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Electric DC motor, commutator, carbon brushes, and magnets

  1. Apr 14, 2016 #1
    1. The problem statement, all variables and given/known data
    upload_2016-4-14_1-16-34.png

    2. Relevant equations
    no equations required

    3. The attempt at a solution
    so here are my answers, i just want to know if they are correct.
    also, im really confused about the function of the commutator in the DC motor, the answer i wrote for part b) was based on research off random websites that didnt really make sense, i dont actually understand how a commutator functions and its purpose, can someone please explain in simple terms.
    thanks for the help!

    78.

    a) the answer is Clockwise,
    i used the right-hand rule for the motor principle to figure out the direction of the magnetic force acting on the conductor. As shown below:

    upload_2016-4-14_1-14-47.png

    b)

    A commutator is a common part of most direct current-rotating machines. The commutator functions by reversing the direction of the current in the coil, in order to produce a steady rotating force. Together with the carbon brushes, the commutator ensures that the current flowing through the coil is in the same direction as the magnetic field, so the motor will rotate continuously.

    c) The Carbon brushes that are pressed against each side of the commutator are there to conduct the current throughout the motor. The main function of carbon brushes is to deliver or collect current from the rotating part of the electric motor. These brushes are pressed against the commutator, but not firmly attached to it, so when the commutator and coil begin to rotate, the brushes remain stationary, while still conducting electricity throughout the circuit.

    c) The whole concept of the DC motor is based on the idea that whenever a current carrying conductor (such as a coil) is placed within an external magnetic field, it will experience a mechanical force. How is this ‘external’ magnetic field established? This is where permanent magnets come into play. The permanent magnets simply create an external magnetic field that the coil’s magnetic field can interact with, resulting in a force that ultimately causes the conductor (in this case, the coil) to rotate.
     
  2. jcsd
  3. Apr 14, 2016 #2

    cnh1995

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    It's actually the left-hand rule. Right hand rule is for the generator principle.
    Just after the coil rotates through 90°, the commutator segments flip their polarity and current direction in the coil-sides is reversed. This ensures same current direction under each pole, which produces a undirectional torque and the motor keeps running. If there were no commutators, the motor would stop after 90° rotation. You can see it using the left hand rule.
    There are animations available on youtube which will help you visualize the motor action.
     
    Last edited: Apr 14, 2016
  4. Apr 14, 2016 #3
    the lesson im doing called it the right hand rule??
    upload_2016-4-14_8-58-2.png
    thanks for the explanation, but i have one question, why would the motor stop after a 90 degree rotation (assuming the commutator isnt there). isnt there still a force acting on the coil that would force it to move in some way, since it is placed withing an external magnetic field?
     
  5. Apr 14, 2016 #4

    cnh1995

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    I don't know where they got it from:wink:. But it's definitely the left hand rule. Right hand rule is for generator action. It follows from Lenz's law.
    The force on the coil reverses its direction just after crossing 90° because current direction remains the same but the coil is now under opposite pole.This pushes the coil back and the motor stops. The sole purpose of motor effect is to bring the coil in such a position that there will be zero torque and it will reach equilibrium. This position is reached after 90° rotation(no net torque since the forces are vertical) . It is like a magnetic dipole placed in external magnetic field, which tries to attain equilibrium. To keep the motor running, direction of current in the coil-sides is reversed by the commutators at the instant of change of pole. Commutators are also called "mechanical rectifiers".
    I believe an animation is worth a thousand words:smile:.
     
    Last edited: Apr 14, 2016
  6. Apr 15, 2016 #5
    thanks so much, the animation really helped! :smile:
    so once the motor spins to 90 degrees, the brushes will lose contact with the commutator, and the current will stop flowing through the motor. So what happens is that the coil spins until the left hand commutator makes contact with the right-hand carbon brush, and the right hand commutator makes contact with the left-hand carbon brush. This means the current is switched, allowing a continuous force to rotate the motor.
    this is my understanding so far, i also used this other helpful video, for anyone having trouble with commutators.
     
    Last edited: Apr 15, 2016
  7. Apr 15, 2016 #6

    cnh1995

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    Practically, the commutators don't lose contact with the brushes, instead, they momentarily "short out" the brushes at the 90 degree position, so no current flows through the coil. But it keeps moving due to its inertia and the commutators move ahead. Once the coil-sides get under the poles, motor action takes place again and the motor keeps on running.
     
  8. Apr 15, 2016 #7
    great explanation thanks :)
     
  9. Apr 15, 2016 #8

    cnh1995

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    Glad it helped!:smile:
     
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