Download Three Phase Power Generation

Three Phase Power Generation
3 Phase Alternating Current
The power of alternating current (AC) fluctuates. For domestic use for e.g. light bulbs
this is not a major problem, since the wire in the light bulb will stay warm for the brief
interval while the power drops. Neon lights (and your computer screen) will blink, in
fact, but faster than the human eye is able to perceive. For the operation of motors etc. it
is useful, however, to have a current with constant power.
Voltage Variation for Three Phase Alternating Current
It is indeed possible to obtain constant power from an AC system by having three
separate power lines with alternating current which run in parallel, and where the current
phase is shifted one third of the cycle, i.e. the red curve above is running one third of a
cycle behind the blue curve, and the yellow curve is running two thirds of a cycle behind
the blue curve
("3 phase alternating current," 2003)
3-Phase Generator (or Motor) Principles
All 3-phase generators (or motors) use a rotating magnetic field.
In the picture we have installed three electromagnets around a circle. Each of the three
magnets is connected to its own phase in the three phase electrical grid.
As you can see, each of the three electromagnets alternate between producing a South
pole and a North pole towards the centre. The letters are shown in black when the
magnetism is strong, and in light grey when the magnetism is weak. The fluctuation in
magnetism corresponds exactly to the fluctuation in voltage of each phase. When one
phase is at its peak, the other two have the current running in the opposite direction, at
half the voltage. Since the timing of current in the three magnets is one third of a cycle
apart, the magnetic field will make one complete revolution per cycle.
("Synchronus generators," 2003)
A Four Pole Generator
The speed of a generator (or motor) which is directly connected to a three-phase grid is
constant, and dictated by the frequency of the grid, as we learned on the previous page.
If you double the number of magnets in the stator , however, you can ensure that the
magnetic field rotates at half the speed.
In the picture to the left, you see how the magnetic field now moves clockwise for half a
revolution before it reaches the same magnetic pole as before. We have simply connected
the six magnets to the three phases in a clockwise order.
This generator (or motor) has four poles at all times, two South and two North. Since a
four pole generator will only take half a revolution per cycle, it will obviously make 25
revolutions per second on a 50 Hz grid, or 1500 revolutions per minute (rpm).
When we double the number of poles in the stator of a synchronous generator we will
have to double the number of magnets in the rotor , as you see on the picture. Otherwise
the poles will not match. (We could use to two bent "horseshoe" magnets in this case).
("Changing generator rotational," 2003)
Sources:
(2003, June 1). Changing generator rotational speed. Retrieved from
http://www.windpower.org/en/tour/wtrb/genpoles.htm
(2003, May 12). 3 phase alternating current. Retrieved from
http://www.windpower.org/en/stat/unitsac3.htm
(2003, September 19). Synchronus generators. Retrieved from
http://www.windpower.org/en/tour/wtrb/syncgen.htm