204 



ELECTRICAL ENGINEERING 



current. The speed equation was found to be 

 n = E ~J [r r.p.s. (Art. 123.) 



Shunt Motor. As the motor is loaded and I increases the speed 

 is affected in two ways, (1) the resistance drop IT increases and 

 tends to cause a proportionate decrease in speed; (2) the flux < 

 is decreased by the armature demagnetizing and cross-magnet- 

 izing m.m.f . ; the demagnetizing effect increases directly with the 

 current but the cross-magnetizing effect increases at a slower rate 

 due to the saturation of the path; therefore the armature reaction 

 tends to increase the speed. If the motor is operated at a point 

 just above the knee of the saturation curve the drop in speed due 

 to the armature resistance will be greater than the rise due to 

 armature reaction and the speed characteristic will fall as shown 

 in Fig. 175, curve 1. 



\ 



( 1 ) Shunt 



( 2 ) Compound 



( 3 ) Differential Compound 



(4) Series 



14 Load H Load % Load Full Load \\\ Load 



Armature Current 1 

 FIG. 175. Motor speed characteristics. 



The speed regulation of a motor is the rise in speed when full 

 Ibad is thrown off expressed as a per cent of full-load speed. 



The speed regulation of shunt motors of large size is from 2 to 

 5 per cent. 



