THE CONTINUOUS CURRENT MOTOR 171 



IR a drop, however, again tends to make the speed decrease 

 as the load is increased so that the shape of the speed-load 

 curve is determined by the relative magnitudes of these 

 two effects. 



If the armature resistance is high and the armature 

 reaction and commutating poles relatively weak, the motor 

 speed will fall off as the load is increased. In a motor 

 having a smaller armature resistance the speed will actually 

 rise with an increase of load; one motor tested increased 

 its speed from 1000 r.p.m. at no load to 1080 at three- 

 fourths load and before full load was reached the motor 

 " ran away," i.e., the speed suddenly increased to such a 

 value that the protecting devices in the armature circuit 

 opened the supply line. 



Of course, the brushes were sparking to some extent 

 when in the position A' B f but not as badly as when in 

 the position A" B". In general it may be said, that, 

 with a backward shift of the brushes, the operation of a 

 commutating-pole motor is unstable; under certain con- 

 ditions the speed of the motor may oscillate violently even 

 when the load on the motor is constant. The motor men- 

 tioned above, with its main field weakened somewhat from 

 its normal strength, when the brushes were in the position 

 A' B r " hunted " between speeds of 800 r.p.m. and 1200 

 r.p.m. until, finally, the protecting fuses blew and opened 

 the circuit. 



We may conclude, therefore, that on a commutating- 

 pole motor the brushes must be accurately set in position; 

 a shift of even one-half the width of one commutator bar 

 from the proper position will often produce sparking and 

 unsteady running. 



37. Motor Starting Rheostat. When a motor armature 

 is stationary, there can be no e.m.f. generated in its wind- 

 ings because there are no conductors cutting lines of force. 

 If, then, the stationary armature of a shunt motor is con- 

 nected directly to the supply line, the current which will 



