86 THK I>lKE(T-(Ti;l;KNT MOTOR ell. II 



such that the current at this point would not injure the 

 motor. If the load is still further increased, the car will 

 descend, and the motion will cause the current to be in- 

 creased. If the load is suddenly taken off, as may happen 

 should the car be jammed on its way down, the speed of 

 the motor will increase to its maximum value. This is a 

 source of danger, as the ropes would be paid off the rope 

 drum and hang slack over the car. A slight shock might 

 then cause the car to fall, unless special safety arrangements 

 are provided. 



To return to the diagram. If the load were nothing, 

 the current also would be nothing, and the motor would 

 run at the maximum speed DO. In practice, it would be 

 impossible to reduce the load on a motor to nothing, since- 

 there is always some load due to friction of bearings and 

 brushes. We might, however, reduce the load absolutely 

 to nothing, by applying a small force in the contrary 

 direction, sufficient to balance the torque due to the 

 friction. If this were done, the current in the motor 

 armature would be nothing, and the speed would be given 



E 

 by M- 



Suppose now that we were to apply more force than 

 was necessary to overcome the frictional resistance, the 

 sign of the load would then be changed, and instead of 

 resisting the motion, the load would be assisting the 

 motion. But we have seen that the torque due to the 

 load must be equal and opposite to that due to the current 

 in the armature, hence if the load changes sign, the 

 current must also change sign. 



This conclusion is of great practical importance ; it 

 shows us that the current and the load change sign 



