Thanks for your question. The 50kV AC suppy is transformed to 2.3kV and 3phases are converted to 2 phases by a Scott-connection ( en.wikipedia.org/wiki/Scott-T_transformer), which is the required supply to the main drive motor. The 2.3kV 2ph is also transformed to 440V 3ph for the supply to various drive motors in the auxilliary equipment. Here are two rotating converters that can produce 500 VDC and 125/250VDC, where the alternating current is rectified by means of a commutator, which allows direct current to be extracted from the rotor (en.wikipedia.org/wiki/Rotary_converter). Fredrik / Alexander association

@@AlexanderSAQ Thank you. Rotary converters are fascinating pieces of equipment. They were used quite a bit in early transit systems to convert 3 phase AC to DC for third rail power.

@@AlexanderSAQ So the motor spinning the HF alternator is an AC motor? How do you adjust the speed of the drive motor to adjust the transmission frequency? It also makes sense to me that a country like Sweden with a small population and relatively little international commercial activity could get by with one transmitting frequency. How about large generators of telegrams like the U.K. or the US?

@@Pamudder Thanks for your questions. To maintain a stable speed of the drive motor is a complex system. The voltage to the motor can be controlled between 1300 and 2300 VAC by using a Transductor, which is an adjustable choke coil, where the magnetic saturation of the metal core is adjusted with short pulses with 500VDC. The pulses are generated by the automatic speed control system which is chopping the field current to the 500VDC converter. Then the rotor of the drive motor is open in three phases. The resistance between the phases will determine the torque of the motor, and can be set using two separate liquid rheostats, one for key up and one for key down. At key down the latter is connected in parallell with the first, allowing for a lower resistance and thus more torque is applied to maintain the speed. Sweden had a massive emigration to the US, one of six moved during the hard years in the late 1800's and early 1900's creating a great need for communication. One can also think that the fact that the inventor, Ernst Alexanderson, was Swedish, played a role in the decision. Fredrik / Alexander association

@@AlexanderSAQ Thank you so much for the explanation.