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DC motors

Discussion in 'Home Power and Microgeneration' started by mark Ransley, Oct 10, 2003.

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  1. Nick Pine

    Nick Pine Guest

    Leviton's 6639 AC motor speed control seems cheap enough.
    Grainger's sells it for $16.38. I think it's essentially a lamp dimmer.

  2. daestrom

    daestrom Guest

    Reduce power factor from 1.0 to 0.5, current to supply the same 'real' power
    must double.

    Losses in windings, wiring, transmission are I^2R, so double the current,
    four times the losses.

  3. daestrom

    daestrom Guest

    Uh, losses are I^2R, so double the current means 4X losses.
  4. daestrom

    daestrom Guest

    These types of AC motor speed control only work with 'universal' motors that
    have brushes (like hand-drills or 'skil' saws). They won't work for
    capacitor-start motors.

    Lowering the voltage to a universal motor (by phase-shifting a triac) works
    by reducing the voltage applied to both the field and armature windings.
    But induction motors will overheat if you try to get rated hp from them with
    much-reduced voltage. To change their speed requires a change in frequency
    (or a very specially build 'high-slip' rotor design).

    And universal motors aren't available above a small fraction of a hp

  5. Nick Pine

    Nick Pine Guest

    The Grainger catalog says they are for "split capacitor or shaded-pole motors."

  6. Steve Thomas

    Steve Thomas Guest

    You are absolutely right.
    I can be a bit foggy before the morning coffee.
  7. Mike Glover

    Mike Glover Guest

    *doh* (slaps head) You're right, or course. Thanks.
  8. Steve Spence

    Steve Spence Guest

    a ac motor may be 80-90 % efficient. if a dc motor was 50 more efficient,
    that's only 90-95% total. dc motors tend to be lower voltage requiring
    heavier wiring for a given wattage. DC is good in mobile applications, or
    off grid cabins where pv is used for generation, where ac does not exist. In
    ac applications, ac would have to be rectified to dc, and incurs a energy
    loss (heat).
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