Re: [CAD_CAM_EDM_DRO] Re[1]: FYI: US Home Voltage system

one thing to keep in mind is the ambiguous term, "180 degree phase shift".

revisiting three phase circuits...each phase is delayed with respect to
each other by one-third of a cycle (or 360 degrees divided by 3 = 120
degrees). the phases are actually delayed (or shifted) by 120 degrees each
so if one phase is the reference, the second is at 120 degrees and the third
at 240 degrees.

but with a center tapped transformer there is no phase shift (or delay)
between the two end points, rather it is their _polarity_ that is opposite.
using the traditional US colors of red, white (neutral) and black, this
means that the polarity of the red waveform with respect to the white
wire is opposite that of the black waveform.

comparing at a drawing of a sine wave (original signal) and a sine wave
shifted (delayed) by 180 degrees, this looks the same as the original sine
wave with inverted polarity. thus common usage has become calling each
of the two 120 volt circuits a "phase".

in the US we do commonly supply 3-phase currents at 120 volts potential
for office buildings where each phase does measure 120 volts to the
neutral wire (the center of a "Y" connected transformer array). but
since the potentials are actually phase-shifted, a voltmeter placed
between two of the wires reads (the vector sum) or 208 volts.

so with true phase-shifted potentials you don't see the simple arithmetic
sum of the two voltages, since the peaks do not occurr at the same time,
you see the relative potential difference between them.

back to the center tapped transformer for home service, you do add each
120 volt potential together and get 240 volts since they are _not_
phase shifted, but merely copies of each other with opposite polarity,
and thus the peaks occurr in both wires at the same time.

it would be as nice to start over again and eliminate the usage of
180 degree phase shift to describe these household circuits, but that will
unlikely never change, so you have to keep in your mind that _in this case_
180 degree phase shift doesn't literally mean a delay of a half cycle,
since inverting the polarity of a signal does not introduce a time delay
at all.

###

note that we do use two-phase circuits to run motors, the stepping motor
being a good example of a two-phase motor with the second phase delayed
by 90 degrees. (the term four-phase applied to a stepper motor does not
properly refer to the power supplied to the motor, but describes the four
"phases" of a motion cycle, i.e. there are 200 "steps" or "phases" with
a 50-pole motor).

when used as a "driving" motor or a syncronous motor, the steppers with
windings designed to operate directly off the 120 volt powerline use
a capacitor to shift the phase so that the power supplied to the motor is
two phase.

single-phase motors in your shop that have capacitors for starting, use
the capacitor to shift the phase of the power supplied to the "start"
winding, thus are more properly called two-phase start, single-phase run
motors. true two-phase power is required to cause the motor to turn.

connecting the run winding to one phase of the household circuit and
the start winding to the other "phase" would not cause the motor to turn
at all since there is no delay in the second "phase". the motor would
simply sit there and hum loudly as it heats up. the household circuit
cannot _properly_ be called "two-phase".

###

i hope this explanation helps a confusing situation where the terms
commonly used do not literally describe the actions of the circuits.

-ron

On Tue, 3 Oct 2000 wanliker@... wrote:

> In a message dated 10/3/00 3:24:38 PM Mountain Daylight Time,
> Ian@... writes:
>
> << realisation from what has been said that you may have two phase power
> to your homes and, even more surprising that both phases may be available at
> a wall outlet! Can this be true?? >>
>
> The power in the US is derived from a center tapped transformer, the center
> tap is grounded, and the centertap and both hots are taken into the power
> panel.
> The two hots are at 120 volts potential with reference to the centertap, and
> 240 volts with reference to each other, 180 degrees phase difference. The
> centertap is grounded, but is a current carrying wire if the loads on the 120
> volt lines are unbalanced. It is the neutral in the system. Its color by
> code is white.