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DC Motor question Please help?

  1. Mar 29, 2010 #1
    DC motor question:

    Do the permanent magnets make a motor more efficient as opposed to using coils for the field magnets? Are the permanent magnets responsible for half of the force in which propels the shaft?
    Do the magnets perform half the work and the electric the other half?

    Thanks so much for any help you can give me.
  2. jcsd
  3. Mar 29, 2010 #2


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    Staff: Mentor

    Welcome to the PF. What is the context of your question? Is it for a school project?
  4. Mar 29, 2010 #3
    No. Its not for a school project. I think I know the answer but I want to be sure.So I figured I would try asking some Electrical Engineers.

    Thanks for any help you can give me.
  5. Apr 3, 2010 #4
    Is there anyone out there who can answer this question?
  6. Apr 3, 2010 #5
    Yes, no, and no. I'm not an electrical engineer, so you should wait for another reply. I'm like you, I just want to see if I'm right. :)

    What do you think is the correct answer?
  7. Apr 3, 2010 #6
    Well its just one question put into different words so I think its NO the magnets don't do anything because Magnet Only motors can never work. But my friends say that the magnets' force makes the shaft spin from both electro magnets and permanent magnets and since the permanent magnets dont require a battery and their force helps make the motor spin, they are why the motor is more efficient because they do half the work. And other freinds say that electric does the work not magnets and there are tiny lightning bolts exploding making the shaft spin just like a gas powered motor has tiny explosions.

    Electrical Engineers don't seem to know the answer either.Only because so far no response from any of them.
  8. Apr 3, 2010 #7
    There is an efficeincy improvement when going from feild windings to magnets, but it's not because of any magic property of the magnets.

    It's simply that feild windings are made of wire and wire has resistance. Thus the current that goes through the feild winding contributes heat. Making a beefer feild winding can improve it's efficeincy, but there's only so much size / weight one can tolerate.

    Does that mean that brushless magnet motors are the best? Well, they are are if you can spare no expense and keep them operating in a specific range (torque vs speed). I've designed a goodly many BLDC motor controls (got one behind me), and I love their efficeincy and the ease at which you can control them.

    But, when it comes to EV's they have a few serious disadvantages. An obvious one is expense. It takes a fairly large BLDC motor and six really large switching transistors to make them go.

    Next, their torque tops out and they have an absolute speed limit for a given battery voltage. Thus, at low speed starts, the series wound motor can kick major behind. It can also contribute when the speed is high without a hard speed limit. Finally, the series wound motor only takes one large switching transistor, this makes for a much cheaper controller.

    There is one advantage of the BLDC that I don't know whether anyone is taking: it can be easily designed for regeneration. With a change to the firing pattern, the six transistor controller can do regenerative braking. This is commonly done in industry to break the motor, though the energy is just dumped into a resistor (heat), because it's a matter of some expense to get it back as useful AC power.
  9. Apr 3, 2010 #8
    The work preformed by the motor shaft is completely supplied by the electric current in the windings.

    You should be aware that 'work' is a reserved word in engineering and physics not necessarily having the same meaning as common usage.
  10. Apr 4, 2010 #9
    Earth Magnets used in DC motors do make motors more efficient. Technically they do contribute to the rotational force known as torque and RPM measurements. With out the Earth Magnets the motor would not operate unless replaced with coil field magnets. Not only is there a loss in the resistance of the copper wire windings. But Electricity must be supplied to them to become magnetic.

    Magnetic force is what propels the rotor whether they are Earth Magnets or Electromagnets. You can build an All Electromagnet motor. You can build a Motor using Earth Magnets and Electromagnets but you can not build an all Earth Magnet Motor simply because you can not “turn On/off” the Earth Magnets as we can with electromagnets. Brushes and commutators are responsible for this in brushed motors.
  11. Apr 4, 2010 #10
    Sure you can. Place pieces of iron in the magnetic paths. Use tiny little motors to insert and remove the pieces of iron, changing the flux around the loop syncronously with the armature. Alternately, heat and cool the iron through it's Curie point to obtain the same effect. You'd have to heat and cool really fast, or expect a very slow motor.
  12. Apr 4, 2010 #11

    "Use tiny little motors" You are still using electromagnets therefore this would not be an all Earth Magnet motor.
  13. Apr 4, 2010 #12


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    Staff: Mentor

    (he was kidding)

    Have you read your PMs?
  14. Apr 4, 2010 #13
    Sorta kidding and sorta not. There are all sorts of hopeful inventers you can watch on you tube trying to get energy from nothing by some clever arrangement of magnets. Some of these guys resort to static shielding of magnetic fields, not realizing they would have to put energy into the shields by moving them around against an opposing force.

    Good point. I'll stick to the slow turning Curie point motor. I imagine one magnet arranged perpendicular to a shaft. We can despense with the field magnets and just use three horseshoe shaped pieces of iron. The ends of each horse shoe are across the diameter traced-out by the magnet poles, and arranged 60 degrees apart. Each horse shoe in sequence, is brought below its Curie temperature. The magnet will turn to align across the horse shoe that is below its Curie temperature.

    This might be difficult to follow. It occurs to me that many soldering irons have a linear magnetic motor in the tip.

    Why isn't this a perpetual motion machine? I think the Curie temperature might be dependent upon the magnitude of an externally applied magnetic field.
    Last edited: Apr 4, 2010
  15. Apr 4, 2010 #14
    Building a motor like that might be fun for some but to me, I see nothing but mechanical issues that can not be over come.

    However, if one actually over came the mechanical issues and build such a motor successfully, would it really be obtaining energy from nothing? Not really. Hypothetically speaking, you would be using an existing force to generate it into another form of energy.

    Hydroelectric-power generation uses Earths gravity and weather. Is that obtaining energy from nothing? Again not really.
  16. Apr 4, 2010 #15


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    Staff: Mentor

    Thread closed temporarily until Hickman and EVmotorhead respond to my PM.
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