Energy is energy. So it should cost the utility the same at the
generator. Three phase motors are somewhat more efficient, so they will
cost you somewhat less to run. But, until recently, having a three phase
motor (or other load) meant having a three phase service (more service
equipment, more expensive) and the utility will have to provide three
phase lines to the customer (higher capital costs).
Recently, with the advent of cheap solid state controls, it has become
possible to build appliances with three phase motors, driven by variable
frequency drives, supplied by a single phase supply. So now we can have
the best of all worlds.
Paul Hovnanian mailto: snipped-for-privacy@Hovnanian.com
It depends on too many things to say. In my area three phase is not even
an option for residences but you can get it in some agricultural
communities and I've heard that in parts of Europe residential 3 phase
is common. Here you only find it in commercial and industrial sites.
Three phase requires more complex gear to supply but the rate structure
is up to the utility.
One more conductor means two additional windings on the transformer,
sometimes two entire transformers, a more complex meter, more bus bars
in the service panel, one more pole on all the breakers. It's quite a
bit more complex.
Each EU country seems to have a single phase limit, i.e. the
max current you are allowed on a single phase supply before you
have to take a 3-phase supply (for which there is no limit).
In the UK, the single phase limit is 100A, which is enough for
most homes, so 3-phase into the home isn't very common. You can
ask for it though as it runs past in the street, and it will
be forced on you if you would otherwise need to upgrade a single
phase supply above 100A. In some other EU countries, the single
phase limit is as low as 20A, in which case just about everyone
has a 3-phase supply.
[email address is not usable -- followup in the newsgroup]
Standard service in North America is 200A 240V split single phase.
Larger houses occasionally have 400A with twin panels, particularly if
they were built in an area without natural gas for heat, and I've seen
as large as 600A with 3 panels in a very large house, all of that single
phase. It would be nice if 3 phase were standard but at least now
inverter drives for motors have become so cheap that we can have most of
the advantages of 3 phase power regardless of what comes into the place.
Are all three phases available for loads like motors, or are the branch
circuits just one phase? It's always interesting to see how this stuff
is done in other places, a friend in England sent me a bunch of pictures
of the electrical system in his house and I was surprised to see how
different it is from what we do here.
The three phases go the the main distribution panel.
Then branch circuits are wired from that as needed.
Normally there are three phase branch circuits and
single phase branch circuits. Three phase branch circuits
are used for heavy loads (stove, sauna, water heater)
and motor loads (more common in industrial applications than homes).
Single phase branch circuits are the most common for
most normal uses like normal electrical outlets, lights etc...
Tomi Engdahl (http://www.iki.fi/then /)
Take a look at my electronics web links and documents at
Oh, you can get three phases in just about ALL "communities." At least
along the major roads or wherever the power company locates its feeders.
Likewise, 3 phases are available for half or so of most important roads.
But the "side roads" only have 3 phases until the next road branch. Then,
typically, 2 phases go down the more important branch, and one phase goes
down the spur.
The same thing is true is more suburban communities and even the
distribution system is moderate density cities. LARGE apartment buildings
often have 3 phases with each unit getting 2 phases (120/208 service).
"Garden" apartments typically get a single phase for each section (120/240
Important/large new developments (hospitals, larger schools, shopping
centers) can justify new 3 phase wiring to the location regardless of the
The utility business is separating the power generation business from the
distribution business. Even some of the distribtion business is separated
with one company running the really high voltage ties and another handlilng
lower voltage distribution.
Any particular "utility" company is likely moving toward not owning both
power and distribution in the same market area. It makes it easier to
But where two distribution companies interchange or power generation
companies meet distribution companies, everyone in the deal perfers nicely
"balanced" loads. Thus, there is a push to get larger customers to
present a balanced load so that the utility doesn't have to mix and match
customers to what phases they are connnected to and in what order.
In the area where I live, almost everyone lives on "side roads" that
branch off the main arterial, nobody wants their driveway to connect to
a busy through-way. Houses are clustered in these neighborhoods, each of
which is supplied by a single phase branching off down at the main road.
I've never seen or heard of a residential 3 phase installation here.
That is common, the apartment complexes are downtown where everything
else is 3 phase too. The single family homes are in the hills and
valleys surrounding and between towns.
Essentially going back about 30-60 years when this was normal- advantages to
both utility and customer arose due to the utility being able to predict
future demand reasonably well I.e. deregulation in the power industry, a
relatively recent phenomena has, in most situations, increased cost to the
consumer because it encourages short term economic planning because of
uncertainty of future loads with multiple suppliers in a region.
Nothing has changed in this area. large customers will have enough load to
take advantage of the actual simplicity of 3 phase and it is also to the
customer's benefit to have balanced loads. In addition, large customers
generally have larger motor loads and single phase motors simply don't cut
the mustard compared to polyphase machines in terms of simplicity,
performance and also with regard to harmonics when electronic control is
There is somewhat additional investment in providing three phase. Single
phase motors will not be as efficient as three phase motors. In small
installation, it is probably simple and cheaper to just use single phase
motor, especially for units under one horsepower.
An old man would be better off never having been born.
And miss all that comes in between birth and old age (where I am now)?
Your probable estimate is right.
The economics involved, along with the history of supply development as well
as mass manufacturing has meant that fractional single phase HP motors are
cheaper than 3 phase motors. Try buying a 3 phase 1/4HP motor. If you can
find it, it will be more expensive than a single phase equivalent
I couldn't find a 1/4HP, but I've bought two 1/3HP 3 phase motors
recently and they were dirt cheap, of course they were surplus.
I suspect fractional HP 3 phase motors will get a lot more common over
the next decade or so as the inverters to drive them from single phase
have become so cheap and are getting cheaper still all the time.
I doubt they'll get too hard to find, they're showing up in consumer
white goods now. The biggest competition they have is the permanent
magnet brushless motor, another 3 phase machine that can be used in
similar applications. Switched reluctance motors are also fairly common,
harder to drive but as with the others, the electronics keep getting
better and cheaper.
That may be but it is likely just as economical to go to brushless DC in
this range as is the case with many applications that now exist- electronics
no more expensive- possibly cheaper in that there is no need for frequency
dependent voltage control and likely little difference in manufacturing
The comparison that would then be valid is that between brushless DC and
inverter driven induction machines in terms of performance and cost.
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