POWER LOSSES IN GENERATORS AND MOTORS. 131 



The prescribed degree of field excitation may be specified in 

 three different ways : (a) In terms of the current that is to be used 

 in the field winding ; (b) in terms of the voltage that is to be 

 applied to the terminals of the field winding ; or (c) in terms of 

 the electromotive force that is to be generated in the armature at 

 the prescribed speed. 



Let us consider, for example, a shunt dynamo of which the 

 stray power loss is to be determined at a speed of, say, 1,000 

 revolutions per minute with its field excited by applying an elec- 

 tromotive force of 1 10 volts to the terminals of the field winding. 



Let the machine be separately excited by connecting its field 

 winding to no- volt supply mains, and let current be supplied to 

 the armature through an ammeter and an adjustable resistance. 

 By adjusting this resistance the machine is made to run as a 

 motor at zero load at the prescribed speed. The armature cur- 

 rent, 7 a , is then read off the ammeter, and the voltage E is read 

 off a voltmeter connected between the brushes. 



Inasmuch as the machine is driven as a motor without load, 

 all of the power, EI at which is delivered to the armature, is 

 used to supply the stray power loss 5, and the armature loss 

 */' so that 



This gives the stray power loss under the prescribed conditions 

 as to field excitation and speed. The stray power loss at full 

 load (slightly different value of E a from that which obtains at zero 

 load) may be approximately calculated from the above value of 

 5 by using equation (30). 



Example. A given dynamo, rated as a loo-volt, 5O-ampere 

 generator, when driven at a speed of 1,200 revolutions per minute, 

 is tested for stray power loss as follows : The field is separately 

 excited by being connected to no-volt mains in series with an 

 adjustable rheostat. The armature terminals are connected to the 

 I lo-volt mains also, through an ammeter and an adjustable rheo- 

 stat, and the machine starts, running as a motor. A voltmeter 



