DC commutating electric motors, if fitted with laminated pole pieces, become universal motors because they can also operate on AC; reversing the current in both stator and rotor does not reverse the motor. But the now-standard AC distribution frequencies of 50 and 60 Hz caused difficulties with <answer>inductive reactance and eddy current losses</answer>. Many railways chose low AC frequencies to overcome these problems. They must be converted from utility power by motor-generators or static inverters at the feeding substations or generated at dedicated traction powerstations.
What problems did AC distribution cause?
inductive reactance and eddy current losses