108 ELECTRICAL MACHINERY 



Rating a Rheostat. A certain field rheostat might be 

 rated on its name plate: Resistance 35 ohms, maximum 

 current 8 amperes, minimum current 4 amperes. This 

 rating signifies that the total resistance of the rheostat is 

 35 ohms, that it will safely carry 4 amperes through all of 

 its resistance and that the first step will safely carry 8 

 amperes. Such a rheostat would be suitable for a field 

 having 28 ohms resistance connected to a 220-volt source 

 of supply. When the rheostat was " all in," the current 

 would be somewhat less than 4 amperes and when only 

 the last step of the rheostat was connected in the circuit 

 the current would be about 8 amperes. 



Relation of Rheostat to Field Resistance. If this, rheostat 

 was used to regulate the field of a small generator (say 

 one having 200 ohms resistance in the shunt field) it would 

 not have much effect. The change in field circuit resistance 

 from the " all in " position of the rheostat to the " all out " 

 position would be from 235 to 200 ohms and quite probably 

 this amount of change would not be sufficient. The field 

 rheostat to be used with any generator must be properly 

 designed for the field circuit of that generator. A rheostat 

 suitable for one field will not be at all suitable for another. 



Shunting the Series Field. It is impossible to design 

 the series field of a compound generator with exactly the 

 right number of turns; these field coils are therefore always 

 " over-designed/' By this is meant that the coil is wound 

 with more turns than are really necessary to give the required 

 number of series field ampere-turns, when all of the gener- 

 ator output current is flowing through them. 



The action of this series field is weakened by putting 

 a shunt across the terminals of the series field. This shunt, 

 generally made of German silver, ribbon-shaped conductor, 

 serves as a by-pass for part of the load current. The 

 division of the load current between the series field and the 

 series field shunt depends upon their relative resistances. 

 By adjusting the resistance of the shunt the number of 



