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Re: [power factor]

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  • jim_klessig
    Many motors do not have Start windings that are thermally suitable for having the winding continuously energized. Jerry wrote: switching power supplies like
    Message 1 of 11 , Jul 1, 2013
      Many motors do not have Start windings that are thermally suitable for having the winding continuously energized.
       
       
      Jerry wrote:
      “switching power supplies like computers have without Power Factor
      Correction are the opposite. They need inductors to balance them or
      more likely a built-in PFC circuit that adjusts its draw to come out
      very close to a perfect power factor (1.0)..”
      BTW this is not really the case.  
      An inductor will not really fix the power factor of a standard (cheap) switch-mode supply.
      The issue there is not just that the input appears capacitive,  but mostly that the current
      draw happens in a short spikes when the line voltage exceeds the capacitor voltage. No amount of phase shifting will fix that.
       
      R James (Jim) Klessig P.E. | Senior Power Systems Engineer |
      Electrical Reliability Services, Emerson Network Power
      jim.klessig@..., jim_klessig@...
      1876 Gwin Rd, Mckinleyville | CA | 95519 | USA
      T  (707) 839 8765 |Cell (707) 616 5509 | Fx (707) 839 8765
       
       
       
    • Jerry Durand
      ... What does what I wrote have to do with leaving start windings powered? -- Jerry Durand, Durand Interstellar, Inc. www.interstellar.com tel: +1 408
      Message 2 of 11 , Jul 1, 2013

        On 07/01/2013 09:45 AM, jim.klessig@... wrote:
        Many motors do not have Start windings that are thermally suitable for having the winding continuously energized.
         
         
        Jerry wrote:
        “switching power supplies like computers have without Power Factor
        Correction are the opposite. They need inductors to balance them or
        more likely a built-in PFC circuit that adjusts its draw to come out
        very close to a perfect power factor (1.0)..”
        BTW this is not really the case.  
        An inductor will not really fix the power factor of a standard (cheap) switch-mode supply.
        The issue there is not just that the input appears capacitive,  but mostly that the current
        draw happens in a short spikes when the line voltage exceeds the capacitor voltage. No amount of phase shifting will fix that.

        What does what I wrote have to do with leaving start windings powered?

        -- 
        Jerry Durand, Durand Interstellar, Inc.  www.interstellar.com
        tel: +1 408 356-3886, USA toll free: 1 866 356-3886
        Skype:  jerrydurand 
        
      • Alan Muller
        ... This is something I ve wondered about. Could you elaborate a bit? My observation has been that (resistance) split phase motors often have small wire in
        Message 3 of 11 , Jul 1, 2013
          At 04:45 PM 7/1/2013 +0000, you wrote:
           
          Many motors do not have Start windings that are thermally suitable for having the winding continuously energized.

          This is something I've wondered about.  Could you elaborate a bit?

          My observation has been that (resistance) split phase motors often have small wire in the start winding and that might heat up.  But most capacitor motors seem to have start windings of similar gauge to the run windings. The insulation system could be different though.  If a motors is "class [whatever]" with respect to the insulation system, does that apply to all the windings?  But all I know is from tinkering and reading Veinott's small motor book.

          I agree with Jerry on the need for power factor correction.  This seems to be an issue with CFL lights.  And inrush:  I put a row of them in my attic and one can hear the arc snap in the switch--I'd not be amazed if the contacts welded.  I haven't tried to measure the inrush but it must be considerable.  What can one expect when the entire lamp sells for a dollar?  But as more and more load is switchers, the issue seems real.  Does the EPA "energy star" program have a power factor requirement?  Do the much-hyped "smart meters" enable utilities to track the power factor of residential accounts?

          Alan

           
           
          Jerry wrote:
          “switching power supplies like computers have without Power Factor
          Correction are the opposite. They need inductors to balance them or
          more likely a built-in PFC circuit that adjusts its draw to come out
          very close to a perfect power factor (1.0)..”
          BTW this is not really the case.  
          An inductor will not really fix the power factor of a standard (cheap) switch-mode supply.
          The issue there is not just that the input appears capacitive,  but mostly that the current
          draw happens in a short spikes when the line voltage exceeds the capacitor voltage. No amount of phase shifting will fix that.
           
          R James (Jim) Klessig P.E. | Senior Power Systems Engineer |
          Electrical Reliability Services, Emerson Network Power
          jim.klessig@... , jim_klessig@...
          1876 Gwin Rd, Mckinleyville | CA | 95519 | USA
          T  (707) 839 8765 |Cell (707) 616 5509 | Fx (707) 839 8765

           
           
           
        • Alan Muller
          Yes, of course. I had a situation with a well pump at the end of a long, inadequately sized line, where adding capacitors made a huge difference in line
          Message 4 of 11 , Jul 1, 2013
            Yes, of course.

            I had a situation with a well pump at the end of a long, inadequately sized line, where adding capacitors made a huge difference in line current, the ability of the motor to start reliably, etc.  The alternative would have been to dig up the line and upgrade the conductors, at considerable cost.  But it took some playing around to size things properly, and I had no luck at all explaining it to people.  To most people, power factor seems a very obscure concept, hard to explain effectively.

            Old factories used to have, often enough, big synchronous motors driving stuff, which could be adjusted to take a leading power factor and balance things out.  But I don't see these motors in new places....

            At 10:22 AM 6/30/2013 -0700, you wrote:
             

            On 06/30/2013 10:08 AM, Alan Muller wrote:

            The increase in current from low power factor is "reactive" power.  It causes line losses, but is not "real," isn't measured by your kilowatthour meter, and you don't pay for it (in residential service) directly.

            But you DO need wires that can handle it.  Your motor may be idling at a few amps of real power but there's a bunch flowing back and forth through the wire all the time.  This is why in commercial use there's a surcharge for this, the power company has to supply wires, transformers, and meters that can handle the flow but can't charge you for it as "used" power.

            I heard from one small shop their new smart meter catches the highest "imaginary" (or "reactive") continuous power (over 10 minutes) and you're billed for the month based on that.  SURPRISE!  Sudden intense interest in capacitors.

            BTW, switching power supplies like computers have without Power Factor Correction are the opposite.  They need inductors to balance them or more likely a built-in PFC circuit that adjusts its draw to come out very close to a perfect power factor (1.0).  These are required in many other countries, eventually in the USA too.

            -- 
            Jerry Durand, Durand Interstellar, Inc. 
            www.interstellar.com
            tel: +1 408 356-3886, USA toll free: 1 866 356-3886
            Skype:  jerrydurand 
            
          • jim_klessig
            The insulation class is almost certainly the same, but irrelevant(ish). Starting windings are intended to handle higher currents, but only for a shorter time
            Message 5 of 11 , Jul 2, 2013
              The insulation class is almost certainly the same, but irrelevant(ish).
              Starting windings are  intended to  handle higher currents, but only for a shorter time period.
              The only thing limiting the current is the winding impedance & capacitor impedance.
               
              R James (Jim) Klessig P.E. | Senior Power Systems Engineer |
              Electrical Reliability Services, Emerson Network Power
              jim.klessig@..., jim_klessig@...
              1876 Gwin Rd, Mckinleyville | CA | 95519 | USA
              T  (707) 839 8765 |Cell (707) 616 5509 | Fx (707) 839 8765
               
               
               
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