[SBE] A/C 3phase vs single phase

[Dan Rapak]

Mike is correct about those current spikes.

When ABC Sports was still around we used single-phase air conditioning systems in our mobile units out of necessity. You occasionally ran into a location where three-phase was not available, but more importantly, if you lost a phase you still needed to keep on running. There are some locations around the country where loss of a phase, whether locally within a stadium or from the utility itself is not all that rare an occurrence. Our mobile units could readily operate on three-phase or single phase power.

To get around the problem of spikes and surges, we operated the air conditioning systems as "hot-gas bypass" systems. The compressors run all the time and the gas is shunted when cooling is not needed. Not very efficient, but it kept the network broadcasts on the air.

Dan Rapak



----- Original Message ----- 
  From: Mike Langner 
  To: sbe member discussion mail list 
  Sent: Thursday, June 11, 2009 11:31 AM
  Subject: Re: [SBE] A/C 3phase vs single phase


  Three phase versus single phase air conditioning continued --

  One further quick observation -- depending on the source impedance of your power and the starting current draw of a/the single phase motors, many of which are capacitor-start "across-the-line" starting, you can get some pretty interesting voltage peaks and valleys on startup with single-phase motors.  I've seen more than one transmitter site with surge damage traced to recurrent motor spikes.  With a high enough lighting load in a studio you may not have quite the problem, but with single-phase "across the line" starting, motors of any serious size will have amazing starting currents.

  Usually single phase fan motors are split-phase, which means they don't have much starting torque, but the also don't draw the horrendous starting current of a single phase, capacitor start motor.

  Just a quick point in history -- yes, there really were 2 phase systems -- although they were generally quite limited to specialized applications.  The RCA 5-F AM transmitter used 892R tubes as finals and as modulators.  The transmitters internally derived their 2 phase from 3 phase power for the 892R filaments.

  Conversion between 2 phase and 3 phase power can be done with a "Scott Connection" of transformers/windings, as can be seen in
  http://en.wikipedia.org/wiki/Scott-T_transformer.

  Interestingly, early in the development of power distribution, both 2 phase and 3 phase systems  were in operation.  3 phase is the most efficient, especially when the cost of copper wire is factored in, and it eventually won the "phase race" as we all know!

  Good luck !

  Mike/

  Mike Langner, CPBE
  Albuquerque, NM
  __________________________

    -----Original Message-----
    From: sbe-bounces at sbe.org [mailto:sbe-bounces at sbe.org]On Behalf Of Robert Denny
    Sent: Thursday, June 11, 2009 8:52 AM
    To: sbe member discussion mail list
    Subject: Re: [SBE] A/C 3phase vs single phase


    Chris,

    It's my opinion that three-phase machines are more efficient than single-phase machines.  For small loads, the difference is negligible and may not warrant the more expensive three-phase distribution system (that's why most residential dwellings use single-phase service), but for big loads, three-phase machines are the way to go, unless you need a servo or something fancy.  I don't think I've ever encountered a single-phase motor larger than 7 to 10 horsepower.

    Bob

     
    On 6/11/09, Chris Hoopes <hoopeschris at yahoo.com> wrote: 
            My hypothetical questions assumes both 3 phase and single phase are available. One being 3 phase 440, or 480 with 3 hots, or 220 with 2 hots and a neutral. For years I always believed 3 phase would produce more cooling BTUs for less power bill then a single phase system. Imagine my surprise when my boss tells me that there is no difference!
            Thanks for your comments.

            --- 

             
              Chris,

              Some of the smaller, inverter-type air conditioners are quite efficient, but they are limited in cooling capacity.  Three-phase motors are smaller and simpler in design than equivalent single-phase motors and present a more balanced load to the source.  For a relatively small cooling load, an inverter-type unit may be the way to go, but for large cooling loads where a lot of shaft power is needed three-phase machines may be the only reasonable choice.

              Note that while there is two-phase power, it is hardly ever used these days.  Commonly found today are three-phase loads and single-phase loads.  Generally, a load connected to two line (hot) wires in a domestic or commercial system (whether or not a neutral wire is used) is single phase load and not a two-phase load.  Three-phase loads have three hot wires and, in the case of a Y-connected load, three hot wires and a neutral.

              Regards,

              Bob Denny

               
              On 6/11/09, Chris Hoopes <hoopeschris at yahoo.com> wrote: 
                      It comes up every now and then that I am called upon to scope out new air conditioning systems. I always figured with both available, that 3 phase was more efficient. Less current is required to start a 3 phase motor. Smaller gauge wires, but more of them. But then the other day I was called down on this. I wondered if advancement in A/C technology had changed the fact. The way 3 phase is measured is a difficult concept, peak demand. What has been your experience? Is 3 phase cheaper to operate and cool with then a 2 phase system? Or is there no difference? 


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