114 THE IHRECT-CURRENT MOTOR CH. V 



motor is placed by itself on the full tension of the line, 

 500 volts. (2) When the motor is in series with one other 

 similar motor. (3) When the motor is in series with three 

 other similar motors. 



This diagram enables us to find at once the speed 

 for any given load with any given arrangement 

 of motors ; the torque and current given is that per 

 motor. Thus for a load of 3,000 inch-pounds per motor, 

 each motor would run at 320 r.p.m. if in parallel ; and 

 at 70 r.p.m. if in series with a second motor. A car 

 equipped with four motors with a load of 3,000 inch- 

 pounds on each could not move if all four motors were 

 thrown in series. 



The curves in Fig. 28 for different tensions may be 

 obtained graphically in the same way as the speed and 

 torque curves were obtained in Fig. 20, or we may calculate 

 the speeds for given current and given torque from formulas. 

 We know the induction factor for any given current, so 

 that we can obtain the speeds for different values of E, the 

 tension on the motor terminals. 



The speed of a motor in series with one other similar 

 motor is less than half the speed of the motor when placed 

 direct on the full tension of the line by an amount depend- 

 ing on the load. The speed in parallel can be expressed 

 thus : E cR 



And the speed in series is 



n s =^-- cR ... (45). 



So that we can put 



i i C XL / \ i *\ 



-n a = ^n -i-^ r (4b). 



j y -6 fl/r \ / 



