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3 pole dc motor with commutator?

Discussion in 'Electronic Basics' started by BobG, Mar 24, 2007.

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  1. BobG

    BobG Guest

    The standard slot car motor was a 3 pole mabuchi motor with two
    magnets on the side of the case. The poles were 120 degrees apart. I
    assume that every 60 degrees, the next pole was energized and pulled
    toward the closest magnet. Now consider replacing the commutator
    segments with slip rings... gnd, and coil a,b,c. Q1) If I energize
    coil a, does it rotate to the center of only one magnet? Either
    magnet? Or to one edge of either magnet? I assume if I energize the
    coils in sequence a,b,c the armature will rotate. I can compute the
    ampere turns of each coil. Q2)Is the max torque when the force of the
    electromagnet equals the force of the permanent magnet? Q3) Is the
    strength of the permanent magnet measured in gauss? webers? teslas?
    How many gauss in a 1"" magnet like this? And now the last stupid
    question... (yaaay!) Q4) How do I convert magnetic strength of the
    permanent magnet to newtons so I can calc torque? I think the force of
    the electromagnet is NIB, but the permanent magnet doesnt have any N
    or I, so I'm confused as usual.
     
  2. Rich Grise

    Rich Grise Guest

    If all you want is speed control, just use PWM. What you're suggesting
    could only have value as a learning experience; you can already get
    brushless DC motors off the shelf.

    Cheers!
    Rich
     
  3. BobG

    BobG Guest

    Hi Rich. I only converted the commutator to slips rings in my head as
    a mental what if. I'm trying to figure out how to compute torque so I
    can get a good tight spreadsheet on electric vehicle kwhrs per mile
    and argue with the ice heads. Let me rephrase my main question....
    there is a big honkin magnet in skycraft that is holding a big steel
    bolt straight up in the air on a chain. You can pull it away from the
    magnet sideways, but aint no way you're gonna pull it away from that
    bad boy on axis. Lets say you have to pull 50lbs just for talking
    purposes. I figure if I built a motor out of those things, and it had
    a 6in radius, There would be 25 lb-ft of torque just from the magnet
    attracting the iron pole piece. Now if I put enough NI (ampere turns)
    in the coil to equal the B from the permanent magnet, I assume I'll
    have 50 lb-ft of torque. If I double the NI in the coil again, if
    thermally possible, I dont get any more torque increase, right? I'm
    'swamping out' the permanent magnet?. So I want to calc how much
    torque I need to accelerate my EV at .3Gs etc.
    Anyone want to agree or disagree with this shade tree mechanic
    analysis of dc motor torque?
     
  4. (snip)

    All coils are energized at all times, except when a brush
    shorts across the gap between two commutator bars. Then the
    coil connected between those two sees no voltage. There are
    3 coils, one connected across each of the 3 commutator gaps.
    As long as the two brushes are in contact with only two
    bars, all three coils are energized in one of two
    directions, with either the whole supplied voltage across
    them, or half the supply (where there are two gaps between
    the brush contact points. When a brush shorts a gap, one
    coil gets no voltage, and the other two coils get full
    voltage (in opposite directions). A brush crosses a gap
    every 60 degrees, so you might make a timing diagram with
    waveforms representing the voltage across each coil, showing
    how every 60 degrees, either coil passes through the shorted
    (pick some number of degrees less than 60, say, 30 that it
    takes for the short to pass across the gap) situation and
    then has its voltage reversed.

    So you have 30 degrees with constant voltage on the coils,
    one getting full voltage and two getting half voltage in the
    opposite direction.

    30 degrees with a short across one of the half voltage coils
    (show that as zero volts) while the other coil that had half
    voltage now gets full voltage.

    30 degrees with the just shorted coil getting half voltage,
    reversed from what it last had, and the one that first had
    full voltage drops to half, also. The coil that popped up
    to full voltage in the last step continues to get full voltage.

    etc.
     
  5. BobG

    BobG Guest

    Thanks John P. I rewound slot car motors back in '68 in hi school, but
    I always thought in my simple mind that one needed 6 poles to get
    pairs energized simultaneously. Anyway, I'm glad to know more about
    how they really work, but can you help me with my torque question?
    That is, is the max torque possible when the pull from the coil
    matches the pull from the magnet?
     
  6. I might not be much help, since I am not very fluent in the
    units of magnetism. I have to look them up every time I
    have a problem to solve. But I can tell you that the peak
    torque from any armature pole (around which a coil is wound)
    occurs when that pole is in the crack between a pair of
    permanent magnet poles. In that place, it is simultaneously
    repelled by one magnet and attracted to the next. This is
    also the place it should be in the middle of its maximum
    voltage phase (in the diagram I talked about, earlier),
    since this is where the peak rate of change of flux through
    the coil occurs (assuming the motor is turning), and so,
    generates the peak generated EMF the supply must exceed to
    keep forcing current through the coil.

    One important limitation to torque any permanent magnets can
    produce is the field that it takes to demagnetize them. So
    if you have high coercivity magnet material, like neodymium
    iron boron, you may be able to push the armature flux above
    the magnet flux (actually pushing flux a little backwards or
    sideways from the permanent field), but if you have a motor
    made with low coercivity material (like some ALNICO that
    will be partially demagnetized if you simply pull the
    armature out of the field), the armature flux may have to
    stay well below the permanent flux to preserve the
    magnetization.
     
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