house outlet ac power

[Caliaxy]

It's not easy to build a generator that delivers square wave. Yeah with inverter from DC power it's easy but not so with an engine or other mechanical source turning an alternator.

And imagine all those harmonics…

[IanB]

Why isnt all ac power a square wave,  makes it all simpler.

Because a generator rotates in a circular fashion, and the sine and cosine functions are called the circular functions. It naturally follows that a generator produces a sine wave output.

If you could work out how to make a square wheel, and how to make the interior of a generator to be square shaped instead of round, then you could make a generator produce a square wave output.

[IanB]

What difference is there between two phase and split phase?

Mathematically, split phase has two phases. If you want N balanced phases, you divide the circle of 360° or 2π into N equal divisions, and that is the angle between phases.

So two phases are 180° apart, three phases are 120° apart, four phases are 90° apart, and so on.

But to electrical engineers, having two phases 180° apart lacks utility, so they reserve the term "two phase" for cases where the two phases are not balanced.

So split phase has two equally distributed phases, but it is not "two phase"

[CaptDon]

IanB, that is an excellent description of 'split phase' vs. 'two phase' and in fact in one of my Tektronix TM (Transport Module 500 series TM506 specifically) operators manual it indicates "If running from 240vac the unit is NOT designed to operate on two phases of a 3 phase circuit, (typically a 208 Y connection) but intended to be operated from single/split phase". We had a transformer (around 3kva if I remember correctly) in a lab and it always ran hot and sounded 'angry' with a weird raspy buzz. Turns out it was a 120/120 dual primary and they wired it for 240 which was o.k. in itself, but they hooked ALL THREE wires, the far ends and the center tap to 208Y !!!! Center tap was connected to Neutral!!! After pulling the center tap off the neutral connection the transformer was much happier!!
I am sure that transport module would get really pissed off with a 3 wire connection to 208Y since it also has a big transformer inside!!!

[Shock]

Single, two and three phase systems are 120 degrees out of phase.

How can a single phase be out of phase?

Depends where you live but you can have two neighbors with out of phase single phase systems. Typically single and three phase systems in the street are derived from the same wiring. Or you might get a whole street on the same phase supplied via a transformer. Next street over, different phase. Two phase systems I suspect are legacy, probably just saves running a wire for a small premises that needs two out of three phases.

[tooki]

Why isnt all ac power a square wave,  makes it all simpler.

Start with the fact that a square wave is a sum of infinite sine waves: a sine wave at the fundamental frequency, plus sine waves at all the odd harmonics (e.g. if your square wave is 100Hz, then it’s composed of the sum of sine waves at 100Hz, 300Hz, 500Hz, etc. theoretically to infinity).

[fordem]

What difference is there between two phase and split phase?

Mathematically, split phase has two phases. If you want N balanced phases, you divide the circle of 360° or 2π into N equal divisions, and that is the angle between phases.

So two phases are 180° apart, three phases are 120° apart, four phases are 90° apart, and so on.

But to electrical engineers, having two phases 180° apart lacks utility, so they reserve the term "two phase" for cases where the two phases are not balanced.

So split phase has two equally distributed phases, but it is not "two phase"

I would change one line in this "definition" - half a line actually - they reserve the term "two phase" for cases where the phase angle is not 180°

Single phase will have a single hot line & a neutral, split phase will have two hot lines & a neutral, the phase angle between the two hot lines will be 180°, two phase will have two hot lines & a neutral, with a phase angle between the hot lines of anything other than 180° (usually 120° or 240°), three phase will have three lines (there may or may not be a neutral) and the phase angle between the hotlines will be 120°

[bdunham7]

two phase will have two hot lines & a neutral, with a phase angle between the hot lines of anything other than 180° (usually 120° or 240°)

That would be two phases of a three-phase system, typically seen in apartment buildings with 208/120 Wye 3-PH where the provide two legs for stoves, water heaters and electric dryers.

