Re: 3 phase power (was Re: Wireless home LAN - WiFi vs Bluetooth?
On Tue 30 Jul 2019 at 11:34:43 (-0400), rhkramer@gmail.com wrote:
> […]
> 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.
Your first paragraph answers the implied question: the lack of any
rotation. Rather similar to a steam locomotive with the motion on
the two sides incorrectly set 180° apart. If the locomotive came to
rest in front dead centre on one side (piston at forward extremity),
it would also be dead centre (backward extremity) on the other side,
and consequently unable to move in either direction.
Like a real locomotive, genuine '2-phase' electrical power has one
phase leading the other by 90°, and motors powered by it will be
self-starting. (Steam locomotives have double-acting pistons, so they
can choose the direction of rotation.)
Hence it's not useful for electrical engineers to call a split
single-phase supply by the term '2-phase' because it doesn't behave
in a similar way to all other n-phase supplies, like interconversion.
Cheers,
David.
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