Ques. Name the kind of break that is difficult to locate.
Ans. A partial break.
Short Circuits.--In a series or compound dynamo a short circuit or heavy load will overload the machine and cause the fuses to blow. A shunt machine will not excite under these circumstances, for the reason that practically the whole of the current generated in the armature passes direct to the external circuit, and the difference of potential between the shunt terminals is practically nil.
Ques. What should be done if it be suspected that the failure to excite arises from this cause?
Ans. The main leads should be taken out of the dynamo terminals, then, if due to this cause, the machine will excite.
Ques. What parts of a dynamo are specially liable to be short circuited?
Ans. The terminals, brush holders, commutator, armature coils and field coils.
Ques. How are the terminals liable to be short circuited?
Ans. The terminals of the various circuits of the machine are liable to be short circuited, either through metallic dust bridging across the insulation, or through the terminals making direct contact with the frame of the machine.
The various terminals should be examined, and if the fault cannot be located by inspection, they should each be disconnected from their circuits and tested with a battery and galvanometer arranged as in [fig. 694].