why were phasors of two phase power chosen to be 90 degrees and not 180
degrees apart?
And also how would you decompose these "balanced" phasors into their
| why were phasors of two phase power chosen to be 90 degrees and not 180
| degrees apart?
if they were 180 deg apart, then it wouldn't be different from single phase
i think two-phase systems were created to provide revolving field for
motor starting
| And also how would you decompose these "balanced" phasors into their
| symetrical components?
|
Va = V /_0
Vb = V /_90
Vc = 0
That was the problem with (3 wire) two phase systems. The neutral carried
current. You could transfer more power by making the third wire another
phase of a 3 phase system. That's why modern power is 3 phase.
(There can be 4 wire and 5 wire 2 phase systems. The 4 wire system is
2 unconnected single phase systems at 90 degrees, the 5 wire is essentially
two Edison systems at 90 degrees connected at their neutrals)
would it be disadvantageous if we were to make power transmission in 4
phases or 5 phases? or is it only the cost factor which prevents us
from doing so?
Certain mine duty power systems were more than three phases in order to
smooth out the torques. There are lots of research going on in
academia on making more than three phase motors. Hamid Toliyat at
Texas A & M has a lot of work fgoing on with five phase machines. This
becomes feasible because we can use a power electronic interface to
convert three phases to four or five phases, whereas we would have to
change the entire distribution net works for the world.
Six-phase is easy to obtain by phase shifting using Y & Delta windings. That
is the principle of 12-pulse converters. Six are from the original voltage,
and six are phase shifted. It eliminates 5th and 7th harmonics, making the
current cleaner and easier to filter.
Ben Miller
I don't think there is any advantage, and the disadvantage is we have more
things to break (4 or 5 hot conductors, 4 or 5 transformers instead of 3
each time we need to step the voltage down or up)
Having said that, polyphase has been done. Somewhere on the net is a
photo of two bizarre transmission line towers. One carries 6 phase, the
other 12 phase. The conductors are arranged in a circle.
On 3/13/06 1:50 PM, in article dv4pf4$hm7$ snipped-for-privacy@pcls4.std.com, "Michael Moroney"
I do not see any advantage in going to more than three phases for power
transmission. It is true, however, that if you need to double up on the
transmission line, going to six phase (two three-phase systems shifted by
60° from one another is not a big deal.
The advantage of more phases arises when rectifiers are used. More phases
will usually end up with less ripple and conductor currents that are closer
to sine waveforms.
Such systems are popular for pulsing particle accelerator magnets. Kinetic
energy is stored in a rotating machine. Upon thyristor rectification, the
voltage is applied across a magnet winding. Current rises linearly with
time. After the peak of the magnetic field is reached, the thyristors are
triggered to provide inversion so that the inductive energy goes back into
replacing energy extracted from the rotating machine.
Bill
-- Ferme le Bush
There have been 6 phase distribution systems built. The advantage is that
the line is more compact than it would be for 3 phase. phase to phase
insulation is the same as phase to ground. I know it was done in some urban
areas but I don't know whether it is still being done.
On Mon, 13 Mar 2006 21:50:28 +0000, Michael Moroney wrote:
The gain from the additional phases drops significantly after three
phases. It takes two wires to carry a single phase, three to carry two,
three for three phase, and above that the number of wires/phase is
constant, IIRC. Thus, there is no advantage to distributing more phases.
Certainly there may be reason to have more than three phases, but cost
isn't it. ...at least AIUI.
If Va=V @0 andVb= V @90 then you only have one component.
if the Va and Vb are unbalanced in phase or magnitude then they can be
resolved into two two phase balanced sets in a manner similar to that for 3
phase.
One is forward rotating and the other backward. This is done for
modelling of a single phase motor as an unbalanced two phase device.
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