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Armature windings?

Discussion in 'Electrical Engineering' started by Airy R.Bean, Feb 3, 2005.

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  1. Airy R.Bean

    Airy R.Bean Guest

    Not having a background that involves electrical
    (as opposed to electronic) engineering....

    In the armatures of motors, each commutator segment
    seems to be the end of more than one coil. In addition
    to the coil that is currently across the brushes,
    another coil is connected; that other coil will terminate
    at an unconnected commutator segment from which a third
    coil is connected and arrives.......at the other segment in
    contact with the other brush.

    Now, I can see that this parasitic path will not give rise to
    any mechanical output, as the two coils are not well-disposed
    towards the magnets, but what effect does the energising of
    the parasitic coils have? Are they responsible for the weird
    wave-shape identified as "Armature Reaction"?

    In the case of an alternator, do these other parasitic
    paths cause a loss of output?
     
  2. daestrom

    daestrom Guest

    No.

    DC Armatures are generally one of two basic winding types.

    One is called 'LAP' winding. A wire comes from a commutator bar (segment),
    stretches over about 1/2 pole pitch (call it CCW), down a slot, back across
    one pole pitch (CW), up another slot, then across about 1/2 pole pitch
    (again CCW) and connects to the bar directly adjacent to the one where you
    started. The next coil starts from this segment, moves the same route, but
    through slots that are one-off from the first. This continues all the way
    around. So when brushes contact two bars one pole pitch apart (brushes
    being spaced 1 pole pitch apart), all the coils between those segments are
    in series, and the coil's MMF over'LAP'.

    The other type is 'WAVE' winding. A wire comes from a bar, stretches over
    about 1/2 pole pitch (again, let's go CCW), down a slot, just as before.
    But now, instead of moving CW across the back side, it moves CCW (in the
    same direction as when we first left the commutator). It then returns
    through a different slot, then continues on (still moving CCW) 1/2 pole
    pitch and connects to a commutator bar about 2 pole-pitches away. But not
    exactly 2 pole-pitch. Then a similar coil carries on from there (still
    moving CCW) 1/2 pole, down slot, 1 pole, up slot, and ~1/2 pole further to
    connect to the bar right next to the one that we started with at the very
    beginning.

    (each can be further categorized as 'progressive'/'retrogressive', as well
    as the exact pitch per coil. Also note that the direction the windings take
    moving around the armature from commutator bar to commutator bar has nothing
    to do with the direction of rotation of the machine.)

    Both of these types of windings have two wires connected to each commutator
    slot. Both of these have load current flowing through almost every coil
    during most of their rotation. While some coils are poorly positioned to
    generate any torque, most are in a position to develop some (depending on
    the local air-gap flux). The sum of all the forces from all the coils
    provides the total torque. It isn't just one 'active' coil at a time with
    the rest being 'parasitic'. In fact, the only 'parasite' is the coil whose
    ends terminate at commutator bars directly under the brush, all other coils
    are contributing to the total torque developed. Ideally, the coil that is
    shorted by having its bars under the brush would have its current reverse
    from full load current in one direction, to full current in the opposite
    direction before its leading bar loses contact with the brush.

    'Armature Reaction' is a term used to describe some of the various affects
    caused by the interaction of the MMF of the current-carrying coils of the
    armature with the MMF of the fixed field coils. The higher the load (thus
    stronger the armature current), the stronger the armature MMF is and the
    more distortion of the air gap flux.

    When a coil(s) is/are shorted by the brush spanning the gap between two
    commutator segments, it is desirable to have as small a voltage induced in
    the coil as possible to avoid sparking/burning of the commutator segments.
    So the coil sides should be moving in as low an air-gap flux as possible.
    This 'neutral' point can be shifted by the distortions of the flux caused by
    the armature's MMF. Various schemes exist to counter-act/minimize this
    problem so that the neutral point doesn't shift with varying load and thus
    sparking/burning of commutator bars is avoided/minimized. (shaded
    pole-tips, inter-poles (also called commutating poles), and compensating
    windings are schemes used in various situations)

    Small machines with few slots per pole often have poor waveforms. This is
    due to the heterogenous nature of the magnetic flux the coils pass through,
    and the larger voltage steps between commutator bars. Large machines, with
    large number of slots and commutator segments have smoother waveforms. The
    type of compensation used to improve commutation also plays a role. Very
    small machines, with no compensation will have very rough waveforms. And of
    course active sparking can generate all sorts of 'noise' including RF.

    daestrom
     
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