Horace wrote:
> One of the reasons 60 Hz was chosen over higher frequencies is the > prevention of transmission line losses. One main problem with using high > freqency 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. Horace has a point, for over long distances even widely spaced high voltage transmission line have shunt capacitance. The frequency standards were set early on when a lot of the AC machines had cast iron magnetic circuits and eddy current losses were greater the higher the frequency. Laminated construction came later. Traction systems still use 25 Hz for commutated motors. Series-wound 'DC' motors will run off AC, with lots of arcing from mechanical switching of the inductance of the field coils. The lower rhe frequency the easier this is. Series wound motors are preferred for traction service because their starting torque can be very high. Mike Carrell
