DIRECT-CURRENT MACHINERY . 159 



ture current (Fig. 123); and compound machines in which the 

 field has both a shunt and a series winding (Fig. 124). 



According to the number of poles machines are divided into 

 bipolar and multipolar machines but the bipolar type is only 

 adapted for small sizes. 



102. Direction of Rotation of Generators and Motors. Fig. 

 125 represents either a generator or motor. The directions of the 

 currents in the armature are shown by the dots and crosses and 

 the directions of rotation by arrows. 



Generator 



Motor 



FIG. 125. Direction of rotation. 



With currents as shown, the direction of rotation of a generator 

 is counter-clockwise and of a motor is clockwise. 



The direction of the e.m.f. of a generator is the direction of the 

 current but the generated e.m.f. in a motor is opposed to the 

 current. 



These results are obtained by examining the fields produced by 

 the armature currents. 



Take for example the conductor A. Its field combines with 

 the main field and produces a- strong field below the conductor 

 and a weak field above it. There is therefore a force / acting on 

 the conductor tending to move it up. This is the force that must 

 be overcome by the engine driving the generator in order to de- 

 velop electric power and therefore the rotation of a generator is 

 against this force and is counter-clockwise. 



In the case of the motor, the force / on the conductor is the me- 

 chanical force developed and the rotation is in the direction of this 

 force and is clockwise. 



To reverse the direction of rotation of a motor it is necessary to 

 reverse either the armature current or the field current but not 

 both. 



