On Sat, 03 Aug 2019 17:28:24 +0100 mick crane <mick.cr...@gmail.com> wrote:
> On 2019-07-30 13:30, 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 > >>> <jhas...@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? 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 don't really understand how 3 phase motors work. Is normally wanted > if you have some heavy equipment like a car crusher. > As I understand each cable has 130 or something volts where the AC is > 120 degrees out of sinc so putting a resistance between one of the > cables and earth would give you just that ~130 volts. > Somehow a 3 phase motor must get the feeds back in sinc so you get > the 415 Volts. A three-phase motor is pretty much three single phase motors with a shared armature. The point about it is that the three magnetic propulsive forces arrive in a definite order, so the motor is always guaranteed to start and turn in the same direction every time. Single-phase motors must use dodges like extra windings linked to capacitors and even a partly short-circuited area of the ironwork (shaded pole) in order to start up and run in a particular direction. The three-phase motor doesn't need anything like this, so is conceptually simpler. Three phases allows lower currents in the cables than a single-phase motor of the same power, and can run on three wires only, without the need for a neutral wire. There's no need for any 'syncing'. In a three-phase power system, the voltage between two phases is always the single phase to neutral voltage multiplied by the square root of three. https://www.raritan.com/landing/three-phase-power-explained -- Joe
