1-40 THE DIRECT-CURRENT MOTOR CU. VI 



c f as 5-5 amperes, we find the maximum efficiency 

 on 500 volts to be 76'7 per cent. If the magnets are 

 now shunted, giving R=\'Q ohm, and c / =6 > 5 amperes, 

 we find the maximum efficiency to be 76 '7 per cent, as 

 before. 



We must not forget that we are here assuming that 

 the torque loss remains constant at all loads. From the 

 torque curves in Fig. 21 we see that this is not strictly 

 true. The torque loss actually increases with the total 

 load. This will prevent our applying the equations strictly 

 in such cases, but they may still serve as a general guide 

 in determining the form of the efficiency curves under 

 different conditions. 



We have also assumed that the torque is proportional to 

 the current ; this is so in a motor with constant induction 

 factor, but not in a series-wound motor. Thus in 

 Example 37, the currents for maximum efficiency are 

 by Equation 58, 47 and 57 amperes for the unshunted 

 and the shunted motor respectively. We see from the 

 curves in Fig. 33 that the currents are actually 25 and 33 

 amperes, the difference being due to the increase of M 

 with the current. 



Curves showing the variation in the efficiency of a 25 

 h.p. Westinghouse railway motor are given in Fig. 35. 

 One curve shows the efficiency when the motor is running 

 in parallel with a second similar motor, and driving a car 

 weighing seven tons, the tension of the line being 500 volts. 

 The other curve shows the efficiency when the two motors 

 are connected in series on the same tension. Equation 58 

 shows that the series curve has been carried too high in 

 the figure. The diagram is obtained from tests made by 

 Mr. H. S. Hering. 



