Fig. 678.--Method of correcting reversed polarity in large shunt dynamo by transposing the shunt field leads, and then starting up the machine. As soon as the voltmeter registers any voltage, the dynamo may be stopped and the field leads restored to their original position, when it will be found that the residual magnetism in the pole pieces will usually bring the dynamo up to its polarity and proper voltage. This method has the disadvantages, of the uncertainty as to the machine building up, and that a temporary wire must probably be run from the switchboard to one terminal of the field circuit, which is usually connected to a terminal back of the dynamo frame, so that the flow of current through the field coils may be reversed. With dynamos having laminated field magnet cores of comparatively low residual magnetism, this method may suffice, but in the case of solid field magnetic cores it is not practical. A better method is to disconnect the shunt field leads and temporarily extend them to some other source of direct current. If the current be of higher voltage than the coils are designed for, as for instance 110 volt dynamo and available current 500 volt, caution must be exercised and a suitable resistance be provided to protect the coils. A 500 volt coil, however, may be supplied from 110 volt circuit, providing the field winding to be energized is equipped with a cut off switch having a discharge resistance, so that it may be used to close and break the circuit when the temporary leads have been connected. If the field windings be not so provided, a bank of lamps or some other non-inductive resistance must be connected across the leads between the field magnet coils and the point at which the circuit is to be opened and closed. This is to provide a path for the discharge of the induced electromotive force. The circuit should not remain closed more than a few seconds if the full voltage can be applied. It is well, however, to leave the current on long enough to run the machine up to about half speed and make sure, by means of a voltmeter, that the polarity has been corrected. When this has been ascertained the dynamo should be stopped and the field winding leads returned to their proper terminals. Then the voltage will be brought up in the right direction, provided the work has been done correctly.
This can be tested by connecting a voltmeter across the terminals of the armature, or by means of a magnetic needle placed at a short distance from one of the pole pieces in such a position that it does not point to the north pole. If the field coils be improperly connected, the current due to the initial voltage will weaken the field magnetism and thus prevent the machine building up, and when the field circuit is closed the voltmeter reading will be reduced, or the magnetic needle will be less strongly attracted.
Ques. What will be the result if the connections of some of the field coils of a dynamo be reversed?
Ans. If one-half the number of coils oppose the other half, the field magnetism will be neutralized and the machine will not build up at all; but if one of the coils be opposed to the others, the machine might build up, but the generated voltage will be low, and there will be considerable sparking at some of the brushes.
Ques. How may it be ascertained which coil is reversed?
Ans. In all dynamos there should be an equal number of positive and negative poles, and in almost all of them the poles should be alternately positive and negative. Therefore, if a pocket compass be brought near the pole pieces, and it show that there are more poles of one kind than the other, the indication is that one or more of the coils are reversed, and the improper sequence of alternation will determine which one is wrongly connected.
Ques. When a dynamo loses its residual magnetism, how can it be made to build up?
Ans. By temporarily magnetizing the field. To do this a current is passed through it from another dynamo, or from the cells of a small primary battery. Usually, this will set up sufficient initial magnetism to allow the machine to build up. The battery circuit should be broken before the machine has built up to full voltage.
Ques. What should be done if a dynamo become reversed by a reversal of its field magnetism due to lightning, short circuit, or otherwise?