200 ELECTRICAL ENGINEERING 



nected across the line in shunt with the armature and therefore 

 has a constant excitation. (Fig. 167.) The compound motor 

 has a series winding carrying the load current in addition to its 

 shunt winding. The excitation therefore increases with load. 

 (Fig. 169.) 



The series motor has its field circuit in series with the armature 

 and has no shunt winding. Its excitation is zero at no load and 

 increases directly with the load current. (Fig. 168.) 



123. Speed Equation of a Motor. The impressed voltage is 



E = & + Ir, 

 and 



g = Zn3> -2 10~ 8 = Kn3> volts; 



Pi 

 therefore 



E = Kn3> + Ir 



and 



this is the speed equation. 



The term Ir is small in comparison to E and can be neglected 

 except at heavy loads. 



The speed equation can therefore be written 



thus, the speed of a motor is directly proportional to the impressed 

 voltage and is inversely proportional to the flux crossing the air 

 gap. 



124. Methods of Varying Speed. The speed can be varied in 

 three ways, (1) by varying the impressed voltage E, (2) by varying 

 the flux 3> by reducing the field current, (3) by shifting the brushes. 



(1) The voltage impressed on the armature can be varied by 

 introducing a resistance R, Fig. 170, in series with the armature. 

 If E, is the supply voltage, the voltage impressed on the motor is 



E = E.- IR. 



By increasing R, E can be reduced to any required value and 

 any speed from standstill to normal speed can be attained but no 

 increase of speed above normal. 



