Pass ONLY first 45 Degrees of AC Sine?

Discussion in 'General Electronics Chat' started by KlackyB, May 26, 2007.

  1. KlackyB

    Thread Starter New Member

    May 26, 2007
    4
    0
    This may be a strange question....

    Years ago at Home Depot I saw a device that claimed to increase efficiency of AC motors, making better use of the AC sine wave, and causing the motors to run MUCH more quietly, by taking standard AC wall voltage like this:

    [​IMG]

    And only outputting the first 90 degrees (push?) like this:

    [​IMG]

    I don't know if they were using SCR's, Triacs?

    But they had a demo unit there, with images of the sine waves like the ones I have included here, and an AC motor.

    One could puch a button for standard AC, and hear the motor's 60 Hz hum... ostensibly because of the downslope of the second 90 degree parts of the wave.

    Then one could punch the button that supplied their modified AC, and the motor would turn like before, but MUCH more quietly!
    Wondering if anyone has ever seen such a circuit, if it is hocus pocus, or if it is a viable, useful device. Would it save energy? Would the same amount of work still be able to be done? What kind of circuit could do this?

    Thanks!
     
  2. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    It may have been quieter but I suspect there was a loss of torque. Did the demo have the motor connected to some sort of mechanical load through a belt or other linkage?

    hgmjr
     
  3. KlackyB

    Thread Starter New Member

    May 26, 2007
    4
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    Not that I remember, I don't think so...
    They seemed to be implying that the second quarters (positive and negative) of the wave were pulling in the opposite, or wrong? direction, causing an internal "fight", wasting energy in the form of heat and noise....
    Thank you for your quick response!
     
  4. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    I would not support such allegation, at least with induction motors. The decreasing portions of the waves is also used to drive the motor. They won't pull the rotor to the oposite direction, but rather acompany the rotor rotation. In induction motors, the current on the stator induces a current on the rotor, with concurent magnetic fields. So, the rotor will always rotate so the magnetic fields will repel each other in a "pursue".

    It is not by chance that a sine wave can be described by a point fixed to a rotating wheel, if you consider only one axis.
     
  5. gootee

    Senior Member

    Apr 24, 2007
    447
    50
    I guess you actually meant 90 degrees, rather than 45.
     
  6. KlackyB

    Thread Starter New Member

    May 26, 2007
    4
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    Yes! 90 degrees... Oops.
     
  7. KlackyB

    Thread Starter New Member

    May 26, 2007
    4
    0
    So, just for curiosity's sake, what would be a simple circuit to duplicate the output as described. Anyone know?
     
  8. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    This could probably be done with MOSFETs and a clever driver circuit. One would need a zero crossing detector to turn on the appropriate transistor and a peak detector to turn it off.

    The motor would be more quiet because only had half as much power is going in. Not a very attractive trade-off.
     
  9. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You might use a GTO, wihch is a thyristor that may be turned off during conduction. Use a peak detector to generated the turn off signal.
     
  10. recca02

    Senior Member

    Apr 2, 2007
    1,211
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    yup,
    possibly a gto(gate turn off thyristor) or a smart use of thyristors perhaps.
    i was thinking abt working on a project abt variable frequency drive,if i do maybe i wud learn something more abt this.
     
  11. John Luciani

    Active Member

    Apr 3, 2007
    477
    0
    Since they are only powering inductive loads my guess is that the first 90deg of the
    voltage cycle a capacitor is being charged and the second 90deg the capacitor is
    used to drive the motor. This will improve the power factor which increases the efficiency.
    To determine the actual efficiency you need to look at the voltage and current at the load.

    Do a search on "power factor correction" and will find more information.

    (* jcl *)
     
  12. recca02

    Senior Member

    Apr 2, 2007
    1,211
    0
    now that i think about it .
    it doesnt make sense to supply only half of the wave to the ac motor.
    this will fail in creating a uniform rotating magnetic field in the motor.
     
  13. techroomt

    Senior Member

    May 19, 2004
    198
    1
    is a gated ac swirch device. perfect for supplying a portion of the sine wave. generally gated "on" at some point then they self commutate "turn off" at zero crossover, sometimes difficult with inductive loads. by reducing the time the power is on, power consumption is reduced and motor output power is reduced as well, but if that's all you need...

    i also have a project circuit schematic which is a motor speed control for small universal motors (ac/dc) with an scr in the circuit.
     
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