EFFICIENCY OF A DYNAMO-ELECTRIC MACHINE 193 



This is a great advantage in the testing of large machines; 

 for example if a 1000-h.p. motor were to be tested, two dif- 

 ficulties would be encountered if it were attempted to 

 actually load it up to its rated capacity. First it would 

 be difficult to find some way of putting a load of 10.00 h.p. 

 on the motor and then of measuring it accurately, secondly 

 it would require about 1000 kw. of power to run the test 

 and even though this power were available, it would all 

 be wasted in running the test unless some special " pump- 

 back " test* was used. 



43. Obtaining Data for the Determination of Efficiency. 

 To determine the losses no facilities for loading the machine 

 are required and but little power is used in making the 

 test. To get the stray power the machine is run as a motor 

 with no load. When the rated voltage is impressed on 

 the armature, the field rheostat is adjusted until the rated 

 speed is obtained and then the input to the armature 

 circuit is measured. 



The resistance of the armature winding is determined 

 and the armature I 2 R loss (with no-load current flowing) 

 is calculated. This subtracted from the no-load input 

 to the armature gives the no-load stray power and this is 

 assumed as the same for all loads. The resistance of the 

 shunt field circuit is measured and the shunt field I 2 R loss 

 is found. The brush contact resistance is calculated after 

 the area of the brush contact has been measured and the 

 brush contact PR loss may be determined from the formula 

 discussed in a previous paragraph. 



* The term " pump-back " test is used to designate a test in which 

 two similar machines are being tested at the same time. In such a 

 case one machine is generally used as a motor to drive the other machine 

 as a generator; the power from the generator is " pumped " back 

 into the line from which the motor is drawing its power, hence the power 

 actually used, even when both machines are operating at approxi- 

 mately full load, is only that amount necessary to supply the losses 

 in the two machines. 



