Railways must operate at variable speeds. Until the mid 1980s this was only practical with the brush-type DC motor, although such DC can be supplied from an AC catenary via on-board electric power conversion. Since such conversion was not well developed in the late 19th century and early 20th century, most early electrified railways used DC and many still do, particularly rapid transit (subways) and trams. Speed was controlled by connecting the traction motors in various series-parallel combinations, by varying the traction motors' fields, and by inserting and removing starting resistances to limit motor current. COUNT 3
QSN1 What is speed limit for railways? ANS1 variable speeds QSN2 Why is DC motor being used more than AC type? ANS2 conversion was not well developed QSN3 What are two types of railway transportation still use DC motor? ANS3 rapid transit (subways) and trams