Need a bit more info concerning PFC

Discussion in 'General Electronics Chat' started by R!f@@, Feb 6, 2011.

  1. R!f@@

    Thread Starter AAC Fanatic!

    Apr 2, 2009
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    Hi there Folks..
    I am quite buzy finishing the wiring of the ground floor. I get a few hours at night before going to bed to do my work.

    Right now I am fixing a PSU of a flat TV. The power switch blew taking the PFC circuit with it. As I was working at my house a thought came to me about PFC, so I am asking this and would be very grateful if any one can correct me or draw some light on to this.

    To my knowledge a PFC is used to correct the power factor or bring it as close to unity, right?
    So by doing this it is changing the load to a resistive circuit. this is the basic idea.

    But my dilemma lies about SMPS being a reactive load. I never gave this a proper thought before or did not do a detail research.

    Is the SMPS a reactive load?

    I am asking this cause from day to day I see the ever increasing use of PFC circuits in flat TV's and PC supplies.

    How does a SMPS produce out of phase current?

    One more question is does PFC circuit can be used to boost the output to 400VDC. Or is this type some thing else?

    Thanks in Advance.
     
  2. marshallf3

    Well-Known Member

    Jul 26, 2010
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    PFC = power factor correction but it's rare to find any signs of it in a TV, it's mainly used when you've got large inductive loads such as large 3 phase motors. Yes, they are starting to put them into PC power supplies but it's merely a cap to help resonate the input &/or output of a transformer.

    SMPS circuitry depends on the reactivity of an inductor to buck or boost a voltage, look it up.

    One more question is does PFC circuit can be used to boost the output to 400VDC. Or is this type some thing else?

    No, PFC is only to try and get the KVAR as close to perfect as possible. I've got a 250,00 ft*sq plant probably running several hundred HP worth of 480V 3 phase motors, most of which are on 24x7. Through careful planning I'm running around a 98% PF during the winter and a 96% during the summer as despite my adding PFC caps to the individual compressors it's hard to get everything perfect due to all the lighting going on and off during operating and non-operating hours. Nevertheless the power company is certainly pleased that I pay attention to it, and the life of my motors are increased by quite a bit having it as perfect as possible. I will get penalized if I go below 85%, this year they're going to try for 90%.

    You can trial/error or they'll normally gladly come out and point out where you can improve. Some will even pay you in the form of a check or a rebate for merely improving on your efficiency. Saves them money by not having to increase the capacity of their main lines, step-down transformers etc.

    http://en.wikipedia.org/wiki/Power_factor

    http://en.wikipedia.org/wiki/Smps
     
    Last edited: Feb 6, 2011
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  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Computer power supplies are now getting PFC to above 80-98% for a little more money.

    Most all PFC info you find concerns inductive loads, such as motors. For PC/Home Electronics Supplies, it has more to do with Not Chopping the Input, so the power is still a nice sinewave, even with a 1kW load from a computer.

    I'm not sure if it can be classified as leading or lagging, but more of a shaping. Older Power Supplies chop the wave into a distorted waveform, causing the supply system to be non-pure sine wave.
     
  4. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    How does that work?
     
  5. R!f@@

    Thread Starter AAC Fanatic!

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    Thanks Marshal

    So what I understood is that a PFC in an SMPS is used because of the choppers.
    The Current does not lag nor lead but is rather non-sinusoidal. Am I right?
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
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    From my limited understanding:

    SMPS requires a DC voltage source to "switch" into other voltages. Usually a simple rectifier or voltage doubler does this job of taking current from the AC mains and charges up a capacitor. The charging current is peaky and the SMPS at a whole does not have a good power factor.

    By using another IC chip to control/switch/gate the charging current to charge this capacitor such that the current taken from the mains is sinusoidal and in phase with the AC mains voltage, the SMPS will "Appear" to be an unity load.

    In short, the PFC circuit charges a DC capacitor using power from the AC mains in a way that appears to be resistive.
     
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  7. R!f@@

    Thread Starter AAC Fanatic!

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    Why does a SMPS does not have a good PF?
     
  8. marshallf3

    Well-Known Member

    Jul 26, 2010
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    You all need to study, if I recall I posted links and feel free to look up other ones.
     
  9. marshallf3

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    Jul 26, 2010
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    and the life of my motors are increased by quite a bit having it as perfect as possible.

    How does that work?


    Quite simple, they take less current input for the same amount of horsepower out, the waveform is closer to sinusoidal thus the internal heating is less.
     
  10. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    I assume your referring to before and after pf correction. I don't want to jump to any conclusions here, but I'm thinking that your thinking that power factor correction alters the pf of the motor?
     
  11. marshallf3

    Well-Known Member

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    It merely makes them more efficient. Less KVAR in = less wasted power = less heat = less wear.
     
  12. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    We must be talking different things.

    I'm talking about a motor that's drawing an apparent power lagging, and then add to the branch circuit corrective caps, who's reactance when combined with motors, result in a net reduction in the reactive component. This MAY, by code, allow me to reduce overloads, overcurrent, wire, and disconnects, on the line side of the correction, but not the load side because the motor is still drawing the exact same apparent power it always has.

    I'm not aware of any correction that alters the pf of the motor itself, but then I don't claim to be a master at the trade. Help me learn more.
     
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