ELEMENTS OF ELECTRICAL ENGINEERING. 



required to overcome friction and windage, and to supply the 

 eddy current and hysteresis losses in the armature core. 



36. Typical examples of series motor characteristics. (a) 

 Series motor supplied with current from constant voltage mains. 

 Fig. 43 shows a typical speed-torque characteristic of a series 

 motor supplied with current from constant voltage mains. When 

 the speed is zero the counter electromotive force of the motor 



Speed 



Speed 



Torque 



Fig. 43. 



Fig. 44. 



is zero ; the current through the motor is then very large, and 

 since this current flows through the field winding of the series 

 motor, the field excitation is large. Therefore the flux <I> is large, 

 and, according to equation (25) Chapter IV., the torque is very 

 large. As the motor speeds up its counter electromotive force 

 increases, the current decreases and the torque decreases. When 

 the motor is unloaded its speed becomes excessive, and the torque 

 drops to the value which is necessary and sufficient to overcome 

 friction, and windage, and, in addition, to supply to the armature the 

 power lost in the armature core by hysteresis and eddy currents. 



The portion cd of the characteristic is the only portion used in 

 practice, inasmuch as the excessive current which flows through 

 the motor at low speeds would produce destructive heating, and 

 the excessive speeds at light loads are dangerous. 



(>) Series motor supplied with constant current. Fig. 44 

 shows a typical speed-torque characteristic of a series motor 



