448 



ELECTRICAL MACHINERY 



being electrically connected to the armature winding, may 

 give a fatal shock to an operator coming in contact with 

 it. 



The frame of any electric machine should be well insu- 

 lated from the winding and it is advisable to actually 

 connect the framework of the machine to a ground con- 

 nection, such as a waterpipe. Then the operator will not 

 receive a shock upon coming in contact with the frame of 

 the machine even though the winding should become 

 grounded on the iron frame of the machine. 



The location of a short-circuited coil, open-circuited 

 coil or ground is easily carried out with the help of very 

 simple apparatus. 



To Locate a Short-circuited or Open-circuited Coil. Remove 



FIG. 287. Connections for Locating a Short-circuited or Open-cir- 

 cuited Coil. 



all the brushes from the commutator but one pair. Con- 

 nect a dry cell to the pair of brushes, as in Fig. 287. Then 

 with a low reading voltmeter (the full range of which is 

 somewhat greater than the e.m.f. of the dry cell) read 

 the IR drop between every adjacent pair of commutator 

 segments. If there are no short-circuited coils or open- 



