When an alternator generates voltage, we always use a multiple of 1.11 because for a pure sine wave the FORM FACTOR is the ratio of rms value of voltage or current with the avg. value of voltage or current and for pure sine wave rms value of current is Imax/root '2' and avg. value is 2Imax/pie and which comes out to be 1.1;

We can't have a combination of other then a multiple of 1.11*.

So we can see all the voltages are made inevitably multiple of this value (1.1, which is the form factor of ac wave).

Also it provides us the best economic construction of step up and step down transformers.

* In the case of a Square Wave ie. a digital wave, the RMS and the average value are equal; therefore, the form factor is 1.

We can't have a combination of other then a multiple of 1.11*.

So we can see all the voltages are made inevitably multiple of this value (1.1, which is the form factor of ac wave).

Also it provides us the best economic construction of step up and step down transformers.

* In the case of a Square Wave ie. a digital wave, the RMS and the average value are equal; therefore, the form factor is 1.