Posted by **Ernie Nemeth**
(here)
Sinusoidal waveforms seem so complicated but it is not really. The title of an average electrical textbook would make it far clearer: Rotating Machines. It is the circular motion of electrical generators that causes the sinusoidal waveform. As the machine turns it pumps out a varying rate of voltage and current, depending on windings and configuration, that completes a 360 degree circle while turning. Each turn or portion thereof can create a full sine wave, again depending on configuration and number of poles in the machine. The sine wave is the natural output of a rotating machine.

It is called the sine wave because it is trigonometry that describes the mechanics of rotation. The cosine coefficient is the relational unit, like miles or kilometers for distance, that relates degrees of rotation to its component effects of current and voltage. In a rotating machine, voltage and current are constantly changing their value as lines of force are cut at varying rates as the rotor turns within the field of the stator. This creates the familiar sine waveform, starting at zero and rising to a maximum positive value, then falling back to zero and reaching a maximum negative value, and then back to zero.

This rising and falling action is typical for all rotating machines. To create DC directly with a rotating machine requires a new piece of equipment - the commutator, which flips the polarity of the connection as the rotor crosses the poles of the machine during rotation. This causes the output to remain a steady polarity (DC), instead of flipping polarity (AC). The resulting DC power from such a generator has a rising and falling characteristic but only in one direction - ie. zero to a max positive value then back to zero then back again to high positive value. This fluctuating DC wave can be smoothed out with electronics but can never be a completely smooth output of a given value. For that you need a non-rotating source like a battery.

Hope the crash course helps

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