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One may distinguish such systems by:
Early electric systems used relatively low-voltage DC. Electric motors were fed directly from mains, and were controlled by using a combination of resistors and relays which connected the motors in parallel or series.
The common voltages are 600 V and 750 V for tramSirio low-floor tram in Athens, Greece passes in front of the Acropolis A tram (or tramway trolley streetcar strassenbahn is a light-rail vehicle for public transport. Trams are distinguished from other forms of light rail in that they travel along trackss and metroThis page refers to urban rail mass transit systems. For other uses see metro (disambiguation metro in Athens, Greece double as museums with displays of antiquities found in excavations of its tunnels in the historic city. A Metro is a form of mass transis, and 1500 V and 3000 V for railways. In the past, Rotary converterRotary Converter refers to a class of electrical machinery that was used to convert one form of electrical power into another form. A very typical use would have been for railway electrification where utility power was supplied as Alternating Current ( acs were used to convert utility (mains) ac power to the required dc voltage. Today, this is usually accompished via semiconductorA semiconductor is a material that is an insulator at very low temperature, but which has a sizable electrical conductivity at room temperature. The distinction between a semiconductor and an insulator is not very well-defined, but roughly, a semiconducto rectifiers.
This system is quite simple, but requires thick wires and short distances between feeding stations; additionally, there are significant resistive losses.
Auxiliary machinery, such as fans and compressors are also powered by motors fed directly from mains. Consequently, these motors are often unusually bulky.
The 1500 V DC system is used in France, The Netherlands, Japan and Australia. The 3000 V DC system is used in Belgium, Italy, Poland, the Czech Republic, Slovakia, former Yugoslavia and in the former Soviet Union. 3000 V DC was also formerly used by the Delaware, Lackawanna & Western Railroad (now NJ Transit, now converted to 25,000 V AC).
Common commutating electric motors can also be fed alternating current, because reversing current in both stator and rotor does not change the direction of torque. However, this creates much stress on the motor. This stress can be reduced if the frequency is low, so many European countries, including Germany, Austria, Switzerland, Norway, and Sweden have standardized on 15 kV 16-2/3 Hz (one-third the normal mains frequency) single-phase alternating current (earlier, 6 kV and 7.5 kV systems have been in use). In the United States (with its 60 Hz distribution system), 25 Hz (an older, now-obsolete standard mains frequency) was commonly used at either 11 kV or 12.5 kV.
Motors are fed through a switching transformer that allows voltage change, so no resistors are required. Auxiliary machinery is driven by low voltage commutating motors, powered from a separate winding of the main transformer, and are reasonably small.
The unusual frequency means that electricity has to be converted from utility power by motor-generators at the feeding stations, or generated at altogether separate electric power stations.
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