2-A] SPEED CHARACTERISTICS. 35 



There is then no sudden discharge of field magnetism and con- 

 sequent liability to damage ; for, as the armature slows down it 

 generates a gradually decreasing electromotive force which main- 

 tains the field excitation so that it too decreases gradually. (If 

 there is an automatic release on the starting box, it opens the 

 armature and field circuits after the field excitation has decreased 



V 



to a low value.) 



The effects of induced electromotive force caused by sudden 

 field discharge can be reduced by absorbing its energy in a high 

 resistance shunt in parallel with the field circuit, or in a short- 

 circuited secondary circuit around the field core. A brass field- 

 spool will act in this way. 



Throwing power suddenly off the line, by opening the supply 

 switch, may cause fluctuations in line voltage, particularly in 

 case of large motors under load. To avoid this, before the sup- 

 ply switch is opened, the starting resistance may first be gradu- 

 ally introduced into the armature circuit, which, however, is not 

 to be opened; then the supply switch is opened. 



13. Compound Motor. In a compound* motor, the series 

 winding strengthens the field as the armature current increases. 

 On starting or under heavy load (i. e., at times when the arma- 

 ture current is large) the motor is accordingly given a very 

 strong field and therefore has for a given armature current 

 a greater torque than it would have with the shuntf winding 

 only. 



* ( I3a). To tell whether a series winding is connected "compound" or 

 " differentially," throw off the belt and start the motor (for a moment) 

 with the series coil only. If the motor tends to start in the same direction 

 as it does with the shunt coil, the winding is " compound " or "cumula- 

 tive;" if in the reverse direction, the winding is "differential." 



t(i3b)- This means a greater torque than it would have with the 

 same shunt winding only. The motor could be given a different shunt 

 winding which would give as strong a field and as great a torque as is 

 obtained by means of the compound winding. Such a shunt winding, 

 however, would give the strong field at all times ; whereas the compound 

 winding gives the strong field only at particular times, i. e., at starting 

 and under load. 



