patri ke is taraf ya us taraf, zindagi mein hum sab RailFan ek taraf - Ananya D'Souza

The OHE of AC sections in India works on 25Kv single phase AC, this is fed to the loco with the aid of a panto where conversion has to take place to feed the three phase motors, older locos used ROTARY Converters where the input voltage was being fed to the Rotor(single phase) while Stator used to be a three phase STAR(wye) connection from which the output used to be taken.
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Modern Locos have STATIC converters which employ solid state equipment to create a rotating magnetic field between the single phase source...
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and star connections to generate a 3 phase output. So practicaly there is no rotating part but technically the field does keep changing. This avoids the need of a convertor with moving parts arcing,wear, losses, maintenance and other trouble.
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Locos with Static convertors fitted have it written on it, hope it helps..