[SBE] A/C 3phase vs single phase

[Robert Denny]

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<http://us.mc329.mail.yahoo.com/mc/compose?to=hoopeschris@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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