The Magnetic Attraction of Electricity
by
“The greatest danger
for most of us is not that our aim is too high and we will miss it, but that it
is too low and we reach it.” –Michelangelo
Have you ever held a magnet up to a television screen and watched it distort the picture? Don’t try this at home; it might damage your TV. Instead, go over to your neighbor’s house and try it.
Magnetism is a wondrous phenomenon. Besides being a cool way to mess with the TV, it’s also the driving force of electricity. Without magnetism, electrons would sit home on Saturday night with nothing to do—in the dark, for that matter. It’s the magnetic attraction between negatively charged electrons and positively charged protons that moves electrons through a conductor to transmit electrical energy.
A conductor, like a copper wire, is made up of atoms with very loosely bound electrons orbiting the nucleus. When the conductor moves at a right angle (perpendicular) to a magnetic field, the magnetic forces act on the electrons to move them through the wire, thus producing an electrical current.
That’s the principle behind an electric generator. It has a magnetic field built into its housing and a rotor that is turned by some type of applied force—it could be a steam turbine, hydraulic pressure, or combustion energy applying the force. The stator, or the part that doesn’t spin, is wound with copper wire, and the rotating magnets cause the windings to cut the lines of magnetic flux, thus generating a current flow. It’s as simple as that.
The windings – or armature – are stationary, but the rotor rotates 360° on its axis. As it spins, the magnetic field travels around the stator in such a way that the windings are sometimes perpendicular to the flux and sometimes parallel to the flux. But a current is only generated when the conductor is moving perpendicularly to the lines of flux of the magnetic field. Consequently, the current fluctuates between zero (when a winding is parallel to the flux) and the peak current (when it’s perpendicular to the flux).
When the rotor has traveled halfway around the axis, it
starts to reverse its direction relative to the winding. Each time that happens,
the current alternates in direction, thus producing an alternating current, or
AC. In
During a single cycle, as the current rises, falls, and then changes directions, it produces a waveform that looks suspiciously like one you probably saw in your high school trigonometry class. If you were paying attention in class then I won’t have to tell you that it’s a sine wave. So let me just tell you now: it’s a sine wave.
A sine wave is a close relative to the circle in that it exhibits certain characteristics in common with or derived from a circle. Both are repetitious and have a cycle of 360° before they start repeating. If you were to plot the x and y coordinates of a circle on the Cartesian plane (an x-y plot), the value of the x-axis would be proportional to the value of a sine wave at the same angle. At 0° the value is zero and at 90° it’s at its peak value. Then it starts falling back to zero at 180° before going into negative values. It reaches its negative peak at 270° and goes back to zero at 360° before starting over again.
The physical location of the windings in a generator with respect to its rotation about the axis is directly related to the sine wave it generates. Just like the circle, when windings are at 0° and 180° relative to the magnetic field, the current is zero. When it’s at 90° and 270° the current is at its positive and negative peaks, respectively.
Some generators have three windings on one rotor. Each winding is spaced 120° apart from the other two windings so that they are each an equal distance from one another. When the rotor spins, each winding generates its own current. When one of the windings is perpendicular to the flux, generating the peak current, the next one is 120° behind it, cutting the flux in the opposite direction at a somewhat indirect angle to the flux. Because it’s not yet traveling perpendicular to the flux, it is not yet generating its peak current. It has just passed its negative peak current and is falling towards zero. The third winding is spaced 240° behind the first winding, or 120° ahead of the first winding and 120° behind the second winding. It has just passed zero current and is rising towards the negative peak.
The three waveforms, just like the windings, are “spaced”
120° apart from one another. They make up the three legs of a three-phase
system. They are called phases because each leg has a distinct phase angle,
which is their angle of rotation. Because there are 360 degrees in a circle or
a revolution, we can think of their physical angle as their electrical phase
angle. When the first leg is at zero phase angle, the
second is 120° behind it and the third is 240° behind
it. In the
Once you understand how magnetism works and how it acts to help generate electricity, you can explain it to your neighbor after you ruin his television. If you really understand the trigonometry, then you might baffle him enough to get a good running head start.
Direct your neighbor’s
complaints to the author at rcadena<at>swamicandela.com.