[SBE] A/C 3phase vs single phase

[Mike Langner]

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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