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Old 11th May 2012, 03:37 AM   #21
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Main reason for three phase power is motor torque is a constant, whereas with single phase motor torque is pulsating. The high vibration leads to low reliability, and in higher power applications the benefit of constant torque becomes a necessity. Conductor size reduction is a side benefit.
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Old 11th May 2012, 03:39 AM   #22
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In Australia we have 200kVA distribution transformers on the pole dropping 66kV delta down to 240V star. (415V between phases) Houses get phase A, B or C but they are all out there in the street. I actually have 3-phase - cost an extra $200 when the house got built 18 years ago. No questions asked. Normal supply is 1 x 80 amps but this is 3 x 60 amps.
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Last edited by Circlotron; 11th May 2012 at 03:41 AM.
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Old 11th May 2012, 03:41 AM   #23
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Originally Posted by benb View Post
For a stereo (or even mono) amp? It seems overkill.
Overkill has no place with diy stuff.
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Old 11th May 2012, 05:52 AM   #24
jitter is offline jitter  Netherlands
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Even though 3 phase has been available for many decades over here to residential areas, its usage is limited. In most (older) houses the main fusebox can hold three fuses but is equipped with only one because the meter, fusebox and wiring are only for single phase. But with 230 V mains, 16 A fuses/circuit breakers allow for 3,680 W (per subcircuit) which is more than enough for ovens, washing machines and laundry dryers.
The aim is to get a balanced load on all three phases, so a third of the houses is connected to L1, a third to L2 and a third to L3.

Having said that, I do know that there are some houses with all three phases connected, esp. newer buildings where the electrical system was wired with the 3 phase system in mind. But domestic usage is very limited to high power devices like induction cookers and the odd hobby workshop with 3 phase equipment (like e.g. >165 A MIG welders) . But the wall sockets in living rooms,etc. are always single phase and my guess is >95% overall residential usage is single phase.

Last edited by jitter; 11th May 2012 at 06:02 AM.
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Old 11th May 2012, 05:23 PM   #25
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hmm intresting, i did not know 3 phase is not available world-wide in homes, it is more than common here. I actualy managed in the mean time to speak with one of the employees of the electric serice company, and according to that person here single phase is highly uncommon
what a surprise.
And we have 60 hz main lines.
So the humm would be 360 hz or so.
My main reason for thinking about 3 phase was not the higher power available, but the chanse to get a smoother DC. But now that i know that 3 phase is uncommon in the majority of the homes world wide it is understandable that high fidelity stuff usualy uses only 1 phase.
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Old 11th May 2012, 05:42 PM   #26
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Quote:
Originally Posted by zigzagflux View Post
Wow, how this audio forum continues to degrade when power systems are discussed.
???

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Originally Posted by zigzagflux View Post
This is the most cost effective way to provide single phase; yes, they could install a transformer suitable for line-line single phase primary, but the extra bushing adds to the cost. Secondary winding simply has a grounded center tap, 120/240V.
On the east coast, this is true. On the west coast, it seems they use two bushing transformers. From SanFran to Napa, all I saw was two bushing..
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And we have 60 hz main lines.
So the humm would be 360 hz or so.
No. 60 hz 3 phase will produce 60 hz hum. What can change is the relative phase of the hum. Non linear devices can still produce harmonic content.

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Old 11th May 2012, 05:48 PM   #27
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I have seen numerous posts claiming that North America does not use the 3phase system and that they have a split phase system.
Poppycock.

Look outside at the cables strewn across the country. They are in groups of three for the three phases. Just like everywhere else that uses a 3phase system.
The lines on the top of the wooden poles along the street run 3-phase 7.2KW to earth (~13KW phase to phase) but the typical residential house transformer primary is connected to only one phase. The secondary is center taped for the Edison split phase 120VAC - neutral - 120VAC. This way only one wire is required to be run to a rural home that is far from the distribution line. For a neighborhood with several houses, they will each be connected to different lines to balance the loading on the 3 phases. There are some line to line residential transformers but they are the minority, extra cost is a reasonable sounding reason.


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On the east coast, this is true. On the west coast, it seems they use two bushing transformers. From SanFran to Napa, all I saw was two bushing..
I wonder if this has to do with seismic issues and grounding, or trying to keep more even loading on the lines.
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Last edited by CBS240; 11th May 2012 at 05:58 PM.
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Old 11th May 2012, 07:21 PM   #28
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I wonder if this has to do with seismic issues and grounding, or trying to keep more even loading on the lines.
I do not know. I do know that the single bushing used on the east coast seems to present a stray current issue that two bushing systems do not have.

I'd suspect grounding by itself...
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Old 11th May 2012, 11:30 PM   #29
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Originally Posted by jneutron View Post
On the east coast, this is true. On the west coast, it seems they use two bushing transformers. From SanFran to Napa, all I saw was two bushing..
You sure you're not looking at regulators or oil switches? They often have that same pole-mount design. Line-line is better from an engineering standpoint, but single bushing is no question the lowest cost (one less switching device, fuse, and arrester). For utilities cost will usually trump optimal design.

Significant application where I see two bushing single phase transformers:
1. Three xfmrs connected delta for a three phase system, where one can fail and the remaining two can be connected in open delta at reduced kVA.
2. Ungrounded or high impedance grounded three phase system, with no neutral available.

Perhaps you can provide pics, because I still have doubts about your claim. We are talking residences here...
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Old 13th May 2012, 12:17 AM   #30
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"bushing" is the kind of word that needs further definition. It's most common usages seem to be "grounding bushing" or "insulating conduit bushing".
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