Why is three phase used?
Several people mentined the reason (motors) , but didn't go further. To
get a motor to turn you have to generate a magnetic field that "turns".
With three phase this can be done. The relative voltage/cirrent between two
phases (lines) can each drive a winding and the result is a rotating
magnetic field inside the motor. This will then "drag" the rotor around.
With the single phase motor, this rotation must be made some other way.
With the capacitor start/run motors, the cap provides some phase shift on a
second winding giving a pseudo two phase supply and therefore some rotation
in the field. The cap-start motors do then run on one phase when the cap is
switched out. Thus the comments on "running a three phase motor on 2 phases
(actually this is just one phase when one of the three lines is cut -- the
"three phases" needed for the motor are not the three lines, but the phase
between each TWO line-pairs...of which there are three pairs) .
Shaded pole motors use a part of the field core which is separated from
the main core.. You will see a single turn of large copper wire wound
around a small section of the core. This "shorted turn" causes a lag in the
field in that section of the core and a resulting rotation of the composite
field. These are usually in small blowers like typically found in bathroom
fans. Go to the hardweare store and look at one of these motors.

Steve K;9;D:C:I
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