Ques. What are the causes of excessive voltage?
Ans. Over excitation of the field magnet or too high speed.
In the former case, resistance should be introduced into the field circuit to diminish the current flowing therein if a shunt machine; or if a series machine, a portion of the current should be shunted across the field coils by means of a resistance arranged in parallel with the series coils; or the same effect may be produced in both cases by reducing the speed of the armature if this be possible.
If due to excessive speed, which will be indicated by a speed indicator, the natural remedy is to reduce the speed of the engine driving the dynamo, or, if this be not easily done, insert resistance into the dynamo circuit, as described above.
Ques. What are the causes of excessive current?
Ans. If the dynamo be supplying arc lamps, the excessive current may possibly be caused by the bad feeding of the lamps. If this be the case, the fact will be indicated by the oscillations of the ammeter needle, and the unsteadiness of the light.
If incandescent lamps be in the circuit, the fault may be caused by there being more lamps in circuit than the dynamo is designed to carry. Under such circumstances, another dynamo should be switched into circuit in parallel, or, if this be not possible, lamps should be switched off until the defect is remedied.
When motors are in the circuit, sparking frequently results at the dynamo commutator, owing to the fluctuating load. In such cases the brushes should be adjusted to a position at which the least sparking occurs with the average load.
Ques. What may be said with respect to reversal of polarity of dynamos?
Ans. When compound or series wound dynamos are running in parallel, their polarity is occasionally reversed while stopping by the current from the machines at work.
Loose Connections, Terminals, etc.--When any of the connecting cables, terminal screws, etc., securing the different circuits are loose, sparking at the brushes, as a rule, results, for the reason that the vibration of the machine tends to continually alter the resistance of the various circuits to which they are connected.
When the connections are excessively loose, sparking also results at their points of contact, and by this indication the faulty connections may be readily detected. When this sparking at the contacts is absent, the whole of the connections should be carefully examined and tested.