28 THE DIRECT-CURRENT MOTOR CH. II 



The forces acting on the shaft of a dynamo 

 may be divided into two classes, those which assist and 

 those which resist the motion. 



Thus, when a dynamo is acting as a generator and 

 driven by a steam engine, the force of the engine assists 

 the motion, while the action of the current in the armature, 

 if any, and the friction resist the motion. 



If the dynamo is acting as a motor, the forces of the 

 current in the armature acting on the magnetic field assist 

 the motion, while the friction and the useful working forces 

 resist the motion. 



We shall use the term load to denote the forces 

 in a motor tending to resist the motion, and we shall 

 express these in terms of inch-pounds of torque. 



A motor may be said to be ' free to move ' when the 

 load is finite. If the shaft is blocked in any way the load 

 is infinite and the motor is not free to move. It does not 

 follow that the shaft will turn if the motor be free to move, 

 since the torque produced by the action of the current in 

 the armature may not be sufficient to cause motion. 



When the magnets of a dynamo are excited and the 

 armature caused to rotate, either as motor or generator, 

 we may divide the torques acting on the shaft as follows : 



1 . The torque on the pulley ; this assists the motion 

 in a generator and resists the motion in a motor. 



2. The action of the current in the armature, given by 

 =l'41cAf ; this assists the motion in a motor and resists 

 the motion in a generator. 



3. The action of eddy currents in the armature and 

 pole-pieces; these oppose the motion in all cases. 



4. Friction of the bearings and brushes, and wind 

 resistance ; these oppose the motion in all cases. 



