CHAPTER V 

 THE EFFICIENCY OF A DYNAMO-ELECTRIC MACHINE 



40. Importance of a High Efficiency. If the amount of 

 electrical power put into a motor is measured and the 

 mechanical power output is measured during the same time, 

 the efficiency of the motor may be obtained by finding 

 the ratio of the output to the input. In just the same way 

 the efficiency of a generator is the ratio of the output to 

 the input; in this case, however, the input is mechanical 

 power and output is in the form of electrical power. Both 

 input and output must be expressed in the same unit before 

 the efficiency may be calculated. Suppose, for example, 

 that the input to a small electric motor is 1 kw. and the 

 output is 1 h.p. The output in turns of kilowatts is .746, 



.746 

 so that the efficiency is = 74.6%. Or we might say 



the input is equal to 1.34 h.p. so that the efficiency is equal 

 10^ = 74.6%. 



Example of the Operating Cost of a Low-efficiency Machine. 

 The efficiency of an electrical machine is one of its most 

 important characteristics. To illustrate the importance of 

 a high efficiency for a motor let us consider the case of a 

 factory requiring 100 h.p. to run the machinery installed 

 in it. If the motor used to drive the shafting has an 

 efficiency of 90% the necessary input when the motor is 

 giving off 100 h.p. is equal to 100^.90 = 111 h.p. which is 

 equal to 83 kw. Suppose the price of power is $.06 per 

 kw,-hr. (prices vary between $.04 and $.15 per kw.-hr., 



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