Two-phase, AKA the original polyphase, AFAIK just had 4 wires and no neutral, with the voltage on one pair of wires 90 degrees out of phase with the other. 

[IanB]

That would be two phases of a three-phase system, typically seen in apartment buildings with 208/120 Wye 3-PH where the provide two legs for stoves, water heaters and electric dryers.

I wouldn't like that system. 208 V has only 3/4 the power of 240 V 

[bdunham7]

That would be two phases of a three-phase system, typically seen in apartment buildings with 208/120 Wye 3-PH where the provide two legs for stoves, water heaters and electric dryers.

I wouldn't like that system. 208 V has only 3/4 the power of 240 V 

They make 208V elements for water heaters, dryers can work fine with a little less heat, stoves and ovens you just have to deal with it.  Electric motors up to 5HP are often rated for 208-240VAC single phase, but I can tell you that they aren't as happy at 208V.  I had a large commercial building that only had one large 208/120 service entrance.

[David Hess]

The last time I changed some water heater elements, they were intended for 208 and 240 VAC operation and marked with different power levels.

[mcz]

Why isnt all ac power a square wave,  makes it all simpler.

Start with the fact that a square wave is a sum of infinite sine waves: a sine wave at the fundamental frequency, plus sine waves at all the odd harmonics (e.g. if your square wave is 100Hz, then it’s composed of the sum of sine waves at 100Hz, 300Hz, 500Hz, etc. theoretically to infinity).

To give a bit more insight into why this is a bad thing:
1. Generators used to supply sinusoidal waveforms, mostly 3 phase AC since a hundred or so years.
2. The inductance and capacitance of the power transmission lines would filter out the higher frequencies of that square wave, rounding it off a bit.
2.1 same goes for transformers on the power grid.
2.2 this also needs a lot more reactive power which means more current on the transmission lines -> bigger lines -> more money
3. The transformer (those things get used a lot in power delivery) does not like high frequency. The higher the frequency, the more losses you get for a given iron core material. Cores are already designed to deal with these losses and reduce them to a point where a transformer has 99% efficiency at nominal power. But introducing higher frequency will up these losses and waste money and heat the transformer.. which would need bigger cooling.
4. Same goes for all the 3-phase (and also 1-ph) motors that get used everywhere in industry and run the western world when it comes to moving things.
5. 3 phase power with square waves would be all weird and wonky.

And it does not really have any significant, real world benefit. You get the downsides of DC with the downsides of AC combined and none of the upsides I guess.

[Brumby]

But to electrical engineers, having two phases 180° apart lacks utility, so they reserve the term "two phase" for cases where the two phases are not balanced.

Ah - so a "convention" is in place.  I've not heard of that being stated explicitly, but it is not unreasonable and it does fit in with previous usage I have encountered.

[Brumby]

Single, two and three phase systems are 120 degrees out of phase.

How can a single phase be out of phase?

Depends where you live but you can have two neighbors with out of phase single phase systems.

A bit outside the box I was thinking about - but that is a fair point.

Typically single and three phase systems in the street are derived from the same wiring.

This has consistently been my observation.  A pole transformer with a 3 phase Y secondary providing 4 wires down the street.  With 3 hot wires and a neutral, every house has a connection to the neutral plus a connection to 1, 2 or 3 of the hot wires.

Or you might get a whole street on the same phase supplied via a transformer. Next street over, different phase. Two phase systems I suspect are legacy, probably just saves running a wire for a small premises that needs two out of three phases.

Can't say I've seen whole streets on the one phase.  Whenever I've looked, they tend to alternate houses so that the number of houses connected to each phase is roughly the same.  Pick any 6 houses together and you'll likely see 2 on each phase.  (It seems these days, single phase house supply is more the norm.  Where I grew up, 3 phase to houses was common.)
Two phase houses are around, but not too common.

