Three waves that cancel
The trick behind the power lines you see on tall towers.
What you are looking at
The dots split into three groups, each moving in a wave. The three waves are shifted a third of a turn from each other.
The idea
Take three wheels that turn at the same speed, but start a third of a turn apart. Watch how high a point on each wheel is. When one is high, the other two are low. When you add up the three heights, you always get zero, at every moment.
This is very useful for electricity. Power lines carry a wave of electricity that goes back and forth. With three waves set up like this, the "back" part of each wave is provided by the other two. You do not need a separate return wire, and the power comes out smooth instead of in pulses.
That is why the big power lines you see on towers have wires in groups of three.
Where else you see it
- High-voltage power lines usually come in sets of three wires.
- Large motors in factories and trains run on three-wave power and start without any extra tricks.
Go deeper: the proper names and the math
For phases 0°, −120°, −240°: sin θ + sin(θ − 120°) + sin(θ − 240°) = 0 for every θ. As phasors, the three vectors form a triangle that closes, so they sum to zero. In a balanced load the neutral current is therefore zero.
The voltage between two lines is √3 times the voltage between a line and the neutral (for example, 400 V and 230 V). The total power delivered by a balanced three-phase load is constant in time, which gives smooth torque in motors. Three coils spaced 120° apart and driven this way also create a rotating magnetic field, the principle of the induction motor (Tesla).