THE MAGNETIC CIRCUIT 



191 



determined, the calculation of the size of wire for a given voltage 

 may be proceeded with exactly as explained in connection with 

 the winding of magnet coils (see Art. 10, Chap. II). The allow- 

 able cooling surface is not quite the same as for the coils of lifting 

 magnets, because the fanning effect of the rotating armature is 

 to some extent beneficial; but it will be convenient to consider 

 the heating effects of the shunt and series coils together, and 

 the question of cooling coefficients for use in predetermining 

 temperature rise will therefore be taken up later. 



Shunt Field Rheostat. Even when the machine is compounded 

 by the addition of a series winding, it is usual to provide an 

 adjustable resistance in series with the shunt winding. This 

 field rheostat allows of the excitation being kept constant not- 

 withstanding the fact that the shunt winding will not have the 

 the same resistance when cold as it will have when a continuous 

 run of several hours' duration has raised the temperature of 

 the coils. The rheostat also allows of final adjustments being 

 made after the machine has been built and tested. 



In compound-wound generators it is customary to allow a 

 voltage drop in the rheostat amounting to 15 or 20 per cent, 

 of the total terminal pressure; and a sufficient number of con- 

 tacts should be provided to avoid a variation of more than J^ 

 to 1 per cent, change of voltage when cutting in or out sections 

 of the rheostat. 



The size of wire for the shunt field coils should therefore be 

 calculated on the assumption that the impressed voltage is 15 

 to 20 per cent, less than the terminal voltage of the machine. 

 It will generally be found desirable to connect the windings on 

 all the poles in series. With shunt-wound dynamos, the field 

 rheostat plays a more important part: it must be designed to 

 give the required variation in field strength between no load and 

 full load, at constant speed, or, in the case of a motor, to provide 

 for the required speed variation. The amount by which the 

 excitation has to be varied apart from the requirements to 

 compensate for the effects of temperature changes may be 

 determined by reference to the saturation curve as drawn in 

 Fig. 74. 



In a well-designed machine, the PR losses in the shunt field 

 winding should not greatly exceed the values given below, where 

 the loss is expressed as a percentage of the rated output of the 

 dynamo : 



