158 



ELECTRICAL MACHINERY 



in Fig. 97 in which the result of a laboratory test is given. 

 The curves are practically straight lines, curve A being for 

 a weak field (all of the field rheostat was in series with the 

 field windings) and curve B for a strong field (field rheostat 



"all-out")- 



Series Motor. The series motor gives a different shaped 

 curve because the field strength of such a motor depends 

 upon the current flowing through its armature, the field 

 and armature being connected directly in series. The 



1C 



11 



10 



24. 6 8 10 12 14 16 18 20 

 Armature current 



FIG. 97. Current-torque Curves for Shunt Motor. 



formula for torque involves the product of the field strength 

 and armature current; when the field strength is directly 

 proportional to the current through the field coils the 

 torque must vary as the square of the current. 



This is the case with the series motor at light loads. 

 At values of armature current near full load (and for all 

 currents of higher value), the field is approaching satura- 

 tion so that the field strength is not proportional to the 

 current through the windings. At very high values of 

 current the strength of the field is practically independent 



