2-B] EFFICIENCY. 4 1 



EXPERIMENT 2-B. Efficiency of a Direct Current Motor* (or 

 Generator) by the Measurement of Losses. 



i. Introductory. Efficiency is the ratio of output to input. 

 The obvious and direct method for determining the efficiency of 

 a motor is, therefore, to measure the outputf and the input and 

 take their ratio. An indirect method, known as the method of 

 losses or stray power method, avoids the measurement of output. 

 In this method the losses are measured and the output obtained 

 by subtracting the losses from the input; the efficiency is then 

 determined. 



This method of losses possesses several advantages over meth- 

 ods that involve the measurement of output. The motor output 

 is in some cases a troublesome quantity to measure, especially if 

 accuracy is essential ; but, even with the same degree of accuracy 

 in the measurement of output and of losses, the efficiency cannot 

 be as accurately determined^ from the former as from the latter. 



* With the appendices, this experiment covers the main features of the 

 usual methods for determining the efficiency of any machine, direct or 

 alternating. The main experiment is explicit for determining the efficiency 

 of a shunt motor, and it is suggested that the student, without reference 

 to the Appendices, first performs this main experiment. The Appendices 

 should then be read and, if desired, a second experiment made (either 

 now or later) under some of the special conditions which are there treated. 



t ( ia). Direct Measurement of Output. The output of a motor can 

 be determined directly by electrical measurement (using for a load a 

 calibrated generator, 24), or by mechanical measurement (measuring 

 torque by means of a Prony brake, Brackett cradle dynamometer, etc.). 

 Power can be readily computed when torque and speed are known ( 3b, 

 Exp. 2-A). There are various forms of absorption and transmission 

 dynamometers conveniently arranged for the direct measurement of power. 

 For description of Prony brake, see Flather's Dynamometers and the 

 Measurement of Power and the usual hand and text books; also Electric 

 Journal, I., 419. For the cradle dynamometer, see Nichols' Laboratory 

 Manual, Vol. II., and elsewhere. 



$ (ib). Let us suppose that the error in measuring the input, output 

 or losses is one per cent., due to inaccuracies in the instruments or in 



