PhilP said:
16.7Hz..?
Now there is a very precise choice..
I would love to know the reason(s) for choosing such a low frequency. - Though it is 1/3 of 50Hz..
My brain is now trying to work out if there is a way to get this from a 3 phase 50Hz supply, and a clever use of transformers. It's a long time since I worked any theory like that!
I can't claim any personal knowledge of the reason for 16.7Hz but I have been told it was reduce the synchronous speed of AC motors (3000rpm for 2 pole at 50Hz). With a four pole motor at 16.7Hz, the speed is down to a more practical 500rpm. Multi pole motors are relatively expensive and I'm not sure if the electric motor technology in the 1920s was sufficiently advanced. It's likely that mechanical limitations did not allow (relatively) high speed reduction gearboxes in those days.
However I suspect that the real reason was to match the generators to the low speed turbines in the hydro power stations. 16.7Hz is the Swiss Federal Railways standard and the railway hydro stations are separate from the general public power system. The same technology regarding poles, frequency and synchronous speed applies to generators and certainly mechanical limitations even more so.
I believe there is a theoretical way of frequency changing using a passive (transformer) circuit but in practical power applications, frequency changers tended to be motor generator sets until electronic power ratings increased to allow rectifier / inverter solutions. Switched mode power supplies are usually far cheaper now than transformers for consumer devices, as well as smaller and far lighter.
Incidentally modern offshore oil installations in the UK North Sea often operate at 60Hz. This allows smaller lighter generators and motors making a significant cost saving in the structure. Again for weight reduction, some aircraft systems operate at 400Hz. Technology has moved on since the Swiss ran out of economical fossil fuel in the early 20th century and hydro electric power was the solution. Whither the next 100 years?