The high frequency generatorThe high frequency generator has a very different design. This is caused by the fact that it is supposed to produce a frequency of 17200 Hz, which is an extremely high frequency for rotating electrical machines. In order to keep the iron losses at a reasonable level, the stator is wound from very thin iron ribbon, unlike traditional alternating current machines, whose stators are constructed from iron sheets that are pressed together. Also the rotor of the high frequency generator has a very different design. Normally, the rotor of an alternating current machine has a winding fed by DC (direct current) in order to achieve a rotating magnetic field in the stator. The rotor of the high frequency generator is a steel disc, 1.6 m in diameter and 7.5 cm thick at the periphery. The steel disc has no winding. Instead, 488 slots are milled at the periphery. The slots are filled with non-magnetic material (brass), in order to reduce the air friction. The magnetic flux is generated by a field coil, situated outside the stator. The stator is designed to encircle the periphery of the rotor. The magnetic flux is conducted through the periphery of the rotor via the stator. The air gaps between the rotor disc and the stator are less than 1 mm wide. On the stator there are 64 armature windings that collect the variations of the magnetic flux. The variations arise when magnetic steel and non-magnetic material passes through the magnetic flux. Each armature winding gives 100 V and 30 A, which is conducted to the output transformers. An interesting detail on the high frequency generator is the device that keeps the rotor disc centred in the middle of the stator, independent of the changes of length of the rotor shaft as a result of varying temperature. Nominally, the high frequency generator has a power of 200 kW. In normal telegram transmission this power was seldom used and only the amount of power needed for good readability in USA was generated. Nowadays when the machine is run it is not excited very hard. When transmitting, the generator power is about 80 kW. |