188 THE DIRECT-CUKRENT MOTOR CH. VI 



The efficiency curves for a motor with two magnet 

 windings are given in Fig. 33 ; one is for a high induction 

 curve and the other for a low induction curve. The 

 difference due to the two windings is well shown. 



If the resistances of two motors are not equal, the 

 advantage gained by the higher induction curve maybe 

 lost by the increase in the resistance. 



Example 37. A motor with a resistance of 1.-245 

 ohms, running on a line of 500 volts tension, with c / =2'5 

 amperes, has a maximum total efficiency of 85 per cent. 

 If the resistance is reduced to 1 ohm, the efficiency is 

 increased to 86 per cent. If the induction curve is 

 lowered, so that c f is 3 amperes, the efficiency, with 

 the reduced resistance, will be again 85 per cent. 



This is illustrated by some tests made on the G.E. 800 

 railway motor, of which the results are plotted in Fig. 34. 

 The curves marked ' magnets full ' are for the motor of 

 which the induction curve is given in Fig. 21, the 

 resistance of the armature being 0*44 ohm, of the 

 magnets 0-805 ohm total, 1-245 ohms. The curves 

 marked ' magnets shunted ' are for the same motor, when 

 the magnets are shunted with a resistance of 1-8 ohms, 

 thus sending 30 per cent, of the current through the 

 shunt. The tension of the line is 500 volts in each 

 case. 



We see from the curves that the maximum efficiency 

 of the shunted motor is higher than for the unshunted 

 motor, in consequence of the reduced resistance of the 

 former ; if the resistances had besn the same, the shunted 

 motor would have had the smaller maximum efficiency. 



Example 38. Taking the resistance of the G.E. 800 

 motor as 1-245 ohms for the unshunted magnets, and 



