THE OPERATION OF THE DYNAMO AS A MOTOR 1 1/ 



becomes excessive. The object of the differential compounding 

 of a motor is to prevent the decrease of speed with increase of 

 load, or even to cause an actual increase of speed with increase 

 of load up to full load. The shunt motor, however, runs at a 

 speed which is nearly enough constant for most purposes, and 

 therefore the differential compound motor is seldom used. 



The cumulative compound motor. The speed of this ttiotor 

 decreases very considerably with increase of load. Such a motor 

 has, however, the great advantage that at starting, when the cur- 

 rent through the armature and series field winding is large, the 

 total field excitation is large and hence the torque is large. That 

 is to say, the cumulative compound motor has a large starting 

 torque like the series motor, as explained later. 



At starting, when the current through the armature and series 

 field winding of this motor is large, the total field excitation is 

 very large. The advantage of this large field excitation (large <1>) 

 at starting is twofold : (a) the torque is large (= \.^\QZ' I a ac- 

 cording to equation 25), and (#) the counter electromotive force, 

 <&Z'n y is larger than it would be with a smaller value of <E>. 

 The result is (a) that the motor accelerates more rapidly, and (^) 

 that during the time that the motor is speeding up its efficiency 

 is greater than it would be if its counter electromotive force were 

 smaller in value. 



The cumulative compound motor has the disadvantage that its 

 running speed decreases very considerably with increase of load, 

 on account of the increase of <l> produced by the series field 

 winding. This disadvantage may, however, be obviated by pro- 

 viding a device for automatically cutting out the series field 

 winding by short-circuiting it after the motor is fairly started. 

 The machine then runs as a simple shunt motor. 



Cumulative compound motors are frequently used for operating 

 elevators and hoists, rolling mills, and other machinery where 

 large starting torque is required, and where constancy of speed 

 under variations of load is not necessary. The advantage of the 

 cumulative compound motor over the series motor for service of 