[TimFox]

Power distribution systems vary locally.
In my residential block in Chicago, a pole transformer (probably fed with 4000 V, single-phase) has a secondary at approximately 240 V, center-tapped, that feeds about 14 houses that share the alley.
At my former employer, a light manufacturing location, the original building was fed with 208 V (three-phase, line-to-line, with neutral) that provided three-phase and 120 V single-phase to the outlets and HVAC.
When the building was expanded, a second feed included 208 V and approximately 480 V three-phase from a second transformer.
At my employer, when we needed to consider options for powering equipment in the US, I found useful information at our local utility's website:  https://www.comed.com/SiteCollectionDocuments/MyAccount/MyService/ServiceAppCommercialIndustrial.pdf
The end of this form has check boxes for the readily-available options on page 3.

[bdunham7]

The end of this form has check boxes for the readily-available options on page 3.

Yikes! They still offer 120/240 3PH 4W, which has inherent power quality problems due to the built-in phase imbalance.  Last I knew at least some utilities would not provide that for new installations and for any decent size commercial building it is pretty standard by now to have 277/480 3PH for the large loads--including lighting--and then 120/240 1PH for the outlets, office equipment, etc.

[TimFox]

The other options on that line are far more common (240 VCT, 208 V + neutral 3-phase, 480 V + neutral 3-phase).
One of my co-workers doing some work on his old church building in Joliet was surprised to encounter the 240 V 3-phase with the center of one pair of lines grounded:  it makes a difference!
Note that one of the options on the second line (480 V, B-phase grounded) is not allowed in the City of Chicago:  this utility company covers much of Illinois.

[themadhippy]

you yanks really like to over complicate things,how many options? Here in the uk you get 230V ( its really    240 but in the name of harmony we pretend) single phase,or 415V 3 phase.If you  really ask nicely they might let you have 11kv.

[TimFox]

The three options I referred to are common here.  I assume the complete list results from the stock transformers available to the utility from manufacturers, to include replacement for older industrial equipment that is long-lived.  Variety is the spice of life, or so they say.

[bdunham7]

you yanks really like to over complicate things,how many options? Here in the uk you get 230V ( its really    240 but in the name of harmony we pretend) single phase,or 415V 3 phase.If you  really ask nicely they might let you have 11kv.

It isn't as difficult as it appears, the actual distribution is typically HV--4kV range on older systems, 12kV range on newer--right up to the customer and then there is a transformer at the point of use.  So the customer's actual service configuration is often just a matter of installing the specific transformer(s) and meter(s) required.  I have a single phase of 12kV right in my backyard.

[BILLPOD]

Good Morning, not to complicate things, but when the primary distribution is 7.2 KV 'Delta', then a 3-phase service has a
' WILD LEG' whose voltage could be anything.

[Zero999]

I was told that two phase has always been 90° out of phase. I've also heard the term being used to describe split phase and two phases, off a three phase supply, which only makes and difference, if there's a neutral.

[TimFox]

In a different application, that is the difference between a “selsyn” (3 phase) and a “resolver” (2 phase), which are AC-excited analog rotation sensors used in servos.  The three phases in the selsyn are 120 deg apart, and the two phases of the resolver are 90 deg apart.

[Shock]

Two phases are any two phases, split phase is one phase split in two.
These days if you are getting provisioned and have 3 phase on the street you will get 3 phase if you ask for it. As bdunham7 mentioned they may give you two phases in an apartment.

You can use phase to phase or phase to neutral. On split phase there is no phase to phase. In apartments you are likely limited to the body corporate and code. So don't be messing with your mains supply if you just discovered you have two or three phases. 

Someone mentioned 208V that sounds like phase to phase 120V. Here phase to phase is 415V.

[Brumby]

These days if you are getting provisioned and have 3 phase on the street you will get 3 phase if you ask for it.

I've just upgraded my service.  New meter box, new smart meters, new breakers .... and 3 phase (was previously single phase).

Reason: New plans requiring upgrade.  Major air conditioning upgrade and solar power projects in the wind - with possible upgrade to garage power and being able to deal with an EV (if that ever eventuates).