2-B] EFFICIENCY. 55 



put of a motor can be determined if for a load it drives a shunt gen- 

 erator whose losses are known; it is best to have the generator 

 separately excited. 



The motor output is equal to the power taken to drive the genera- 

 tor, that is, to the measured generator output (El) plus generator 

 losses. The losses are the copper losses and the rotation losses 

 picked from curves (as in Fig. 2) for the particular speed and exci- 

 tation; to this should be added the belt losses,* a small but uncer- 

 tain quantity. If the generator is separately excited, no account need 

 be taken of field copper loss. 



25. Calibrated Motor for Measuring Power to Drive a Generator. 

 The power used in driving a generator can be determined if it be 

 driven by a shunt motor whose losses are known. The power taken 

 to drive the generator is equal to the motor input (El for the arma- 

 ture) less armature RP, less W 9 for the particular speed and exci- 

 tation, less belt loss (243). 



26. Return of Power to Line by "Loading Back." If a direct 

 current generator of suitable voltage is used as a load for a direct 

 current motor, the current from the generator may be " pumped 

 back" into the motor supply line (or into any other supply line). 

 Used as a method of loading, it saves power, avoids the necessity of 

 providing load resistances for the generator and introduces little 

 complication. 



The variation in load put upon the motor in driving the generator 

 is obtained by varying the generator field current. First let us sup- 

 pose that this is adjusted until the generator generates a voltage 

 equal to the line voltage. When connected to the line (the positive 

 terminal to the positive line), the generator will now neither give 

 nor receive current, that is, will neither give power to nor receive 

 power from the line. (At a lower excitation, it will receive power 

 as a motor.) If the field current of the generator is now increased, 

 it will generate a voltage higher than that of the line and will supply 

 power to the line. This power can be increased by a further increase 



* (243). Belt Losses. Cotterill (Applied Mechanics, p. 265) says: "In 

 ordinary belting this loss is small, not exceeding 2 per cent" The belt, 

 on account of its tension, also increases the journal friction of both 

 motor and generator. 



