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Nathanael
Guest
What Dutch said. But I can go into a bit more detail. AC traction motors run off *variable frequency* AC and furthermore, controlling the frequency is how you control the speed of the motor. Commercial AC power and AC overhead power is *fixed frequency* (60 Hz for US commercial power and the northern part of the NEC, 25 Hz for the southern part of NEC)Thanks for the explination, but one question: Why is all power converted to DC when most of it, the traction component, is then converted back to AC?
Variable-frequency AC is very different from fixed-frequency AC. The easiest way to convert between the two is to have an intermediate DC stage (since DC doesn't have a frequency).