the sine wave is exists from nature,when we use signals
like ramp,step,impulse, dirac comb,rect,etc..,originating
from sudden change so system components like
diodes ,resistors will destroy due high flow of current.due
to this sinewave is prefferble
any waveform showing change in magnitude and direction is
ac wether it is square triangular or whtever, but
calculation using sinusoidal(sine and cosine as well)
becomes easy. for exmpl sum of two sine waves is a sine
The reason is that electromechanical alternator outputs sine-wave AC.The voltage produced by the stationary coils by the motion of the rotating magnet is proportional to the rate at which the magnetic flux is changing perpendicular to the coils (Faraday's Law of Electromagnetic Induction). That rate is greatest when the magnet poles are closest to the coils, and least when the magnet poles are furthest away from the coils. Mathematically, the rate of magnetic flux change due to a rotating magnet follows that of a sine function, so the voltage produced by the coils follows that same function.
Basically because every periodic function can be represented as a
function of sine and cosine waves (Fourier expansion).
f(x)= a+ sin nx + cos nx (n varies from 1 to infinity)
Therefore a sine (or cosine) wave of AC is the most flexible form
of usage since it can be converted into any other form as required.
Remember, a cosine wave is also a sine wave that is phase shifted
by 90 degrees..
if we apply any wave form other than sine wave to the transformer the output is spikes due differentiation and devices dont work for those spikes and damage all home appliances so we use sine wave only