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Types of opposing forces in a generator, and a motor

  1. Dec 6, 2015 #1
    I keep going back to this, but I get confused when imagining everything. I'll start off with a motor:
    Basic set-up fixed magnets of a uniform magnetic field and a conductive armature that has current flowing, the armature experiences a torque(due to the the Lorentz force), and the armature gains speed, the initial opposing "force"(not actual force, just found the word easier to describe the system) to the change in magnetic flux(area in this case) would be counter-electromotive force that would reduce the applied current and the motor reaches high speeds.
    Since there is current flow within the armature there is a generated magnetic field surrounded by the exterior magnetic field of the magnets, is there any form of magnetic repulsion/attraction going on? I keep thinking about it a lot... they are basically two magnets, the armature experiences two simultaneous phenomenons occurring, the Lorentz force, and back-emf, is the magnetic attraction/repulsion a third? The same goes if we switched it around, making the magnets rotate around and the electromagnets fixed.

    Would the electromagnets/magnets creating the uniform magnetic field undergo any forces? I always ignore the sources the create the magnetic field, all the action happens to the conductive rotor!

    Likewise, for a generator when there is a load. And current begins to flow, there is a Lorentz force opposing the change in flux, is there a form of magnetic force here two?

    I feel that sometimes I don't account for everything in such systems, which I'm planning to work on in the future.
  2. jcsd
  3. Dec 6, 2015 #2


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    Yes, what do you think it is that causes the armature to start to rotate ?

    Keep in mind there is NO current flow TILL there is a load. There is just a voltage developed across the output terminals of the generator winding

    so now connect a load, and current flows, what do you think happens ?

  4. Dec 6, 2015 #3
    Lorentz force?

    A Lorentz force is created(ILxB) from the induced current, and will oppose the change that created it.
    I feel like the Lorentz force is a form of magnetic repulsion/attraction..?
  5. Dec 6, 2015 #4


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    yes, but look at it more simply ...

    take the motor scenario....
    forget about what is generating the magnetic fields for a moment
    just consider the 2 magnetic fields the one from the permanent magnets and the one from the armature winding(s)
    what effects are they going to have on each other

    Last edited: Dec 6, 2015
  6. Dec 6, 2015 #5


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  7. Dec 7, 2015 #6
    Thanks for the link Dave, this made me remember how the explanation of a motor's functionality is odd.
    This is now more confusing, fundamentally, the main component to focus on the most that generates the force(therefore, torque) is the Lorentz force, that each charge experiences a force that pushes it away from the magnetic field. I don't understand how "attraction" is possible, when the Lorentz force is force that pushes a conductor away... I'm thinking about this like how you attract to magnets or repel them from one another, but in a motor we focus mainly on the Lorentz force(##IL \times B##)...
  8. Dec 7, 2015 #7


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    as I said in post #4 ... at the most basic level, it is still a magnetic attraction and repulsion system. the interaction between the magnetic fields of the
    permanent magnets and the fields generated by the current flowing in the armature windings
  9. Dec 8, 2015 #8

    jim hardy

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    maybe there's a simpler approach

    draw a free body diagram of the motor
    torque coming out shaft must be balanced by torque against motor frame
    follow that second torque back - to forces on your field magnets or field turns.....

    have you never energized an automobile starter on the workbench? With a good battery it'll leap across the bench , so don't do it.
  10. Dec 19, 2015 #9
    I realized what my mistake or source of confusion was.
    If I think of a very basic system, where the stationary magnet is a solenoid, and the moving part(rotor) are just some wires... where current flows in both of them they both generate their own separate magnetic fields(Ampere's law), and due to the current flow in an exterior magnetic field(solenoid's field to the rotor or vice versa) the charges will experience a Lorentz force.

    I broke it down, diagrammed it... thought about every single constituent's interaction to the fields and cleared my confusion, @davenn thanks for sharing the links and explaining things, but going back to the fundamentals and reviewing the system was the best solution for me, thanks @jim hardy for the contribution as always.

    That explanation was based on a system with a solenoid as the magnet(specifically electromagnet) and the rotor being a conductor, there is no magnetic attraction at all if it's all conductors and they repel each other due to the Lorentz force.

    The interaction between a moving charge(current flow) and an exterior magnetic field, will only lead to a Lorentz force so far that I know of, however, if we replaced the solenoid with a permanent magnet things change, there is a magnetic attraction force, and repulsive force depending on the polarity of the rotor's magnetic field.
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