128 ELEMENTS OF ELECTRICAL ENGINEERING. 



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in which R g is the resistance of the shunt winding including the 

 field rheostat, / is the current in the shunt winding, and E x is 

 the electromotive force between the terminals of the machine. 

 The resistance of the shunt winding increases considerably after 

 the machine is started, on account of rise of temperature. In 

 calculating field loss, the resistance of the winding at the steady 

 running temperature of the machine should be used ; this is fre- 

 quently called the hot resistance, for brevity. 



The field loss of a shunt dynamo (generator or motor) is ap- 

 proximately constant. Thus the field winding of a shunt motor 

 is usually connected directly to the constant voltage mains, and, 

 after the steady running temperature has been reached, the field 

 loss is strictly constant whatever the load on the motor may be. 

 On the other hand, the terminal voltage of a shunt generator 

 varies more or less with its current output and, inasmuch as the 

 field winding is connected across the terminals of the machine, 

 the field current, and therefore the field loss also, vary slightly. 

 When the terminal voltage of a shunt generator is kept constant 

 with increase of current output by the manipulation of a field 

 rheostat, the field current and also the field loss change consid- 

 erably. 



(2) For a compound dynamo : 



F= R,I? + RJ* (28) 



in which R s and / have the same significance as in equation 

 (27^), R c is the resistance of the series field winding, and / is 

 the current in the series field winding. 



In applying equation (28) to " long shunt" or "short-shunt" 

 compound dynamos, careful attention must be given to the diagram 

 of connections to insure that the values of / arid / are cor- 

 rectly derived from such data as may be given. 



(3) For a series dynamo equation (28) may be used by dropping 



