Why Supply Frequency Is Either 50 Hz or 60 Hz ??

Ever wondered why the supply frequency is kept fixed at 50 Hz or at 60 Hz. yes the probably answer everyone would give is for the sake of uniformity, or it is adopted as a standard to have an understanding between the consumers and the generation side. Yes I agree completely, and I am giving a table of power frequencies used by some nations:

Here if you have observed the supply frequency is fixed to two values, either 50 or 60. The real question is why 50 or 60 only, why not 75 or 100 Hz??

The reason is because of the limitations of synchronous generators. We use synchronous generators (AKA turbo alternators) in thermal plants. The turbines of these turbo alternators have maximum efficiency at 3000 rpm to 3600 rpm. So when we calculate the frequency, (min of 2 poles) we get f= 50 or 60.

frequency, f = PNs / 120

P = no. of poles

N_s = Synchronous speed in rpm

The poles are suitably reduced to 2, because a large number of poles means, larger diameter of the rotor, which in turn means more centrifugal forces are developed during such high speed rotation.

And we all know most of the power plants are thermal based. 

Hence the power frequency is fixed at either 50 Hz or 60 Hz.