1 82 ELEMENTS OF ELECTRICAL ENGINEERING. 



heavy load. It is generally due to under-excitation of field. 

 The displacement of the brushes from their proper positions 

 causes a shunt motor to run faster than its normal speed if its 

 load is not too great, but this is always accompanied by sparking 

 and this trouble, therefore, is covered by the discussion under 

 class I. 



84. Tests for faults in armatures.* In the four tests described 

 below, the field of the machine is not excited and the armature 

 is not run. 



(a) Tests for broken leads. Clean the commutator and sub- 

 stitute for one of the regular brushes a piece of sheet metal which 

 touches one commutator bar at a time. Connect a low voltage 

 and low resistance battery through an ammeter to the metal 

 brush and to the carbon brush, and turn the armature very slowly 

 by hand causing the metal brush to make good contact with each 

 commutator bar in succession. Usually there are two leads sold- 

 ered to each commutator bar. A break in any one of these leads 

 will be indicated by a reduced deflection of the ammeter, and a 

 break in both leads will be indicated by zero deflection of the 

 ammeter when the metal brush touches the commutator segment 

 to which the faulty leads belong. 



(b) If no broken lead is found by test (a), a broken circuit in 

 the armature winding itself may be located as follows : Connect 

 the low resistance battery through an ammeter to two metal 

 strips which touch adjacent commutator bars, and turn the arma- 

 ture very slowly by hand. An armature section which is broken, 

 or which has an abnormally high resistance, will be indicated by 

 a reduced deflection of the ammeter. In this test a short-circuited 

 armature section will be indicated by an increased ammeter de- 

 flection. 



In both of these tests (a) and (&) the ammeter circuit will be 

 repeatedly broken as the metal brush (or brushes) rides over the 



*See paper on "Location of Grounds in Armatures, Fields, etc.," by C. E. Gif- 

 ford, Trans. Am. I. E. E., Vol. XII., pages 260-267. 



