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External magnets to PMDC motor

  1. Jan 8, 2016 #1
    generally in the pmdc motor the armature will rotate.As it works with the help of magnets ,if i provide external magnets to motor ,there will be any change in the rotation of armature?
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  3. Jan 8, 2016 #2


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    It might seem surprising but the stronger the magnets the slower it will go.
  4. Jan 8, 2016 #3


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    In principle: Yes.

    If you strengthen the magnetic field in the airgap, you will increase the motor constant, Km . Thus you will increase the back-emf in the motor [ V/ω ], which will make the motor turning slower at constant supply voltage ( unloaded ).

    At the same time you will increase the torque [ Nm/A ] at constant supply current.

    However I don't think you can improve the motor performance significant, because the manufacturer has already optimized the magnetic circuit of the motor.
  5. Jan 8, 2016 #4


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    Increasing the motor constant allow you to reduce the number of turns and yet achieve the same rpm. Reducing the number of turns on the armature reduces the resistance and that reduces the heat loss ("copper losses"). This is why motors with powerful rare earth magnets are more efficient than motors with weaker ferrite magnets.
  6. Jan 8, 2016 #5
    thank you
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