or just continue thread for another 1000 posts:thumbsup:
ahem.... should i paste this? oh, why not......
http://en.wikipedia.org/wiki/Two-phase_electric_power
and for those of you who are so twizzled at this point that clicking
on the link is more than you can bear..... here is the text....
Two-phase electric power
From Wikipedia, the free encyclopedia
Two-phase electrical power was an early 20th century polyphase alternating current
electric power distribution system. Two circuits were used, with voltage phases
differing by 90 degrees. Usually circuits used four wires, two for each phase.
Less frequently, three wires were used, with a common wire with a larger-diameter
conductor. Some early two-phase generators had two complete rotor and field
assemblies, with windings physically offset by 90 electrical degrees to provide
two-phase power. The generators at Niagara Falls installed in 1895 were the
largest generators in the world at the time and were two-phase machines.
The advantage of two-phase electrical power was that it allowed for simple,
self-starting electric motors. In the early days of electrical engineering, it
was easier to analyze and design two-phase systems where the phases were
completely separated.[2] It was not until the invention of the method of
symmetrical components in 1918 that polyphase power systems had a
convenient mathematical tool for describing unbalanced load cases.
The revolving magnetic field produced with a two-phase system allowed
electric motors to provide torque from zero motor speed, which was not
possible with a single-phase induction motor (without extra starting means).
Induction motors designed for two-phase operation use the same winding
configuration as capacitor start single-phase motors.
Three-phase electric power requires less conductor mass for the same voltage
and overall amount of power, compared with a two-phase four-wire circuit of
the same carrying capacity.[3] It has replaced two-phase power for commercial
distribution of electrical energy, but two-phase circuits are still found in certain
control systems. Power transfer in a three-phase system with balanced loads is
constant, whereas it pulsates at twice the line frequency in single-phase systems.
These power pulsations tend to cause increased mechanical noise in transformer
and motor laminations due to magnestriction and torsional vibration in generator
and motor drive shafts.
Two-phase circuits typically use two separate pairs of current-carrying conductors.
Alternatively, three wires may be used, but the common conductor carries the
vector sum of the phase currents, which requires a larger conductor. Three-phase
can share conductors so that the three phases can be carried on three conductors
of the same size. In electrical power distribution, a requirement of only three
conductors, rather than four, represented a considerable distribution-wire cost
savings due to the expense of conductors and installation.
Two-phase power can be derived from a three-phase source using two transformers
in a Scott connection: One transformer primary is connected across two phases of the supply.
The second transformer is connected to a center-tap of the first transformer, and is
wound for 86.6% of the phase-to-phase voltage on the three-phase system. The secondaries
of the transformers will have two phases 90 degrees apart in time, and a balanced two-phase
load will be evenly balanced over the three supply phases.
Three-wire, 120/240 volt single phase power used in the United States and Canada is
sometimes incorrectly called "two-phase". The proper term is split phase or 3-wire single-phase.
The two live outputs of a 3-wire single phase transformer secondary winding are properly called "legs".
true two phase 4 wire systems are like a two cylinder engine, with the power pulses 90 degrees
apart.
think harley.
