Ans. The residual magnetism should be reversed by a current from another dynamo, or from a battery; but if this be not convenient, the connections between the machine and the line should be crossed so that the original positive terminal of the dynamo will be connected to the negative terminal of the line, and vice versa.
Fig. 679.--Method of correcting reversed polarity in compound wound dynamo. The polarity may be reversed without disconnecting or changing the wire. The figure shows two compound dynamos, and essential connections. The current from any machine connected to the equalizer bar by its equalizer switch will divide, a portion going through the series field winding of the other machines connected to the bus bar, the division being determined by the resistance of the different sets of coils. For instance, assume that No. 1 dynamo has had its polarity reversed and that No. 2 is running connected to the bus bar. The method of reversing the polarity of No. 1 machine is as follows: No. 1 machine should be at rest and then make sure that the circuit breaker and negative switch are open and that any other special connections to other machine or station lighting circuits are open. Then close the positive and equalizer switches, thus allowing a part of the current from the other dynamo to pass through the equalizer connection and through the series field winding of No. 1 machine in the usual direction, which will magnetize the magnetic core. If No. 1 machine be a large unit and No. 2 a small unit, it will be necessary to cut out the resistance of the shunt field circuits by means of the rheostat, if it be desired to maintain its bus bar voltage at its normal point. This will rob the series winding of any other machines which may be connected to the bus bars and will lower the voltage slightly. No. 1 machine is then brought up to full speed when it will be found to have recovered its correct polarity. The positive switch may be readily opened, watching the bus bar voltage closely as it will rise when the current is restricted again to the series field winding of the other machines. The dynamo will then be ready to cut in with the other machines as soon as the voltage has been brought up to the proper point, or it may be shut down until required.
Ques. Can a dynamo be reversed by reversing the connections between the field coils and the armature?
Ans. No, for if these connections be reversed, the machine will not build up.
Ques. Will a dynamo build up if it become reversed?
Ans. Yes.
Ques. Then what is the objection to a reversed dynamo?
Ans. Since the direction of current of a reversed dynamo is also reversed, serious trouble may occur if it be attempted to connect it in parallel, with other machines not reversed.
Attention while Running.--When a dynamo is started and at work, it will need a certain amount of attention to keep it running in a satisfactory and efficient manner. The first point to be considered is the adjustment of the brushes. If this be neglected, the machine will probably spark badly, and the commutator and brushes will frequently require refitting to secure good contact.