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Increasing motor's RPM + Torque with stronger magnetic fields?

Discussion in 'Sensors and Actuators' started by Dretron, Aug 2, 2012.

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  1. wannabegeek

    wannabegeek

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    Aug 17, 2011
    Sorry for late reply...
    I need to read this thread a bunch more times...

    1) Doesn't, back emf occur as a scalar times the angular velocity of the unloaded motor ?
    This force needs to be overcome and requires more torque. To make the forces balance, net force the motor provides is at least equal to the back emf plus force to turn a constant load.

    2)I was thinking that if the load is too much for a given motor, we use gears to get more torque and in exchange loose angular velocity, and KE_rot = 1/2mw^2 .

    @Dretron, I can't easily go back get the quote sorry, but I liked what you wrote about when the motor and load are at a steady state rpm, then the need for torque is only to overcome the back emf and the friction, which is presumed to be less force than to get it rotating from rest.

    I think this whole thread needs to move to physics.stackexchange.com
    because we need to see the equations and work out the frames of reference for the forces. It's pretty hard to do that with words. I know that the ideal cases can be analytic but that most of the time the torque equations for real loads are computed numerically.

    wbg
     
  2. john monks

    john monks

    693
    1
    Mar 9, 2012
    1) the back emf is directly proportional to the rotor speed.
    In an ideal motor with no load and no friction force is not even a factor. Your back emf is still the same as your rotor speed.
    2) is correct.
     
    Last edited: Aug 13, 2012
  3. Dretron

    Dretron

    101
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    Jun 9, 2012
    Ok, now I know the torque is the result of magnetic force. What about RPM? Isn't it from the same source? "MAGNETIC FORCE"?
     
  4. john monks

    john monks

    693
    1
    Mar 9, 2012
    The RPM is from the same source.
    Maybe another analogy will help.
    Suppose you have an ideal motor with no load. The RPM's will rise to a level such that the counter emf will equal the source voltage . At that point the source current will drop to zero amperes and the rotor will be turning at a fixed amount.
    Now suppose that we Forget about voltage and put in 1 watt of power. The shaft will be increasing in speed, the kinetic energy of the shaft will be increasing at a rate of 1 joule per second, and the counter emf will be increasing the same as the supply voltage. In physics we call this the conservation of energy, a proven law of nature.

    Now let's hold the shaft still. In this case the input current will be steadily increasing and the voltage will be decreasing in order to maintain 1 watt in. This energy will increase in the windings in the form of magnetic energy like in an ordinary conductor. And in the long haul the motor will look like a dead short because no counter emf is being generated because the shaft is not turning.

    So as you can see in an ideal motor the input begins to look like and open circuit with no load and the input begins to look like a short when the shaft is held still. These are the effects of counter emf.
     
    Last edited: Aug 13, 2012
  5. Dretron

    Dretron

    101
    0
    Jun 9, 2012

    Thank you! All I needed to hear that BOTH Rpm& Torque are from the same source which is the magnetic force acting on the rotor. It depends on certain circumstance how they work and are rated. :D
     
    Last edited: Aug 13, 2012
  6. Dretron

    Dretron

    101
    0
    Jun 9, 2012
    You can also add that :granted if the magnetic field's strength increased so as the C-EMF if the previous "weaker" magnetic field was capable of generating 2V of C-EMF and the magnetic field doubled so will the C-EMF so it would be 4V of C-EMF, now as the C-EMF voltage increases, instantly MORE current is drawn from the input, main reason for that is because the C-EMF acts as a resistance, sometimes it can be helpful but now its clearly acting as a resistance and the motor HAS to overcome it. So theoretically, I can say that increasing magnetic field would certainly increase rpm. HOWEVER, it would cause more input current due to the increase of C-EMF that would lead to a inefficient improvement and more heat to be generally generated.

    By the way, when increasing the magnetic flux does that change the speed or torque?


    Dre
     
  7. john monks

    john monks

    693
    1
    Mar 9, 2012
    Increasing the magnetic flux will increase your speed and force depending on how you increased your magnetic flux.
    You can increase your magnetic flux by increasing the supply current by means of increasing you supply voltage. This will increase your speed and force. Or you can apply a greater load to the shaft. This will cause the c-emf to decrease and consequently your current to increase thus increasing your magnetic flux.
    The magnetic flux in an ordinary motor with no permanent magnets is due mostly because of the current in the coils.
    So increasing the voltage or loading the shaft increases the magnetic flux to increase.
     
  8. Dretron

    Dretron

    101
    0
    Jun 9, 2012
    Good point. Increasing the permanent magnet's magnetic field would also increase magnetic flux, however, would limit the motor due to the C-EMF generator by it.
     
  9. john monks

    john monks

    693
    1
    Mar 9, 2012
    That is absolutely correct.
     
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