The first attempts to use standard-frequency single-phase AC were made in Hungary as far back as 1923, by the Hungarian Kálmán Kandó on the line between Budapest-Nyugati and Alag, using 16 kV at 50 Hz. The locomotives carried a four-pole rotating phase converter feeding a single traction motor of the polyphase induction type at 600 to 1,100 V. The number of poles on the 2,500 hp motor could be changed using slip rings to run at one of four synchronous speeds. The tests were a success so, from 1932 until the 1960s, trains on the Budapest-Hegyeshalom line (towards Vienna) regularly used the same system. A few decades after the Second World War, the 16 kV was changed to the Russian and later French 25 kV system.
How could the locomotives run on four speed levels?
motor could be changed using slip rings