On 2/27/2012 7:40 PM, Edward Ned Harvey wrote:
Actually... I'm quite sure 120V is a single phase (relative to ground), and
240V are just two 120V circuits that are directly out of phase with each
other (2-phase) and 208V is three 120V signals that are each 120deg out of
phase with each other.
120V = 1 phase
240V = 2 phase
208V = 3 phase
This is almost correct. 208V 3 phase can be distributed in two ways. One
is what is called Delta distribution in which there are 3 legs of 120V
arranged in a triangle (delta) and a ground. You can only get 208V from
this circuit by going from one leg to another, and you balance across
the 3. You do NOT go from 208V to ground, and I think this point isn't
clear from your post. The ground is there for safety and is only
supposed to carry current during normal operation.
The second distribution is a called a WYE distribution and is because
the 3 phases + ground are supplemented with a 5th wire called a neutral.
The neutral rides in the space between the 3. You get 120V by going from
any 3 of the 120V legs to the neutral. The neutral is often sized
slightly larger than the 120V legs because of the inefficiency in power
supplies vis-a-vis the power factor correction and sampling. More modern
power supplies have a PF closer to 1, which is good. If power supplies
were perfectly efficient than the phase-to-neutral would cancel out on
the neutral as you balanced phases across the 3 for 120V machines. But,
this is not the case and a proper plan electrical distribution needs to
take this into account. However, nearly *all* modern computer power
supplies are auto-sensing and wide input able to accept from 100v to
140V and 200V to 250V at either 60hz or 50hz without any difficulty.
Running at 208V just makes sense for reasons mentioned earlier:
1) because converting every time you convert down you lose a small 1-2%
as heat.. You lose power as during every step and minimizing those
losses by running your equipment at the highest voltage it will take
makes sense.
2) higher voltage means lower amperage for the same number of watts,
this means less heat loss in the wire and also that you can use thinner
wire for the same amount of power. Put another way, you can have almost
twice as much* power over the same gauge of wire at 208V as at 120V.
(*the actual ratio is the square root of 3 or 1.73x)
references:
http://www.federalpacific.com/university/transbasics/chapter3.html -
this is a good reference for understanding the difference between Delta
and Wye. Wye requires an extra conductor and drives up your
infrastructure capital cost but gives you more flexibility.
Functionally, they both deliver 208V. There are differences in how the
windings come off of the transformer during the 480V to 120V conversion.
http://www.phaseconverterinfo.com/phaseconverter_deltawye.htm - this one
also adds 120v to 240v which is correctly identifies as two 120v that
are 180 degrees out of phase. (whereas 208V provides 3 legs that are 120
degrees out of phase)
So, why 3 phase anyway?
http://en.wikipedia.org/wiki/Three-phase_electric_power
It turns out that 3 phase is tremendously useful for motors. It makes
them self starting. Almost all commercial buildings, industrial or not,
have 3 phase power to them. Most homes have 2 phase.
http://commons.wikimedia.org/wiki/Category:3-phase_motor_schemes - this
is a brilliant image moving gif image showing how the rotating phases of
3 phase power are used in motors.
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