Sixty-cycle synchronous converters

The paper describes some 60-cycle, six-phase, 600-volt, synchronous converters used in railway substations in the suburbs of Cleveland. The advantages of the use of 60-cycle apparatus include standardization of frequency, low cost and high efficiency. The converters are of the commutating pole type and a number of special features of their design are described. The improvement of commutation due to commutating poles makes it possible to use fewer poles and higher armature speed, but this has the objection of increasing the noise, which would have been objectionable in neighborhoods where these machines were used. To overcome the noise the armature ends were sealed up and a shield provided behind the front leads, thereby cutting down the ventilation to a considerable extent. The field windings were compounded in order to hold the d-c. voltage constant, and in order to maintain the power factor of the supply circuit at unity the converters were over-excited to produce a leading wattless current sufficient to overcome the reactance drop. In order to overcome the heating effect due to restricted ventilation and over-excitation the value of 3500 amperes per square inch of armature conductor based upon d-c. output at rating was adopted. To permit of a-c. starting the copper dampers are interconnected between the poles to produce a squirrel cage connection all around the field structure. Extracts from the specifications under which these machines were built are appended, as well as extracts from the contract under which the power supplied to the substations is purchased.