What if on third rail, instead of DC there was AC? Would it affect the train somehow? Would it save money?
Changing every motor in every car does not sound like a way to save money.
MAYBE. AC current and DC current have advantages and disadvantages depending on distance train goes and number of stops along the line. AC is better for Long distances.
All third rail systems throughout the world are energised with DC supplies. The use of AC is not feasible because of the skin effect. The geometry of the third rail and return running rail is very large compared with the skin depth (around 0.3 millimetres for steel), which means that the resistance to AC is a few orders of magnitude higher than it is to DC.
AC is dangerous. If someone contacted it, it would caused fibrillation and they couldn't let go. DC causes muscles to contract, a victim would jump off involuntarily. So the last scene in Taking of Pelham 123 is bogus. 600 volts wouldn't penetrate a rubber tennis shoe anyway, it would have to be soaked in salt water. It is also much easier to change direction with DC than AC. That's why DC was used , and Still used. AC would be of little advantage. . Short of a reverse gear, an extra winding would be needed to reverse an AC motor. An induction motor doesn't have brushes, true; but, if designed right, a DC motor loses little to sparking; and an induction motor would be very large and heavy for the power output compared to a DC series wound motor. And cost more and slow the train/tram down, actually wasting power. AC would not affect anything at 60 cycles nor on a straight rail or wire. REF: Took tram course on 4th st, down town. Electronics tech since' 78, First class FCC license plus Ham Extra. My daddy was electrician's mate 20 years in USN, fixed motors./dynamos, AC and DC.
