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Ohm's law and using motor as generator

  1. Aug 11, 2016 #1
    hello i was wondering about how a dc motor would work as a generator and how the voltage and current would scale with ohms law. for example lets say i had a 350 watt motor thats 36 volts and generates 9.72 amps now lets say i use that motor as a generator but only generate 200 watts how would the current and voltage scale in proportion
  2. jcsd
  3. Aug 12, 2016 #2


    Staff: Mentor


    You would know that the voltage times current equals 200. But the actual voltage and current depends on the load connected to this generator.

    When acting as a motor, you had to supply the 36 volts and enough mechanical load to draw that current.

    Acting as a generator, you must supply mechanical power to the shaft and electric load to the terminals.

    Think of motors and generators as devices that transform mechanical power to/from electric power. Ohms Law is not much help.
  4. Aug 12, 2016 #3
    hello thanks for the reply, but i think i may have to rephrase my question. basically what i was asking is if i use a dc motor in reverse as a generator the max capacity of that generator would be 350 watts at 36 volts and 9.72 amps now if i use the generator and decide to only generate 200 watts considering i keep the voltage and current proportional what would the voltage and current be when only generating 200 watts. for example lets say a generator generates 100 watts at 10 amps and 10 volts and now lets say you only generate 50 watts then the current and voltage would be equal to 7.07 because 7.07 x 7.07 equals 50 and if i generated 25 watts it would be 5 amps x 5 volts which would equal 25 watts. sorry if my question is long hopefully its easier to understand now.
  5. Aug 12, 2016 #4


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    Generally the voltage is held constant and the type of load determines what wattage is delivered to the load. A 100 watt incandescent light bulb with the Edison style base commonly used (formerly now due to energy conservation laws) is designed to run on 120 volts and will draw approximately .833 amps. Take the same type of bulb that is a 25 watt bulb will draw only .2083 amps while it is still designed to run at 120 volts. So to answer your question we would first need to know what type of motor you have. A lot of permanent magnet motors work fine as simple generators but some DC motors are not designed to work in reverse. They have on board electronics that turn them into AC induction motors for the purpose of eliminating the commutator and brushes. How much voltage a permanent magnet motor will produce unloaded when used as a generator depends upon how fast you spin it and of course will have some dependence on what voltage it was designed to operate with as a motor. How much current it is able to deliver will depend upon the thickness of the windings and things of this nature. Also keep in mind that you could spin it up and get your unloaded target voltage but when you apply the load you will have some sag in the voltage. There will not be an abrupt stop. There will be a point where the voltage falls below the useable level. Best thing to do is get some cheap hobby motor and start experimenting.
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