Changing the Frequency of an AC Main Supply

Rotary converter made to out put you required frequency. (Motor generator.)

Solid state converter using SCR's.

In the original construction of the motor-generator set, the number of windings/poles in the motor portion determines the speed of rotation, while the number of windings/poles in the generator portion determines the frequency of the output. More poles in motor means lower speed, so lower output frequency. More poles in output means higher output frequency. There is a limited number of ratios possible, or at least practical.

If you need variable frequency, forget the rotating device and get a variable frequency drive (solid state).

I remember but don't recall the full Description of "Fero-Resonent-Sub-Harmonics" protocole used in freq-changers to sub-multiple [30Hz for 60 Hz & 25 Hz for 50 Hz] and then multiplying by a factor of 3, 5 etc & even sub-sub-harmonic ie 15 & 12-1/2 Hz

A System was there & we used those systems prior to Digital era

better check the oilfield boys. using variable freq ac equipment for some time now!!!!

Yes

There are two way's to achieve this: an electronic and a mechanical.

The electronic is mostly used for low power applications: rectify and remodulate the power signal to the frequency you want (an audio amplifier is a perfect example: you feed it with 50Hz (or 60) and a range of 20 to 25 kHz comes out, without you hearing the original 50Hz)

The mechanical solution is to drive a syncrone motor and couple it through a gear box to a generator. Disadvantage: designed for only one conversion ratio.

The frequency of AC 50/ 60 cycles power supply can be converted to AC voltage of any required frequency by using frequency converters. The following are common methods used in Industry .

!. Motor generator sets ( MG sets ).

A mechanically coupled 3 phase 50 cycles Induction Motor is run at it's maximum synchronous speed of 3000 RPM .( N = 120 f /P , f being 50 cycles) The coupled generator , generates the required frequency as per the pole design as f= NP/120 where N is 3000 RPM .( So to obtain 4 K HZ ,number of poles have to be 480 and for 8 K HZ 960 poles) .The design of these generators are very special.

2. Electronic Generators

In this case 3 phase 50 cycles AC is converted to DC Voltage with the help of Thyratron's or Thyristors ( to control the power output) and then supplied to Oscillators where output frequency is decided by LC circuit placed in the grid of the oscillators.

I might be old fashioned but in my time we used diodes to rectify the AC to DC.

The Thyristor is used to recreate an AC signal. (driven by the occilator)

Regards but the Thyristor is used in Phase-controlled rectifiers, though these are also used in Inverters what you call "recreate an AC signal" but not (driven by the occilator) but a complicated Cct in which "Commutation" isrequiered as these are running on DC & SCR once triggered cannot be switch-off till the voltge across A & C is brought to zero for sufficient time to discharge its Storage.

Now Power-FETs are being used in Inverters or "DRIVES" they are called now