edited by a moderator: May 4, 2017
  4. Mar 3, 2010 #3
    Every motor is different. In general, if you don't trust manufacturer's specifications for rpm versus voltage or max torque or what have you, your best bet is just to create a quick empirical fit for your motor's speed as a function of voltage AT the load that you intend to use.
  5. Mar 4, 2010 #4
    Resistors are not suitable because they are current limiting and a motor draws varying currents depending on the load.
    Use diodes instead as they are voltage limiting not current.
    The forward voltage drop on a diode is 0.75 volt so if you put eight diodes in series you drop from 12v to 6v.
    The diodes used would have to be able to carry a bit more than your maximum load of 10 amps.
    I have used this to run the windscreen wiper motor of an old car that I upgraded from 6v to 12v.
    The load on the motor depended on how wet (or dry) the windscreen was. If I had used resistors the wipers would have stalled.
    Experiment by adding or removing a diode to obtain the speed that you require.
  6. Mar 4, 2010 #5
    Yes, alternatively use a Zener diode if you don't want a string of diodes. You can find them for different voltages.

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