3 phase power (was Re: Wireless home LAN - WiFi vs Bluetooth?
I was trained as an electrical engineer, and I'd love to be able to clarify
the situation, I'm not sure I can without using far too many words.
3 phase power (around the world as far as I know) consists of (usually) 3
wires carrying electrical power at a given frequency (in the US, 60 Hz., in
some other countries 50 Hz., and, once upon a time (and maybe still in some
places (including at one time parts of the US (or industries therein) at 25
hz.
A fourth wire may be a neutral wire (grounded or not), and there may be a 5th
wire as a safety ground).
The key thing is that those hot wires are carrying power that is 120 degrees
out of phase with the other two wires. (One of the key advantages of this
arrangement is that it makes it easy to create a rotating magnetic field which
is the key thing that makes lots of motors work (there are DC motors and
single phase motors that work on somewhat different approaches).
Most residential power in the US is created using a single phase transformer
(so called because (1) it only takes power from one of the 3 phases mentioned
above and (2) darn -- it's a bitch getting old. The secondary of that
transformer is center tapped with the center tap almost always grounded, such
that the other two taps from the secondary both produce 120 volts (RMS
nominal), but out of phase with each other by 180 degrees.
From the standpoint of the English language (at least the common American
dialects), I see nothing inherently wrong with calling those two hot wires
(the non-grounded end taps) as phases, because, they are, indeed, 180 degrees
out of phase, but that is not the common terminology.
More commonly it is just called (or what I call it) 240/120 volt single phase
power. If you need 120 volts, you connect from one hot tap to the center tap.
If you need 240 volts, you connect to the two end / hot taps.
I have seen diagrams in NEC code books for a different arrangement to get 120
volt 3 phase power, but I don't recall ever actually encountering that in real
life. In that case, iirc, 120 volt loads are connected from a hot tap to the
neutral wire (the 4th wire of the 3 phase arrangement), and you get
(nominally) 208 volts (RMS) connecting phase to phase. I have seen things
like motors that were rated like 240 / 208 volts (or something like that).
I hope this is at least somewhat helpful.
On Tuesday, July 30, 2019 10:26:23 AM David Wright wrote:
> On Tue 30 Jul 2019 at 14:30:50 (+0200), Matthew Crews wrote:
> > On 7/29/19 12:57 PM, David Wright wrote:
> > > On Mon 29 Jul 2019 at 20:43:04 (+0100), Joe wrote:
> > >> On Mon, 29 Jul 2019 10:26:14 -0500 John Hasler <jhasler@newsguy.com>
wrote:
> > >>> They don't have to be on the same branch circuit: just on the same
> > >>> "phase"[1]. There is probably a gadget available that bridges the
> > >>> signal between phases.
> > >>>
> > >>> [1] They aren't really phases but everyone calls them that.
> > >>
> > >> They are in my country. 3-phase, 240V RMS each phase to neutral, 415V
> > >> RMS between phases.
> > >
> > > Irrelevant in a domestic setting: it's illegal to have more than one
> > > phase in an ordinary house. Houses will have one phase each, so you'll
> > > share your phase with various neighbours scattered along the street.
> >
> > How do you figure?
>
> As I pointed out elsewhere, "my country" is the UK where domestiv
> voltages are twice those in the US. There are people who run small
> workshops where they have managed to install 3 phase supplies. If
> you read up their accounts, most of them are operating very much at
> the edge of legality with respect to building regs, planning laws
> (US zoning) and running businesses in domestic premises. I never
> got the impression that enforcement in this area was thorough.
>
> In large residential blocks, you get 3 phase supplied to the block,
> but they're typically split so that individual floors in a block
> of flats will all be on one phase. But you're bound to get exceptions:
> for example, our house was temporarily (over a year) connected to
> supplies from two different streets so running a vacuum cleaner across
> the bridge could be mixing phases. However, that was at US voltages,
> not UK ones.
>
> > In the US most 240V outlets are 3 phase, and they are
> > relatively common. You need them for most ovens, washing machines, and
> > electric cars.
>
> I'm not sure where you get that impression, but I suspect it's from
> counting pins on the appliance plugs. But look at the plate on the
> back of the appliance or the installation leaflet and you'll see
> they're 240v and they just take both 110v hot lines from both sides
> of the breaker box. That's two pins; the third is neutral and the
> fourth is earth. Obsolete 3-pin appliances have earth and neutral
> combined on a single pin.
>
> For cars, that makes them charge at level 2 (level 1 is through a
> normal 110v outlet, so slow). As for washing machines, in my
> experience, most here are hot and cold fill, whereas the UK has
> gazillions of cold fill. I would hate to pay for heating up a
> top-loading cold fill washing machine with electricity.
>
> That's not to say there are no 3-phase supplies in the US, but
> I've never come across them. As they're still 110v, you wouldn't
> see any difference as a visitor, of course. But you have a problem
> with 240v items like those above, because the voltage between any
> two hot lines is only just over 200v. As I say, I've not knowingly
> encountered it; perhaps the woman in Austin TX has. People who've
> spent all their lives here might know more.
>
> Cheers,
> David.
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