Phase rotation
Let's take the three-phase alternator
design laid out earlier and watch what happens as the magnet rotates:
The phase angle shift of 120o
is a function of the actual rotational angle shift of the three pairs of
windings. If the magnet is rotating clockwise, winding 3 will generate
its peak instantaneous voltage exactly 120o (of alternator
shaft rotation) after winding 2, which will hits its peak 120o
after winding 1. The magnet passes by each pole pair at different
positions in the rotational movement of the shaft. Where we decide to
place the windings will dictate the amount of phase shift between the
windings' AC voltage waveforms. If we make winding 1 our "reference"
voltage source for phase angle (0o), then winding 2 will have
a phase angle of -120o (120o lagging, or 240o
leading) and winding 3 an angle of -240o (or 120o
leading).
This sequence of phase shifts has a
definite order. For clockwise rotation of the shaft, the order is 1-2-3
(winding 1 peaks first, them winding 2, then winding 3). This order
keeps repeating itself as long as we continue to rotate the alternator's
shaft:
However, if we reverse the
rotation of the alternator's shaft (turn it counter-clockwise), the
magnet will pass by the pole pairs in the opposite sequence. Instead of
1-2-3, we'll have 3-2-1. Now, winding 2's waveform will be leading
120o ahead of 1 instead of lagging, and 3 will be another 120o
ahead of 2:
The order of voltage waveform sequences
in a polyphase system is called phase rotation or phase
sequence. If we're using a polyphase voltage source to power
resistive loads, phase rotation will make no difference at all. Whether
1-2-3 or 3-2-1, the voltage and current magnitudes will all be the same.
There are some applications of three-phase power, as we will see
shortly, that depend on having phase rotation being one way or the
other. Since voltmeters and ammeters would be useless in telling us what
the phase rotation of an operating power system is, we need to have some
other kind of instrument capable of doing the job.
One ingenious circuit design uses a
capacitor to introduce a phase shift between voltage and current, which
is then used to detect the sequence by way of comparison between the
brightness of two indicator lamps:
The two lamps are of equal filament
resistance and wattage. The capacitor is sized to have approximately the
same amount of reactance at system frequency as each lamp's resistance.
If the capacitor were to be replaced by a resistor of equal value to the
lamps' resistance, the two lamps would glow at equal brightness, the
circuit being balanced. However, the capacitor introduces a phase shift
between voltage and current in the third leg of the circuit equal to 90o.
This phase shift, greater than 0o but less than 120o,
skews the voltage and current values across the two lamps according to
their phase shifts relative to phase 3. The following SPICE analysis
demonstrates what will happen:
phase rotation detector -- sequence = v1-v2-v3
v1 1 0 ac 120 0 sin
v2 2 0 ac 120 120 sin
v3 3 0 ac 120 240 sin
r1 1 4 2650
r2 2 4 2650
c1 3 4 1u
.ac lin 1 60 60
.print ac v(1,4) v(2,4) v(3,4)
.end
freq v(1,4) v(2,4) v(3,4)
6.000E+01 4.810E+01 1.795E+02 1.610E+02
The resulting phase shift from the
capacitor causes the voltage across phase 1 lamp (between nodes 1 and 4)
to fall to 48.1 volts and the voltage across phase 2 lamp (between nodes
2 and 4) to rise to 179.5 volts, making the first lamp dim and the
second lamp bright. Just the opposite will happen if the phase sequence
is reversed:
phase rotation detector -- sequence = v3-v2-v1
v1 1 0 ac 120 240 sin
v2 2 0 ac 120 120 sin
v3 3 0 ac 120 0 sin
r1 1 4 2650
r2 2 4 2650
c1 3 4 1u
.ac lin 1 60 60
.print ac v(1,4) v(2,4) v(3,4)
.end
freq v(1,4) v(2,4) v(3,4)
6.000E+01 1.795E+02 4.810E+01 1.610E+02
Here, the first lamp receives 179.5 volts
while the second receives only 48.1 volts.
We've investigated how phase rotation is
produced (the order in which pole pairs get passed by the alternator's
rotating magnet) and how it can be changed by reversing the alternator's
shaft rotation. However, reversal of the alternator's shaft rotation is
not usually an option open to an end-user of electrical power supplied
by a nationwide grid ("the" alternator actually being the combined total
of all alternators in all power plants feeding the grid). There is a
much easier way to reverse phase sequence than reversing alternator
rotation: just exchange any two of the three "hot" wires going to a
three-phase load.
This trick makes more sense if we take
another look at a running phase sequence of a three-phase voltage
source:
1-2-3 rotation: 1-2-3-1-2-3-1-2-3-1-2-3-1-2-3 . . .
3-2-1 rotation: 3-2-1-3-2-1-3-2-1-3-2-1-3-2-1 . . .
What is commonly designated as a "1-2-3"
phase rotation could just as well be called "2-3-1" or "3-1-2," going
from left to right in the number string above. Likewise, the opposite
rotation (3-2-1) could just as easily be called "2-1-3" or "1-3-2."
Starting out with a phase rotation of
3-2-1, we can try all the possibilities for swapping any two of the
wires at a time and see what happens to the resulting sequence:
No matter which pair of "hot" wires out
of the three we choose to swap, the phase rotation ends up being
reversed (1-2-3 gets changed to 2-1-3, 1-3-2 or 3-2-1, all equivalent).
- REVIEW:
- Phase rotation,
or phase sequence, is the order in which the voltage waveforms
of a polyphase AC source reach their respective peaks. For a
three-phase system, there are only two possible phase sequences: 1-2-3
and 3-2-1, corresponding to the two possible directions of alternator
rotation.
- Phase rotation has no impact on
resistive loads, but it will have impact on unbalanced reactive loads,
as shown in the operation of a phase rotation detector circuit.
- Phase rotation can be reversed by
swapping any two of the three "hot" leads supplying three-phase power
to a three-phase load.
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