ROTARY CONVERTERS 95 



Constant Ratio 



The standard shunt wound rotary converter has a 

 very nearly constant ratio of alternating to direct 

 current voltage, so that any fluctuation in the voltage 

 of the alternating current supply will show directly 

 on the direct current voltage delivered. Such machines 

 are unsatisfactory when such variation in load occurs. 

 When the direct current volts have to be varied on a 

 standard machine, the impressed alternating current 

 volts must be altered. This is generally done by using 

 transformers provided with dial switches, by means 

 of which the transformer ratio is changed. 



If a series field winding is added to the standard 

 machine, a practically constant voltage can be obtained 

 with sudden changes in load by introducing reactance 

 into the circuit, or in some cases by using the inductance 

 and resistance inherent in the feeder circuit. This is 

 possible, for the reason that an alternating current 

 passing over an inductive circuit will decrease in potential 

 if lagging, and increase in potential if leading. 



A rotary converter running as a synchronous motor 

 requires a certain definite field excitation to effect the 

 minimum input current to the armature. Varying 

 the excitation either way changes the input current, 

 so that by using sufficient reactance in the alternating 

 current circuit from which the converter receives its 

 power, the alternating current voltage at the converter 

 terminals may be increased or decreased by increasing 

 or decreasing the field current. By adjusting the shunt 

 excitation of the compound wound machine to give a 

 no load lagging current of about 25 per cent full load 

 current, and the series field to give a slightly leading 

 current at full load, the impressed voltage at no load 

 will be automatically lowered and that at full load 

 increased. Hence, a practically constant direct current 

 voltage will be delivered at all loads. 



Variable Ratio Machines 



The split pole rotary differs from the ordinary rotary 

 in that the poles consist of two separate and independent 

 parts, each provided with its own field coil. The 



