The short circuit may be in the terminals or connections, and these should first be examined and tested.
Some series dynamos are provided with a resistance, arranged in parallel or shunt with the field coils, to divert a portion of the current therefrom, and thus regulate the output.
When making a series dynamo excite, all resistances and controlling devices should be temporarily cut out of circuit by opening the shunt circuit. Series machines have frequently a switch which short circuits the field coils. Care should be taken that this is open, or otherwise the machine will not excite.
Fig. 699.--Watson armature complete. The armature coils are form wound, heavily insulated and so mounted on the core as to insure rapid dissipation of heat by ventilation. Each coil is protected by an insulating sheath and tape covering before mounting. The armature is baked after the coils are mounted to drive out all moisture, then, while hot, is treated with insulating compound and again baked twelve hours.
Wrong Connections.--When a machine is first erected, the failure to build up may be due to incorrect connections. The whole of these latter should therefore be traced or followed out, and compared with the diagrams of dynamo connections given in figs. 190 to 198.
Sometimes errors are made in connecting the field coils, causing them to act in opposition. This may occur when the dynamo is a new one or the coils have been removed for repairs. It may be caused either through the coils having been put on the field cores the wrong way, or through incorrect coupling up. Under these circumstances, the dynamo, if bipolar, will fail to excite; and if multipolar, poles will be produced in the yokes, etc. It may be remedied by removing one of the coils from the core and putting it on the reverse way, or by reversing its connections. The correctness of connections of all the coils should be verified.
In compound dynamos it sometimes happens that the machine will excite properly, but that the series coils tend to reverse the polarity of the dynamo, thus reducing the voltage as the load upon the machine increases. This may be detected when the machine is loaded by short circuiting the series coils, not the terminals. If the voltage rise in doing this, the series coils are acting in opposition to the shunt coils, and the connections of the series coils must be reversed.
Reversed Field Magnetism.--This is sometimes caused by the nearness of other dynamos, but is generally due to reversed connections of the field coils. Under such conditions the field coils tend to produce a polarity opposed to the magnetization to which they owe their current, and therefore the machine will refuse to excite until the field connections are reversed, or a current is sent from another dynamo or a battery through the field coils in a direction to produce the correct polarity in the pole pieces.
CHAPTER XXXII
ARMATURE TROUBLES
A large proportion of the mishaps and breakdowns which occur with dynamos and motors arise from causes more strictly within the province of the man in charge than in that of the designer. The armature, being a complex and delicately built structure, is subject in operation to various detrimental influences giving rise to faults.
Many of the faults which occur are avoided by operators better informed as to the electric and magnetic conditions which obtain in the running of the machine, especially the mechanical stresses on the copper inductors due to the magnetic field and the necessity of preserving proper insulation.