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244 it has, however, some minor defects. The whole of the power has to undergo a double conversion. In the central station the mechanical power of the steam engines must be converted into the electrical power represented by the three-phase high-pressure current; this at the sub-station has again to be converted into mechanical power by means of the electric motors, and, finally, this mechanical power must be again reconverted into the electric power of the low-pressure continuous current which is supplied to the users of electricity. This chain of repeated conversions lowers the efficiency. At full load the efficiency of a motor generator set as represented by Fig. 25 scarcely exceeds 83 per cent. It should further be noted that for every kw. capacity in the D.C. side of the set, about 1.2 kw. capacity must be provided in the A.C. side, so that the total dynamo capacity in the sub-station must be more than twice the output capacity. This makes the system expensive in capital outlay, whilst the low efficiency makes it expensive in working.

Both these defects are to a large extent overcome by the use of so-called "rotary converters," illustrated diagrammatically in