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Electric Motors/Generators

  1. Jun 20, 2007 #1
    we have recently started to learn about motors and generators. A DC motor can be a DC generator and vice versa, but is this the case with AC (an Ac motor being an AC generator)? Why or why not?

    Also is the speed of the motor dependent upon the voltage and the torque of a motor due to the current supplied to the motor? or is it the opposite? If so why is this the case?

    Many thanks to all whom chose to reply,
  2. jcsd
  3. Jun 20, 2007 #2
    Yes, an AC motor is definitely possible. There are tricks that you can do with AC power, such as rectifying (that is to say slice up) the wave with a diode, mutual inductance (the idea of transformers), and many other things that open up many possibilities.

    Are you asking whether the DC motor speed is proportional to current or voltage? Most likely the answer is yes, but there are so many types of motors out there that I don't want to discount anything.
  4. Jun 21, 2007 #3


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    On a practical level the typical AC motor won't act as a generator. Without some current already flowing through it there is no magnetic field to start the ball rolling as far as generating electricity from a varying magnetic field. DC motors, especially the small kind have permanent magnets.

    Now if you have an AC induction motor running and you somehow kicked it into reverse by applying enough torque it will cause a backlash current. But it won't (I don't think) sustain itself as an AC generator. You'll have to design the whole thing differently.

    There are AC motor designs which do use permanent magnets and which do like their DC counterparts work both ways as motor and generator. They however aren't as efficient and cost more to build so you won't find them in your typical commercial application.

    But in the most general terms your question can't be answered simply as there is a broad range of motor designs with varying ability to work in reverse as generators. (Including the DC ones).
  5. Jun 21, 2007 #4
    thank you very much for both replies, they have both been of great help, only wish i had a physics teacher as good as
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