One main problem with using high
frequency transformers in power supplies until fairly recently was
rectification. Diodes drop in efficiency with frequency. These days the
availability of high current low voltage FETs (with switching logic to
achieve the rectification) permits efficient rectification, but even FETs
still have frequency limitations, just much higher AFAIK.
Regards,
Horace Heffner
Not so. Common variable speed drives now use switching transistors not diodes. They commonly switch at about 10 kilohertz. This is done to reform the sin wave using a bunch of square waves. Rectification at megahertz frequencies is not a problem.
This country started off with several standards. 25 hertz was used in the steel mills and coal mines until the early 1980s. The low frequency produced noticeable flicker in lights. I could see it out of the corner of my eye. It was not visible looking forward.
60 hertz produces no flicker, however, it is quite a feat to make a large steam turbine that can spin at 3600 rpm. This was not possible in the early days.
Aircraft commonly use 400 hertz. The spinning mass of the generator is small and transformers are of a much lighter weight.
Frank Znidarsic
